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Week 24 - Measuring Forces and Designing Structures to Resist Forces

Exercise 2.1

Lesson 2.1: Measuring Forces

ACTIVITY A: Mass verses Weight 

Textbook Readings

Science in Action 7
pages none

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
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?


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!

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!

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.

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

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

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