Week 24 - Measuring Forces and Designing Structures to Resist 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

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

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.


Which teepee structure can withstand the strongest wind?


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


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
elastic or string


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