Print bookPrint book

Week 24 - Measuring Forces and Designing Structures to Resist Forces

Tab: Exercise 2.1


Lesson 2.1: Measuring Forces


ACTIVITY A: Mass verses Weight



Textbook Readings

Science in Action 7
pages none
or

Science Focus 7

pages 298 to 300


At one time or another we have all wanted to walk on the moon. When we see pictures from the moon we notice how easy the astronauts walk or bounce on the surface of this Earth-orbiting satellite. Why do they look lighter on the Moon than on Earth?

In 1969, Neil Armstrong and Buzz Aldrin made a historical walk on the moon. Since you were not alive in 1969, you can view his walk in the Quicktime movie below.

You must be connected to the Internet to view this animation
or
http://www.firstscience.com/site/video/history/spaceflight2kb100.wmv

In order to understand why these astronauts appeared to weigh less we have to understand the difference between mass and weight. We need to compare a few points:

1. Mass is a measurement of the amount of matter something contains, while Weight is the measurement of the pull of the force called gravity on an object.


When you were studying Heat and Temperature you learned that all objects are made up of tiny particles. A horse is made up of many of these particles, so it has a large mass. A gold bar contains fewer particles and therefore has a smaller mass. However, the gold particles are packed somewhat closer together than many other objects of the same size. If you compared a gold bar and a slice of bread the same size, which of these objects has particles packed closer together, and therefore has more mass?

WEIGHT
=
MASS
X
PULL
OF
GRAVITY

When we weigh something we are actually measuring the pull of gravity on a mass. If you multiply your mass by the pull of Earth's gravity, you get your weight. By eating or exercising, you are actually changing the number of particles you are made up of; this is your mass. The earth's gravitational pull, on the other hand, decreases as you move farther away from the Earth. Therefore, you can lose weight by changing your elevation, but your mass remains the same. You can also lose weight by living on the moon as it has less gravitational pull, but again your mass is the same.
2. Mass is measured by using a balance comparing a known amount of matter to an unknown amount of matter. Weight is measured on a scale.


A balance is the most common instrument for measuring mass. A balance compares the mass of the object being measured with the standard masses. The most common type of balance used in the school setting is the Triple Beam Balance.
When we measure weight we are measuring the mass multiplied by the force of gravity. We have a number of common measuring devices that assist us in measure weight. When we step on the bathroom scale, we are measuring the amount of force our body mass pushes on the springs in the scale. When we measure the weight of a bag of apples at the store, we place them in the tray of a large spring scale. The mass of the apples will cause the force of gravity to pull down on the scale, therefore giving us a measurement. The only question remaining is: Are the units of measure on the bathroom scale and the produce scale at the grocery store the proper units to measure force?
3. The mass of an object doesn't change when an object's location changes. Weight, on the other hand, does change with location.


Mass only changes if the number of particles in an object changes. When you move around or change locations, you do not change the number of particles in an object. In space, you are still made up of the same number of particles. The effect of the force of gravity is the only thing that has decreased.
Anywhere in the Universe, there are forces of attraction between objects. These forces try to pull objects together. The larger the object the more attraction or gravitational force it has. If you look at the attraction between the Earth and an apple, the Earth has much more pull due to its size than the apple. The further an object gets from another object, the less pulling force it has. When the space shuttle takes off from the Earth, it needs the added force of booster rockets to overcome the gravitational pull of the Earth. Once the shuttle is far enough away it can drop the boosters and carry on with less force. Even in space the Earth has some gravitational pull, however the force is much less than on the surface of the Earth. The Space Shuttles use Earth's gravity to stay in orbit around the Earth. If there was no gravitational force the shuttle would not orbit the Earth, but instead travel away from it in a straight line.
Exercise 2.1A
Complete the questions below and then use the pop up key to correct your work.

1. In your own words, provide a definition and an example of mass.
<
>
2. In your own words, provide a definition and an example of weight.
<
>
3. Weight is dependent on mass and what other variable?
<
>
4. If you had two cups of the same size, which would have more mass, one made out of glass or one made out of styrofoam? Why?
<
>
5. What are the differences between a balance and a spring scale? What are they used to measure?
<
>
6. Does an elephant on the moon have the same amount of mass as it would on the Earth? Why or why not?
<
>
7. Using the resources around you (such as the Internet, parents, fellow students) find out what SI units are used to represent the mass of an object.
<
>
8. A bunch of apples are placed on a triple beam balance and the three counter masses displayed the following when a balance was reached: 60g, 400g, and 7g. If each apple had an individual mass of approximately 67g, how many apples were placed on the balance?
<
>
9. What is the weight of an object if it has a mass of 300g and the gravitational pull is approximately 10 m/s2. Make sure to include the units.
<
>
10. Explain how you know that the Earth is affected by the gravitational pull of the Sun.
<
>
11. Using the following diagram of the Solar System, estimate if a person would weigh more or less than they do on the Earth on each of the following planets.

Section: Key for Exercise 2.1A


    Exercise 2.1A
      1. In your own words, provide a definition and an example of mass.

      (2 marks)

      Answer: Mass is the measure of the number of particles in an object and has nothing to do with force. For example: An apple has 67 grams of mass

      2. In your own words, provide a definition and an example of weight.

      (2 marks)

      Answer: Weight is a measurement of the force of gravity on an object. For example when you step on a bathroom scale you are measuring the amount of force your mass applies to the bathroom scale.

      3. Weight is dependent on mass and what other variable?

      (1 mark)

      Answer: Pull of gravity

      4. If you had two cups of the same size, which would have more mass, one made out of glass or one made out of styrofoam? Why?

      (2 marks)

      Answer: The glass would as the particles are closer together. In a styrofoam cup the particles are not as tightly packed. Air makes up much of the space in a sytrofoam cup. Air molecules are know for being loose packed.

      5. What are the differences between a balance and a spring scale? What are they used to measure?

      (4 marks)

      Answer: A balance is used to compare two different masses where one of the masses is a predefined or standardized value. A spring scale measures the force of gravity pulling on a mass. A balance measures mass and a spring scale measures weight.

      6. Does an elephant on the moon have the same amount of mass as it would on the Earth? Why or why not?

      (2 marks)

      Answer: Yes, it does. When an elephant goes to the moon the number of particles in the elephant stays the same; this is the elephant's mass. Only the gravitational pull changes and not the mass.

      7. Using the resources around you (such as the Internet, parents, fellow students) find out what SI units are used to represent the mass of an object.

      (2 marks)

      Answer: grams (g) or kilograms (kg)

      8. A bunch of apples are placed on a triple beam balance and the three counter masses displayed the following when a balance was reached: 60g, 400g, and 7g. If each apple had an individual mass of approximately 67g, how many apples were placed on the balance?

      (2 marks)

      Answer: 7

      9. What is the weight of an object if it has a mass of 300g and the gravitational pull is approximately 10 m/s2. Make sure to include the units.

      (2 marks)

      Answer: 3000 gm/s2

      10. Explain how you know that the Earth is affected by the gravitational pull of the Sun.

      (2 marks)

      Answer: The Earth is affected by the Sun because the Earth rotates around the Sun. If the Sun did not have a gravitational affect on the Earth, the Earth would continue to move through space in a straight line instead of circling the Sun.

    11. Using the following diagram of the Universe, estimate if a person would weigh more or less than they do on the Earth on each of the following planets.

    • Mercury
      (1 mark)

      Answer:
      Weigh Less; Mercury is a smaller mass and therefore has a weaker gravitional pull
    • Saturn
      (1 mark)

      Answer:
      Weigh more; Saturn is a larger mass and therefore has a stronger gravitational pull
    • Jupiter
      (1 mark)

      Answer:
      Weigh more; Jupiter is a larger mass and therefore has a stronger gravitational pull
    • Mars
      (1 mark)

      Answer:
      Weigh Less; Mars is a smaller mass and therefore has a weaker gravitational pull
    • Pluto
      (1 mark)

      Answer:
      Weigh Less; Pluto is a smaller mass and therefore has a weak gravitional pull
  • Mercury
    <
    >
  • Saturn
    <
    >
  • Jupiter
    <
    >
  • Mars
    <
    >
  • Pluto
    <
    >


ACTIVITY B: Sir Isaac Newton and Force



Textbook Readings

Science in Action 7
page 284


Science Focus 7
pages 298 to 300


Making famous scientific discoveries seems like a difficult thing to accomplish, but once discoveries are made they often seem obvious. In 1687, Sir Isaac Newton became the first person to discover the "law of universal gravitation". Some say he made this discovery while watching an apple fall from a tree. This law states that every object in the universe attracts every other object. The force of attraction is based on their masses and on the distance between them. He used his theory to calculate the orbits of planets. Although many other people understood the law of gravity, Sir Isaac Newton was the first to prove it mathematically.

As Sir Isaac Newton was the first to mathematically prove a force, he is honoured by the standard (SI) metric unit of force being called a newton (N). One newton is defined as the amount of force required to hold up 100 g of mass. If you placed an orange in your hand, you would have to push back on the orange with 1 N of force to hold up the orange.

Take a look at the examples below:

Holding a 800 g textbook requires 8 N of force

Lifting a 300g pop can requires 3 N of force
Lifting a 1 litre milk jug requires 10 N of force

Consider this question:
If weight is a force, are the units that are displayed on your bathroom scale actually showing your weight or your mass?
Exercise 2.1B
Complete the questions below and then use the pop up key to correct your work.

1. What does the "law of gravitation" state?
<
>
2. The gravitional force of attraction is based on what two things?
<
>
3. How many Newtons of force are required to lift a 250 g cell phone?
<
>
4. What is the mass of a full 2 litre milk carton? Explain how you arrived at your answer.
<
>
5. While walking home from the store, you must push up on the handle of a bag of groceries with 15.5 N of force to keep the groceries from dragging along the ground. How many kilograms of mass does the bag have?
<
>
6. As your bathroom scale measures weight, what units should your bathroom scale display?
<
>
7. If you have a mass of 45 kg, what is your weight? (Hint: Weight is a force, what units represent force?)
<
>
8. Using the Internet, library, or local encyclopedia find one other great discovery made by Sir Isaac Newton.
<
>

Section: Key for Exercise 2.1B


    Exercise 2.1B
      1. What does the "law of gravitation" state?

      (1 mark)

      Answer: It states that every object attracts every other object.

      2. The gravitional force of attraction is based on what two things?

      (2 marks)

      Answer: Mass of the object and the distance between them.

      3. How many newtons of force are required to lift a 250 g cell phone?

      (2 marks)

      Answer: 2.5 N

      4. What is the mass of a full 2 litre milk carton? Explain how you arrived at your answer

      (3 marks)

      Answer: 2000 g or 2 kg. If the force required to lift one litre is 10 N then that makes the mass 1000g. If you extend this then you know that a 2 litre carton has a mass of 2000g.

      5. While walking home from the store, you must push up on the handle of a bag of groceries with 15.5 N of force to keep the groceries from dragging along the ground. How many kilograms of mass does the bag have?

      (2 marks)

      Answer: 15.5 N x 100 g =1550g which is 1.55 kg

      6. As your bathroom scale measures weight, what units should your bathroom scale display?

      (1 mark)

      Answer: newtons (N)

      7. If you have a mass of 45 kg, what is your weight? (Hint: weight is a force, what units represent force?)

      (2 marks)

      Answer: 45 kg x 10N = 450 N

      8. Using the Internet, library, or local encyclopedia find one other great discovery made by Sir Isaac Newton.

      (2 marks)

    Answer: answers many vary, but might include: Calculus or his laws and theories of mechanics. The three laws of motion.

ACTIVITY C: Force: Magnitude, Direction, and Location



Required Readings

Science in Action 7
pages 280 to 283
or

Science Focus 7

pages 298 to 301


In the centre of the room, lying on a table is a textbook. Is the book being pulled or pushed by a force? What kind of forces affect the book? How would the same book and table act in space?
In science, a force is a push or pull that causes a change in the movement or shape of an object. A change in movement may be from standing still to moving. It may be from moving one speed to a faster or slower speed or from one direction to another. However, an object that does not appear to move or change shape can also experience force. The book lying on the table is being pulled down towards the centre of the Earth by a gravitational force, yet the table exerts a counter force against it, causing the book to come to rest on the table. The book is said to be balanced by the two or more equal forces being exerted on the book.

Forces can be experienced from many different sources. Some of the forces that affect you every day are gravity, friction, magnetism, electrostatics, and buoyancy. For this module, we will focus on those forces which have the strongest effects on structures.

Forces may push and pull things up and down and side to side, or in any direction. Forces can even bend, squeeze and twist things. As a person, the two kinds of forces that you can exert are a push and pull. When you kick a ball you are exerting a push. When you take the milk out of the fridge you are exerting a pull on the milk carton. Our muscles are made for pushing and pulling.




When considering how force affects different structures, the following three things must be considered:
  • The size (magnitude) of the force.

Your dad asks you to take the garbage out to the back alley. You find that you can barely lift it, so you ask your brother to give you a hand. The two of you easily carry the bag to the alley. What changed about the amount of force being applied to the load?
  • The direction of the force.

When biking up a hill it takes more effort or force on the peddles than when you go down a hill. What other forces are playing a part?
  • The location where the force is applied to an object.

It is now spring cleaning time in your house. Your mom wants to vacuum behind the piano and asks you to move it. You place your arms against the top of the piano and push; the piano barely budges. Dad tells you to put your whole body into it so you move your hand placement down the piano to a mid point and push; the piano rolls easily away from the wall. What did you change about the force you applied?
Exercise 2.1C
Please answer the questions and then use the pop up key below to correct your work.

1. When an object is not moving the forces affecting it are said to be __________.
<
>
2. Name three forces, other than push and pull, which effect your everyday life. Give an example of how each affects your life.
<
>
3. Why is it more difficult to climb a hill than to come down the hill?
<
>
4. What are the three factors which affect force?
<
>
5. Which of the factors above would be the most important to consider when designing the following structures? Explain your answers.

Section: Key for Exercise 2.1C


    Exercise 2.1C
      1. When an object is not moving the forces affecting it are said to be __________.

      (1 mark)

      Answer: balanced

      2. Name three forces, other than push and pull, which effect your everyday life. Give an example of how each affects your life.

      (6 marks)

      Answer: Gravitational Force, Magnetic Force, Buoyancy, Electrostatic Force, Frictional Force. Examples will vary.

      3. Why is it more difficult to climb a hill than to come down the hill?

      (2 marks)

      Answer: Gravity is pulling you down when you are climbing. It also helps you come down as it is a pulling force.

      4. What are the three factors which affect force?

      (3 marks)

      Answer: size (magnitude), direction, and location of the force.

    5. Which of the factors above would be the most important to consider when designing the following structures? Explain your answers.
    • a shopping bag
      (2 marks)

      Answer:
      size and direction of force. When designing a shopping bag you must consider how heavy of a load it will carry. You must also consider that the force will be acting downwards as gravity pulls on the bag.
    • shoes
      (2 marks)

      Answer:
      location, direction, and magnitude. Location - most of the frictional force will happen along the bottom of the shoes so a good sole is require. Direction - Most of the force will be applied to the front and bottom of the shoe. Magnitude - the mass of the person wearing the shoes could required design changes.
    • a diving board
      (2 marks)

      Answer:
      magnitude, direction, and location. Magnitude - different size people will create different amounts of force on the diving bord. Direction - The force will be applied to the top side of the board. Location - the untethered end will need to flex under the force while the tethered end needs to stay put.
    • a bridge
      (2 marks)

      Answer:
      magnitude and location. Magnitude - how much force can be applied to the bridge without causing a collapse. Location - which locations on the bridge are more effected by force?
    • a wind mill
      (2 marks)

      Answer:
      Direction, Magnitude. Direction - which direction is the wind coming from. How strong is the force of the wind.
  • a shopping bag
    <
    >
  • shoes
    <
    >
  • a diving board
    <
    >
  • a bridge
    <
    >
  • a wind mill
    <
    >

ACTIVITY D: Frictional Force


Friction is the force that resists the movement of one object over another. It occurs when objects or surfaces rub together. It acts against the direction of the motion, causing objects to slow down or stop. The amount of friction depends on the texture of a surface and the force pressing them together. Friction is common to everyday life. Without Friction, you would not be able to walk. Think about the different seasons. In the summer it is easy for your shoes to grip the sidewalk as rough surfaces create more friction and friction gives us traction. In the winter, the same sidewalk may ice over after a snowfall making it very difficult to walk without sliding and falling. On the ice, there is much less friction between the shoes and the ice.



As you sit by your computer, try to slide your mouse with one finger. When you start to push the mouse along the table, the mouse does not move at first. A force has to be applied to overcome the friction between the mouse and the table. The mouse only moves once the the force of friction have been overcome. Once the mouse is moving, it slows down if the force is removed. Without friction, the smallest force can cause the mouse to move. Trying moving your mouse on smoother surface to get a feel for friction.
In many situations friction is a good thing. When building structures we want there to be friction between different building materials. When building a house, we are always pounding nails into wood. What keeps the nail in the wood? When we pour cement sidewalks, what keeps them from sliding on the lawn? Try to think of a structure that is built that does not use friction.
Exercise 2.1D
Watch the movie "Friction: There's the Rub!" from the Module 4 CD (Sc7CD4-Mod4)
in the "FORCEOFF" folder provided with this course or connect to the Internet and then
use the button below.

Please answer the questions below and the use the pop up key to correct your work. 1. What wears down quicker, two rough surfaces rubbing together or two smooth surfaces?
<
>
2. What is the relationship of friction to heat?
<
>

Section: Key for Exercise 2.1D


      1. What wears down quicker, two rough surfaces rubbing together or two smooth surfaces?

      < Two rough surfaces - the increase in friction causes it to wear faster. >

      2. What is the relationship of friction to heat?

    < Friction produces heat. >


ACTIVITY E: Force Vector Diagrams


Required Readings

Science in Action 7
pages none
or

Science Focus 7

page 304


Since we can not see forces, we can only measure them by the effect that they have on the objects around us. Forces are best represented by arrows or vectors. By using arrows we can show the direction as well as the strength of the force acting on an object.

At any one time there will always be at least two forces (sometimes more) acting on an object. Lets say you are playing soccer and you just gave the ball a hard kick, what forces are acting on the ball?



There can be a number of forces acting on the ball. If you kick the ball along the ground, the ball will have momentum from the push of the kick, but the force of friction from the grass will work in the opposite direction to slow the ball down. If you kick the ball into the air, the push from the kick will once again be one force, but the ball will also be pulled to the ground by the force of gravity and slowed by the friction caused by the air and grass.
Exercise 2.1E
Draw in or explain the force vectors for each of the following pictures, showing the direction and strength of the forces. Use the pop up key to correct your work. 1. < >
2. < >
3. < >

Section: Key for Exercise 2.1E


    Exercise 2.1E
    Draw in or explain the force vectors for each of the following pictures, showing the direction and strength of the forces.
      • 1.

        (2 marks)

        Answer: Arrows should be drawn to show the direction the elevator is moving with a smaller arrow in the opposite direction to represent the friction from the air.

        2.

        (2 marks)

        Answer: Two arrows can be drawn to show the force from the jump and the force which will cause the ball to go towards the basket.

        3.

        (2 marks)

      Answer: Two arrows can be drawn to show the force that each person is applying to the box.

Tab: Exercise 2.2


Lesson 2.2: External Forces Acting on Structures


ACTIVITY A: External Forces



Required Readings

Science in Action 7
pages 285 to 289
or

Science Focus 7

pages 305 to 306

We all remember the story of the Three Little Pigs. How the wolf huffed and puffed and blew the pigs houses down. Well as forces go, the wolf could be considered an external force, a force that is applied to a structure by something other than the materials in the structure. Wind, gravity, static and dynamic loads can all act as external forces on structures.

When the wind blows it places a force on a structure. If the structure is an empty garbage can, even a strong breeze could have enough of a force to blow it over. When we build structures, we must consider which forces the structure will have to stand up to.



Gravity is an external force that affects all structures. Gravity pulls things towards the Earth. Over time, the effects of gravity can be seen on older structures. How can you predict the effects of gravity on a structure?



Even though gravity acts on all parts of a structure, there is a balancing point in a structure. This point is called the centre of gravity. If a structure is supported at this point it will stay balanced. For example, pick up a pencil and balancing it on one finger. Where is the centre of gravity? It should occur at a point where the mass on either side of the point is equal. If this is the case, the pull of gravity on both sides of the pencil will also be equal. This means the pencil has symmetry of mass or force.

When designing structures and buildings, engineers must also consider those forces not created by nature. A buildings own weight and the materials placed in the building will also creating pushing and pulling loads on the structure. One of the loads can be the structure itself. We would never want to design a structure that crumbled under its own weight so we have to consider the static (dead) load of the structure. This would include the non-moving or permanent force acting on the structure. The weight of the structure itself. Overtime the structure can sag, tilt, or compress under its own weight.



Dynamic or live loads are other external forces which act on a structure. This can include wind, rain, snow, furniture, and the people who move around a structure. Cars on a bridge are a live load. It is very important to consider live loads when building structures. When engineers forget this factor, the end result can be structural failure. At the University of Calgary, the main library consists of numerous floors of books. It is rumoured that when the building was originally designed the engineers did not factor in the weight of all the books the library would hold. What do you think might happen to this library over time?


Another way to picture lives loads is to think of new and old furniture. If you take a new couch it is firm and the cushions and springs bounce back to their original shape with ease. However, if you look at a couch that has been used for 20 years the cushions and the springs are more worn and depressed into the seats from the years of compression from the weight of the people sitting on it. The couch has had to deal with live or dynamic loads.
Exercise 2.2A
Look at the pictures below and determine at least two external forces which are acting on the structures. For each force categorize it as static or dynamic load. Use the pop up key to correct your work. 1. < >
2. < >
3. < >
4. < >


Section: Key for Exercise 2.2 A


    Exercise 2.2A

      (4 marks)

      Answer:

      wind - dynamic load

      buoyancy of the water - dynamic load

      gravity - static load

      weight caused by the boat's mass - static load

      weight of the boat - static

    weight of the person - dynamic load

      (4 marks)

      Answer:

      Gravity - Static load

      Materials the wheelbarrow is constructed of - Static Load Push of the person - Dynamic load

      Wind - Dynamic load

    Weight of the materials in the wheelbarrow - Dynamic load

      (4 marks)

      Answer:

      Gravity - Static load

      Materials used to build the house - Static Load

      Wind - Dynamic load

    Snow on the roof- Dynamic load

      (4 marks)

      Answer:

      Gravity - Static load

      Weight of the cars - Dynamic load

      Wind - Dynamic load

    Weight of the materials in the bridge - Static load
    Look at the pictures below and determine at least two external forces which are acting on the structures. For each force categorize it as static or dynamic load.
    • 1.
      2.
      3.
      4.


ACTIVITY B: Teepee Investigation



Textbook Readings

Science in Action 7
page 287
or

Science Focus 7

pages none


Native people have been building structures in North America for more centuries than we can remember. Their structures have had to withstand the forces of nature; wind, water, rain, and snow.

QUESTION

Which teepee structure can withstand the strongest wind?

HYPOTHESIS

Prior to doing the lab, form a hypothesis about which teepee will stand up to the force of the wind the longest.

MATERIALS REQUIRED



8 bamboo skewers, 25 cm in length
plastic grocery bags
1 large skid mat, such as a damp towel or throw carpet
fan with at least 2 speeds
ruler
elastic or string
tape
scissors

PROCEDURE

Teepee 1

1. Tie four 25 cm skewers together with a string or an elastic about 2 or 3 cm from one end.
2. Splay out the four ends to make a base on the non-skid mat that gives the shape of a square or rectangle. Space the skewers approximately 15 cm apart.
3. Take a plastic grocery bag and construct a shell to cover the skewers. Use scissors and tape where needed.
4. Secure the plastic bag to the bottom of the skewers.
5. Place a fan about 50 to 60 cm away from the teepee. Please use caution with the fan. Tie back long hair, and water or wet hands away from electrical outlets. Turn the fan to the lowest setting and record how the teepee responds to the force in the data chart below.
6. After you have recorded your observations increase the speed of the fan and record how the teepee responds. Continue to increase the speed until you have reached the highest settings.
Teepee 2

Using two 25 cm skewers and two 20 cm skewers, tie them together with string or an elastic 2-3 cm from one end and follow through on steps 2 - 6 once again.
Exercise 2.2B - Complete in your lesson
Once you have read through the lab above, you need to predict what you think will occur in the lab by answering question 1. Then complete the lab and fill in your observations. Your work will be marked out of /25 based on completeness and detail provided.
1. State your hypothesis:< > /2
OBSERVATIONS 2. Fill in the data chart below and provide a title for the chart.< >


Lowest Speed
Medium Speed
Highest Speed
Teepee 1
< > < > < >
Teepee 2
< > < > < >
/11
ANALYZE 3. Which structure had a more secure centre of gravity? Why?
<
> /2
4. How could these designs be modified to better withstand the force of the wind?
<
> /2
5. What are the dynamic and static loads that affect these structures?
<
> /2
CONCLUSIONS 6. Which teepee structure can withstand the strongest winds?
<
> /2
7. Which of the variables in this lab were manipulated and which were held constant? Name at least three variables.
<
> /4


ACTIVITY C: Designing Bridges



Textbook Readings

Science in Action 7
pages 290 to 293
or

Science Focus 7

pages none


Why did the human build the bridge? To get to the other side! One has to wonder where the original chicken jokes actually began. Perhaps it was with the desire of humans to be able to move about freely. Humans have been building bridges since as far back as history can go. The only things that have changed is our ability to builds longer and larger bridges and the materials with which they are built.

There are three major types of bridges

  • the Beam bridge
  • the Arch bridge
  • the Suspension bridge
The principals of these bridges also apply to other structures with similar designs.

The largest difference between the three bridge designs is the distance they can cross in a single span. For example, a beam bridge made from modern materials is likely to span a distance of up to 60 meters, while a modern arch can safely span up to 300 meters. A suspension bridge, the ultimate in bridge technology, is capable of spanning up to 2100 meters. Forces are always acting on bridges. Depending on how they are designed and built they react to forces in different ways. How can you overcome forces when building your bridge?

For more information about bridge design check out: www.howstuffworks.com

Bridge Design Program

YOUR CHALLENGE

To design, build, test, and evaluate a simple beam bridge that will support the greatest possible load, using the least expensive materials and methods, under the following conditions:

  • The bridge must be 45 to 55 cm long.
  • The bridge deck must be no more than 5 cm wide and 5 cm tall.
  • The bridge must span a distance of 40 to 45 cm.
  • There has to be a way to fasten a load to the centre of the bridge for testing purposes
  • You can only construct your bridge with the following materials:
    • cardboard
    • wire or string
    • straws
    • aluminium foil
    • plasticine or modelling clay
    • books or blocks for the span supports
    • tape
Exercise 2.2C
Complete the following section regarding your bridge. Your work will be marked out of /50.

DESIGN STAGE 1. List the materials you will use to build your bridge.
<
> /6
2. What will you use as weights to test the dynamic load on the bridge?
<
> /3
3. Draw your prototype design and label the different materials and dimensions of your bridge.
<
> /15
PROTOTYPE BUILDING 4. Changes are often needed when a design goes to the prototyping stage. What modifications had to be made to your original design when building your prototype? Why or why not?
<
> /5
TESTING YOUR BRIDGE 5. Measure the dimensions of your completed model and provide the following measurements:
  • length of bridge < >
  • bridge span < >
  • width of bridge < >
  • height of bridge deck < > /6
6. Does your bridge meet the required specifications outlined in the original plan? Why or why not?
<
> /3
7. As you tested the ability of your bridge to carry a dynamic load, what was the maximum load that your bridge was able to carry without causing the bridge to bend more than 5 cm?
<
> /3
EVALUATING YOUR BRIDGE 8. Now that you know how your bridge performed under testing, evaluate the materials used for your design. Are you satisfied that you choose and used the materials in the best possible manner in the design and construction of your bridge? Why or why not?
<
> /5
9. What was the weakest point in your bridge? Where did the bridge bend or break? What modifications could you make to this area to allow it to sustain more force?
<
> /5

Tab: Exercise 2.3


Lesson 2.3: Internal Forces within Structures


ACTIVITY A: Internal Forces - Compression, Tension, Shear, and Torsion



Textbook Readings

Science in Action 7
pages 296-298
or

Science Focus 7

pages 307 to 310

Internal forces are the forces that act within the materials from which structures are built. Some forces are pulling forces and some are pushing

Compression is the force that acts to push things together. Think of the word compress - this means to squeeze things together.

Watch the "13tension.mov" movie from your Module 4 CD (Sc7cd4-Mod4) or use the button below and read the section in the textbook before answering the questions that follow.



Tension is the force that acts to pull things apart. Materials will stretch to carry a load. At the point where the material can not take any more force the material can snap or break due to the force of tension.

Watch the "12compression.mov" movie from your Module 4 CD (Sc7cd4-Mod4) or use the button below to access the video. Then read the section in the textbook before answering the questions that follow.



These forces do not have to act alone. Often these forces act together at the same time. When two internal forces act at the same time they are said to be complementary forces. The bending of a beam as you apply your weight to it is an example of complementary forces. The top of the beam is being compressed or squished and the bottom of the beam stretches or tries to pull apart due to the tension in the beam.

Shear is another affect that is simply the combination of tension and compression. It is the force that pushes parts that are in contact with one another apart in opposite directions. Now this may seem a little confusing, but the short movie should help to clear things up.

Watch the "14shear.mov" movie from your Module 4 CD (Sc7cd4-Mod4) or use the button below and then read the section in the textbook before answering the questions that follow.



The last internal force to point out is torsion. Torsion forces twist a material by turning the ends in opposite directions.

Next, watch the movie "15torsion.mov" from your Module 4 CD (Sc7cd4-Mod4) or use the button and then read the section in the textbook before


answering the questions below.

Exercise 2.3A
From the movies and the textbook, answer the following questions. Use the pop up key below to correct your work.
1. When you walk across a beam and it bends, what forces are acting on a beam?
<
>
2. When you pull an elastic too far and it snaps, what force causes it to break?
<
>
3. When you wring out a dish cloth what internal forces are being applied?
<
>
4. How do scissors work to cut a piece of paper? What forces are involved?
<
>
5. The pulling force is called ____< >______ and the pushing force is called ____< >______.
6. Give an example from your home where the internal force of tension is at work.
<
>

Section: Key for Exercise 2.3 A


    Exercise 2.3A
    From the movies and the textbook, answer the following questions.
      • 1. When you walk across a beam and it bends, what forces are acting on a beam?

        Tension and Compression

        2. When you pull an elastic too far and it snaps, what force causes it to break?

        Tension

        3. When you wring out a dish cloth what internal forces are being applied?

        Torsion

        4. How do scissors work to cut a piece of paper? What forces are involved?

        Shear, or a combination of tension and compression

        5. The pulling force is called ____Tension______ and the pushing force is called ____Compression______.

        6. Give an example from your home where the force of tension is at work.

      Answers will vary. An example could be the cord on your phone being pulled taut or a clothes line or the bottom of the floor joists, etc.

Tab: Exercise 2.4


Lesson 2.4: Designing Structures to Resist Forces and Maintain Stability


ACTIVITY A: Strong Structural Shapes



Required Readings

Science in Action 7
pages 300 to 302
or

Science Focus 7

pages none

When taking the time and money to build a structure, we ultimately want them to last as long a possible in a safe manner. How can you ensure that a structure is stable?

For thousands of years humans have been building structures. If we look at those structures that have lasted for centuries what are the common characteristics?

When looking at two dimensional structures what is the strongest structure? Is it a rectangle, a square, or a triangle. Read page 301 in Science in Action and try the simple exercise with a drinking straw to assess these different structures.




By gently pushing on the upper corner of each shape, which structure was the strongest?

TRY THIS!

From what you have learned about strong structural shapes, build a model of a tower using mini-marshmallows and toothpicks. How many stories high can you build?




Build some floors with triangular cross beams and some without. Which is more stable? Which has more deformation?



For those toothpicks used in the vertical and horizontal directions, cut 1 cm off. Leave those toothpicks which will be used as diagonal cross beams at the full length of the toothpick.
Exercise 2.4A - Complete in your lesson.
Complete the questions below. Your work will be marked out of /20 based on the completeness and detail provided.

1. When looking at traditional structures in your textbook which have stood the test of time, what characteristics and materials are common to many of them? List at least two.
<
> /3
2. When you gently pushing on the upper corner of each shape in the exercise above, which structure was the strongest?
<
> /2
3. If you were to build a gate to your backyard what could you do to add to the strength of the normally rectangular structure?
<
> /3
4. What shapes make up tall communication towers such as the one pictured on page 305 in the Science in Action textbook or the Eiffel Tower in Paris?
<
> /2
5. How many stories high did you build your tower without it failing structurally?
<
> /2
6. Why did you have to cut 1 cm off the vertical and horizontal beams in your tower?
<
> /3
7. What difference did the cross beams make in your tower?
<
> /3
8. What alterations could you make to your tower to add extra stability?
<
> /2


ACTIVITY B: Structural Components



Required Readings

Science in Action 7
pages 302 to 303
or

Science Focus 7

pages 321 to 324

Bridges and buildings are usually composed of many components which make up the entire structure. Some components are built to add aesthetic value and others to add strength. Lets take a closer look at arches and beams.

Using your textbook and the How Stuff Works Website answer the questions below which talk about key structural components.

Arches can carry very large loads because the force of the load is carried down through the arch. Arches have been constructed for thousands of years. Many of these arches still stand today.

If you would like to build your own arch, watch the movie below. The "52ARCH.MOV" movie is located on the SC7CD7_MOD4 CD in the "BEAMSAND" folder or you can access it through the button below.



Exercise 2.4B
Complete the questions and use the pop up key below to correct your work.

1. In what type of structure is a keystone required?
<
>
2. What is the difference between box beams and I-beams?
<
>
3. How is an I-beam stronger than a simple beam?
<
>
4. Why would you want to use an I-beam if a solid beam is stronger?
<
>
5. What is a truss and where would you find one?
<
>
6. A diving board is a good example of what kind of structure?
<
>


Section: Key for Exercise 2.4B


    Exercise 2.4B
      1. In what type of structure is a keystone required?

      An arch

      2. What is the difference between box beams and I-beams?

      Box beams are strong, but also tend to be heavier. I-beams are strong, but with less weight.

      3. How is an I-beam stronger than a simple beam?

      The I-beam allows for much less compression as it is essentially a double simple beam.

      4. Why would you want to use an I-beam if a solid beam is stronger?

      Less mass and easier to work with.

      5. What is a truss and where would you find one?

      A truss is a framework of beam. This can be seen on bridges around Alberta.

      6. A diving board is a good example of what kind of structure?

    A cantilever.


ACTIVITY C: Structural Stress, Fatigue, and Failure



Required Readings

Science in Action 7
pages 303 to 306
or

Science Focus 7

pages 315 to 318

No structure is perfect. With enough force acting on a structure over time it will begin to fail. Sometimes bad design will speed up the process. In some cases, when certain factors are not considered when building a structure, failure can happen much faster than original designs would have suggested.

This is what happened to the Tacoma Narrows bridge in November 1940. Nicknamed "Galloping Gertie", the Tacoma Narrows bridge was prone to sway in the wind even when it was being constructed. The natural forces of the winds were not considered when the original architectures designed the bridge.


When a combination of external and internal forces act on a structure the stress can weaken it. When a structure under goes repeated stress over time it leads to fatigue.




Metal fatigue is a common source of structures failing. Take for example, a strip of metal that is bent in half and then straightened and bent in half over and over again. Eventually, the seam in which the metal is bent along will become weak and eventually fail.
Exercise 2.4C - Complete in your lesson
Use your textbook and the movies in the "TENSIONA" folder on the Science 7 Module 4 CD (or use the buttons below) to answer the questions that follow. Please note that the 4 videos in Exercise 2.3 of this lesson are from the TENSIONA folder - please access them from 2.3 if you wish to review the content. Your work will be marked out of /10 based on completion and accuracy.


1. Take the example of a paper clip. Explain how you would cause the paper clip stress, fatigue, and failure.
<
> /3
2. What is another way to describe torsion?
<
> /3
3. Give an example of when the following structural failures have occurred in your surroundings. 1. Buckling < >
2. Torsion < >
3. Shearing < >
4. Deformation < > /4

Site: AB Course Sharing Hub
Course: Science 7 LearnNet
Book: Week 24 - Measuring Forces and Designing Structures to Resist Forces
Printed by: Guest user
Date: Saturday, 16 February 2019, 4:18 PM

Exercise 2.1


Lesson 2.1: Measuring Forces


ACTIVITY A: Mass verses Weight 



Textbook Readings

Science in Action 7
pages none
or

Science Focus 7

pages 298 to 300


At one time or another we have all wanted to walk on the moon. When we see pictures from the moon we notice how easy the astronauts walk or bounce on the surface of this Earth-orbiting satellite. Why do they look lighter on the Moon than on Earth?

In 1969, Neil Armstrong and Buzz Aldrin made a historical walk on the moon. Since you were not alive in 1969, you can view his walk in the Quicktime movie below.

You must be connected to the Internet to view this animation
or
History of Spaceflight

In order to understand why these astronauts appeared to weigh less we have to understand the difference between mass and weight. We need to compare a few points:

1. Mass is a measurement of the amount of matter something contains, while Weight is the measurement of the pull of the force called gravity on an object.


When you were studying Heat and Temperature you learned that all objects are made up of tiny particles. A horse is made up of many of these particles, so it has a large mass. A gold bar contains fewer particles and therefore has a smaller mass. However, the gold particles are packed somewhat closer together than many other objects of the same size. If you compared a gold bar and a slice of bread the same size, which of these objects has particles packed closer together, and therefore has more mass?

WEIGHT
=
MASS
X
PULL
OF
GRAVITY

When we weigh something we are actually measuring the pull of gravity on a mass. If you multiply your mass by the pull of Earth's gravity, you get your weight. By eating or exercising, you are actually changing the number of particles you are made up of; this is your mass. The earth's gravitational pull, on the other hand, decreases as you move farther away from the Earth. Therefore, you can lose weight by changing your elevation, but your mass remains the same. You can also lose weight by living on the moon as it has less gravitational pull, but again your mass is the same.
2. Mass is measured by using a balance comparing a known amount of matter to an unknown amount of matter. Weight is measured on a scale.


A balance is the most common instrument for measuring mass. A balance compares the mass of the object being measured with the standard masses. The most common type of balance used in the school setting is the Triple Beam Balance. We measure mass in grams or kilograms.

When we measure weight we are measuring the mass multiplied by the force of gravity. We have a number of common measuring devices that assist us in measure weight. When we step on the bathroom scale, we are measuring the amount of force our body mass pushes on the springs in the scale. When we measure the weight of a bag of apples at the store, we place them in the tray of a large spring scale. The mass of the apples will cause the force of gravity to pull down on the scale, therefore giving us a measurement. The only question remaining is: Are the units of measure on the bathroom scale and the produce scale at the grocery store the proper units to measure force?

Read this website to find the answer!

http://www.mathsisfun.com/measure/weight-mass.html

3. The mass of an object doesn't change when an object's location changes. Weight, on the other hand, does change with location.


Mass only changes if the number of particles in an object changes. When you move around or change locations, you do not change the number of particles in an object. In space, you are still made up of the same number of particles. The effect of the force of gravity is the only thing that has decreased.

Visit this website to practice some exercises about calculating weight and mass on different planets!

http://www.nyu.edu/pages/mathmol/textbook/weightvmass.html

Anywhere in the Universe, there are forces of attraction between objects. These forces try to pull objects together. The larger the object the more attraction or gravitational force it has. If you look at the attraction between the Earth and an apple, the Earth has much more pull due to its size than the apple. The further an object gets from another object, the less pulling force it has. When the space shuttle takes off from the Earth, it needs the added force of booster rockets to overcome the gravitational pull of the Earth. Once the shuttle is far enough away it can drop the boosters and carry on with less force. Even in space the Earth has some gravitational pull, however the force is much less than on the surface of the Earth. The Space Shuttles use Earth's gravity to stay in orbit around the Earth. If there was no gravitational force the shuttle would not orbit the Earth, but instead travel away from it in a straight line.


ACTIVITY B: Sir Isaac Newton and Force



Textbook Readings

Science in Action 7
page 284


Science Focus 7
pages 298 to 300


Making famous scientific discoveries seems like a difficult thing to accomplish, but once discoveries are made they often seem obvious. In 1687, Sir Isaac Newton became the first person to discover the "law of universal gravitation". Some say he made this discovery while watching an apple fall from a tree.

This law states that every object in the universe attracts every other object. The force of attraction is based on their masses and on the distance between them. In the apple story, the apple was pulled to the ground its particles are attracted to the particles in the Earth. If an object has more mass (and therefore more particles), like a watermelon, it would fall faster because of a higher force of attraction.

He used his theory to calculate the orbits of planets. Although many other people understood the law of gravity, Sir Isaac Newton was the first to prove it mathematically.

Look at this website to learn more about gravity.

http://www.physics4kids.com/files/motion_gravity.html


As Sir Isaac Newton was the first to mathematically prove a force, he is honoured by the standard (SI) metric unit of force being called a Newton (N). One Newton is defined as the amount of force required to hold up 1 kg of mass. If you placed an 1 kg weight in your hand, you would have to push back on it with 10 N (approximately) of force to hold it up.

To calculate the amount of force exerted by a certain mass we use the formula:  W = M x A

W stands for weight - this is measured in Newtons.

M stands for mass - this is measured in kilograms.

stands for acceleration - this is the force of gravity on Earth. This is considered a constant because it never changes on Earth. It is 9.80665 m/sbut we can just round this to 10.

Take a look at the examples below:

Holding a 3 kg weight requires 30 N of force
Holding a 200 g package of noodles requires 2 N of force
Holding a 1 Litre milk jug requires 1 N of force


ACTIVITY C: Force: Magnitude, Direction, and Location



Required Readings

Science in Action 7
pages 280 to 283
or

Science Focus 7

pages 298 to 301


In the centre of the room, lying on a table is a textbook. Is the book being pulled or pushed by a force? What kind of forces affect the book? How would the same book and table act in space?
In science, a force is a push or pull that causes a change in the movement or shape of an object. A change in movement may be from standing still to moving. It may be from moving one speed to a faster or slower speed or from one direction to another. However, an object that does not appear to move or change shape can also experience force. The book lying on the table is being pulled down towards the centre of the Earth by a gravitational force, yet the table exerts a counter force against it, causing the book to come to rest on the table. The book is said to be balanced by the two or more equal forces being exerted on the book.

Forces can be experienced from many different sources. Some of the forces that affect you every day are gravity, friction, magnetism, electrostatics, and buoyancy. For this module, we will focus on those forces which have the strongest effects on structures.

Forces may push and pull things up and down and side to side, or in any direction. Forces can even bend, squeeze and twist things. As a person, the two kinds of forces that you can exert are a push and pull. When you kick a ball you are exerting a push. When you take the milk out of the fridge you are exerting a pull on the milk carton. Our muscles are made for pushing and pulling.




When considering how force affects different structures, the following three things must be considered:
  • The size (magnitude) of the force.

Your dad asks you to take the garbage out to the back alley. You find that you can barely lift it, so you ask your brother to give you a hand. The two of you easily carry the bag to the alley. What changed about the amount of force being applied to the load?
  • The direction of the force.

When biking up a hill it takes more effort or force on the peddles than when you go down a hill. What other forces are playing a part?
  • The location where the force is applied to an object.

It is now spring cleaning time in your house. Your mom wants to vacuum behind the piano and asks you to move it. You place your arms against the top of the piano and push; the piano barely budges. Dad tells you to put your whole body into it so you move your hand placement down the piano to a mid point and push; the piano rolls easily away from the wall. What did you change about the force you applied?

ACTIVITY D: Frictional Force


Friction is the force that resists the movement of one object over another. It occurs when objects or surfaces rub together. It acts against the direction of the motion, causing objects to slow down or stop. The amount of friction depends on the texture of a surface and the force pressing them together. Friction is common to everyday life. Without Friction, you would not be able to walk. Think about the different seasons. In the summer it is easy for your shoes to grip the sidewalk as rough surfaces create more friction and friction gives us traction. In the winter, the same sidewalk may ice over after a snowfall making it very difficult to walk without sliding and falling. On the ice, there is much less friction between the shoes and the ice.



As you sit by your computer, try to slide your mouse with one finger. When you start to push the mouse along the table, the mouse does not move at first. A force has to be applied to overcome the friction between the mouse and the table. The mouse only moves once the the force of friction have been overcome. Once the mouse is moving, it slows down if the force is removed. Without friction, the smallest force can cause the mouse to move. Trying moving your mouse on smoother surface to get a feel for friction.

In many situations friction is a good thing. When building structures we want there to be friction between different building materials. When building a house, we are always pounding nails into wood. What keeps the nail in the wood? When we pour cement sidewalks, what keeps them from sliding on the lawn? Try to think of a structure that is built that does not use friction.




ACTIVITY E: Force Vector Diagrams


Required Readings

Science in Action 7
pages none
or

Science Focus 7

page 304


Since we can not see forces, we can only measure them by the effect that they have on the objects around us. Forces are best represented by arrows or vectors. By using arrows we can show the direction as well as the strength of the force acting on an object.

At any one time there will always be at least two forces (sometimes more) acting on an object. Lets say you are playing soccer and you just gave the ball a hard kick, what forces are acting on the ball?



There can be a number of forces acting on the ball. If you kick the ball along the ground, the ball will have momentum from the push of the kick, but the force of friction from the grass will work in the opposite direction to slow the ball down. If you kick the ball into the air, the push from the kick will once again be one force, but the ball will also be pulled to the ground by the force of gravity and slowed by the friction caused by the air and grass.
Exercise 2.1: External Forces


Exercise 2.2


Lesson 2.2: External Forces Acting on Structures


ACTIVITY A: External Forces


Required Readings

Science in Action 7
pages 285 to 289
or

Science Focus 7

pages 305 to 306

We all remember the story of the Three Little Pigs. How the wolf huffed and puffed and blew the pigs houses down. Well as forces go, the wolf could be considered an external force, a force that is applied to a structure by something other than the materials in the structure. Wind, gravity, static and dynamic loads can all act as external forces on structures.

When the wind blows it places a force on a structure. If the structure is an empty garbage can, even a strong breeze could have enough of a force to blow it over. When we build structures, we must consider which forces the structure will have to stand up to.



Gravity is an external force that affects all structures. Gravity pulls things towards the Earth. Over time, the effects of gravity can be seen on older structures. How can you predict the effects of gravity on a structure?


Even though gravity acts on all parts of a structure, there is a balancing point in a structure. This point is called the centre of gravity. If a structure is supported at this point it will stay balanced. For example, pick up a pencil and balancing it on one finger. Where is the centre of gravity? It should occur at a point where the mass on either side of the point is equal. If this is the case, the pull of gravity on both sides of the pencil will also be equal. This means the pencil has symmetry of mass or force.

When designing structures and buildings, engineers must also consider those forces not created by nature. A buildings own weight and the materials placed in the building will also creating pushing and pulling loads on the structure. One of the loads can be the structure itself. We would never want to design a structure that crumbled under its own weight so we have to consider the static (dead) load of the structure. This would include the non-moving or permanent force acting on the structure. The weight of the structure itself. Overtime the structure can sag, tilt, or compress under its own weight.



Dynamic or live loads are other external forces which act on a structure. This can include wind, rain, snow, furniture, and the people who move around a structure. Cars on a bridge are a live load. It is very important to consider live loads when building structures. When engineers forget this factor, the end result can be structural failure. At the University of Calgary, the main library consists of numerous floors of books. It is rumoured that when the building was originally designed the engineers did not factor in the weight of all the books the library would hold. What do you think might happen to this library over time?


Another way to picture lives loads is to think of new and old furniture. If you take a new couch it is firm and the cushions and springs bounce back to their original shape with ease. However, if you look at a couch that has been used for 20 years the cushions and the springs are more worn and depressed into the seats from the years of compression from the weight of the people sitting on it. The couch has had to deal with live or dynamic loads.


ACTIVITY B: Teepee Investigation


Textbook Readings

Science in Action 7
page 287
or

Science Focus 7

pages none


Native people have been building structures in North America for more centuries than we can remember. Their structures have had to withstand the forces of nature; wind, water, rain, and snow.

QUESTION

Which teepee structure can withstand the strongest wind?

HYPOTHESIS

Prior to doing the lab, form a hypothesis about which teepee will stand up to the force of the wind the longest.

MATERIALS REQUIRED



8 bamboo skewers or sticks, 25 cm in length
plastic grocery bags
1 small towel, damp with water
electric fan with at least 2 speeds
ruler
elastic or string
tape
scissors

PROCEDURE

Teepee 1

1. Spread out your damp towel to create a non-skid surface for your teepee.

2. Tie four 25 cm skewers together with a string or an elastic about 2 or 3 cm from one end.

3.  Spread out the four skewers to give the shape of a square or rectangle. Space the skewers approximately 15 cm apart.

4. Take a plastic grocery bag and construct a shell to cover the skewers. Use scissors and tape where needed. Make sure the plastic bag is secured to the structure.

5. Place a fan about 50 to 60 cm away from the teepee. Please use caution with the fan. Tie back long hair, and water or wet hands away from electrical outlets. Turn the fan to the lowest setting and record how the teepee responds to the force.

6. After you have recorded your observations increase the speed of the fan and record how the teepee responds.

Teepee 2

1. Use scissors or a knife to cut 2 skewers/sticks to measure 20 cm each.

2. Use your two 20 cm skewers and two 25 cm skewers. Tie them together with string or an elastic 2-3 cm from one end.

3. Go to step 3 above and follow the steps with Teepee 2. You will cover the new teepee with plastic and test its ability to resist forces. Remember to keep notes on how the new teepee design reacts to the forces.

 Exercise 2.2: Teepee Investigation

Exercise 2.3


Lesson 2.3: Internal Forces within Structures


ACTIVITY A: Internal Forces - Compression, Tension, Shear, and Torsion


Textbook Readings

Science in Action 7
pages 296-298
or

Science Focus 7

pages 307 to 310

Internal forces are the forces that act within the materials from which structures are built. Some forces are pulling forces and some are pushing

Compression is the force that acts to push things together. Think of the word compress - this means to squeeze things together.

Tension is the force that acts to pull things apart. Materials will stretch to carry a load. At the point where the material can not take any more force the material can snap or break due to the force of tension.

Watch the "What is Tension/Compression?" video then read the section in the textbook before answering the questions that follow.


 

These forces do not have to act alone. Often these forces act together at the same time. When two internal forces act at the same time they are said to be complementary forces. The bending of a beam as you apply your weight to it is an example of complementary forces. The top of the beam is being compressed or squished and the bottom of the beam stretches or tries to pull apart due to the tension in the beam.

Shear is another effect that is simply the combination of tension and compression. It is the force that pushes parts that are in contact with one another apart in opposite directions. 

The last internal force to point out is torsion. Torsion forces twist a material by turning the ends in opposite directions.

Next, watch the video, then read the section in the textbook before answering the questions below.

 

Exercise 2.3: Internal Forces Within Structures

Tab: Exercise 2.4


Lesson 2.4: Designing Structures to Resist Forces and Maintain Stability


ACTIVITY A: Strong Structural Shapes


Required Readings

Science in Action 7
pages 300 to 302
or

Science Focus 7

pages none

When taking the time and money to build a structure, we ultimately want them to last as long a possible in a safe manner. How can you ensure that a structure is stable?

For thousands of years humans have been building structures. If we look at those structures that have lasted for centuries what are the common characteristics?

When looking at two dimensional structures what is the strongest structure? Is it a rectangle, a square, or a triangle. Read page 301 in Science in Action and try the simple exercise with a drinking straw to assess these different structures.




By gently pushing on the upper corner of each shape, which structure was the strongest?

TRY THIS!

From what you have learned about strong structural shapes, build a model of a tower using mini-marshmallows and toothpicks. How many stories high can you build?




Build some floors with triangular cross beams and some without. Which is more stable? Which has more deformation?



For those toothpicks used in the vertical and horizontal directions, cut 1 cm off. Leave those toothpicks which will be used as diagonal cross beams at the full length of the toothpick.
Strong Structural Shapes (optional)

Which shape was the strongest: with crossbeams or without?

Which tower could you build taller: with crossbeams or without?

Why do you think that was your result?

What do you think this means for modern architects?



ACTIVITY B: Structural Components


Required Readings

Science in Action 7
pages 302 to 303
or

Science Focus 7

pages 321 to 324

Bridges and buildings are usually composed of many components which make up the entire structure. Some components are built to add aesthetic value and others to add strength. Lets take a closer look at arches and beams.

Using your textbook and the How Stuff Works Website answer the questions below which talk about key structural components.

Arches can carry very large loads because the force of the load is carried down through the arch. Arches have been constructed for thousands of years. Many of these arches still stand today.

If you would like to build your own arch, watch the video below.





ACTIVITY C: Structural Stress, Fatigue, and Failure


Required Readings

Science in Action 7
pages 303 to 306
or

Science Focus 7

pages 315 to 318

No structure is perfect. With enough force acting on a structure over time it will begin to fail. Sometimes bad design will speed up the process. In some cases, when certain factors are not considered when building a structure, failure can happen much faster than original designs would have suggested.

This is what happened to the Tacoma Narrows bridge in November 1940. Nicknamed "Galloping Gertie", the Tacoma Narrows bridge was prone to sway in the wind even when it was being constructed. The natural forces of the winds were not considered when the original architectures designed the bridge.

When a combination of external and internal forces act on a structure the stress can weaken it. When a structure under goes repeated stress over time it leads to fatigue.




Metal fatigue is a common source of structures failing. Take for example, a strip of metal that is bent in half and then straightened and bent in half over and over again. Eventually, the seam in which the metal is bent along will become weak and eventually fail.

 

Section 2 Notes

You have two options for your Section 1 Quiz - a multiple choice quiz or a written response quiz. You may choose to write the multiple choice or the written response. You only need to write one of them, so the choice is up to you! You are allowed to try both quizzes if you want, but it is not required.

The quiz attempt with the highest grade will be the one that is recorded on your report card.

The multiple choice quiz has 10 multiple choice, matching, and true/false questions. You have 15 minutes to complete it. As soon as the quiz submitted it will be auto-graded and you will receive a grade immediately.

Click the image above to start your quiz

The written response quiz has 5 short answer questions. Your responses must give a complete and detailed answer to the question. You have 20 minutes to complete the quiz. This quiz needs to be manually marked by your teacher, so you may have to wait a few days to get feedback and a grade.

Click the image above to start your quiz