Look around inside the building you are in right now. If you can see outside, look at buildings from the outside. These structures do not fall down and hold the weight of a lot of people and withstand the forces of wind and snow. If you were watching a person hold a lot of weight like a weightlifter or gymnast, you would see muscles tensed and their faces would show stress. Buildings hold tremendous weight and hold their shape against large forces but you don’t see the stress and strain, but it is there!! These are called static forces because there is no motion. We are used to thinking about forces causing motion. You can push very hard against a car, for example, but there is no motion even with all that force. The car pushes back against you with the same force you push against it, so no motion!
The structures around you contain many parts that are connected to hold the building together, to be strong. These parts are beams, studs, rafters, joists and so on. They hold against forces from the top, the bottom, sideways, and twisting forces. This activity helps you “see” the forces at work in a structure that are otherwise invisible because normally there is no motion in a structure. The lines you draw on the foam bricks in this activity will move to show the effects of forces. It turns out, that the bricks, wood, and steel of buildings also show stress but you must have special equipment in order to detect this stress. This is what structural engineers do to check the integrity of buildings and bridges. You will see up close and personal the structure of a major Interstate bridge in the video on Mechanics and Materials.
* Felt-tip pens
* Foam bricks (you can often find these at your local dollar store)
Take a foam brick and draw a grid of uniform lines on it with a felt-tip pen. The brick now looks like it has graph paper wrapped around it. Bend the brick slowly as if there was a weight in the middle on top, like a car in the middle of a bridge. Observe how the straight lines deform (bend or move) with the forces. Make a drawing of the deformed brick. Notice the difference in the spaces of the lines on the top of the brick where the simulated weight is compared to the bottom of the brick. Use the terms tension and compression to describe what you see.
Squish the brick slowly from both ends and watch the lines. Make a drawing. Use the term compression to describe what you see. Construct your description so that the meaning of the term compression is clear.
Hold the long dimension of the brick so that it is straight up, vertical. If the brick is holding the end of a bridge, think what happens to the brick with the wind blows sideways on the bridge. Slowly twist the brick as might be done when there is a strong wind on a bridge or the roof of a house but not so much at the bottom. Heavy snow may build up on one part of a roof exerting large forces on one side of the house but not the other causing a twist in the wooden frame. The lines on the twisted brick show you the forces that the wood and nails must withstand in order for the house to remain standing.
Take a long piece of wood. Putting someone at each end twist in opposite directions. How far can you rotate the twist before the wood breaks? With this skinny piece of wood, you can see the torquing force on the foam brick. Think about the vertical pieces of wood in your house or on a bridge. How much could these pieces twist before breaking? Watch the video Galloping Gerdie linked on the SELS site theselsproject.org/catablog-items/galloping-gertie-video-for-shear/. Using the terms torque, torsion, bending, tension, and compression describe how the twisting and bending forces finally broke this bridge.
Bending and Torsion
Next Generation Science Education Standards
Scientific and Engineering Practices:
– Planning and carrying out investigations
Compare the patterns of lines on the foam bricks when there is a force at both ends (squishing force) with the pattern of lines when there is a bending force. What characteristics must materials have to be able to handle ALL of these forces? See how much squishing force a strand of dried spaghetti can take (without letting it bend) and then see how much bending force it can take.
Ask someone skilled in the construction trades or a civil engineer about what forces concrete can withstand. You may surprised in what ways concrete is weak.
Think about the various ways you sit in a chair. Think about sitting on the arm of that chair or on its back. Think about tilting back on two legs. Examine the structure of the chair and explain what kind of forces (bending, twisting, or compression or squishing) are on the various parts of the chair. Draw pictures to show where the largest forces are. From your analysis of the structure of the chair, make an argument for where the chair will fail (break) if there is too great a force.