Bending forces a material into the shape of a curve or angle. Torsion is the act of twisting or being twisted. Bridges and support beams often experience the forces of bending, torsion or both. The next time you pass over a bridge, think about how the structure of the bridge was designed to withstand the forces of bending and torsion. When designing a bridge or building, engineers must consider and design against forces such as bending and torsion in order for the structure to be safe for people to use. In order to understand what can happen if a structure is not designed to properly withstand these forces, watch the Galloping Gertie video in the Shear Activities section of this website. Although shear forces played an important role in this bridge’s collapse, the forces of bending and torsion were also a part of the bridge’s ultimate failure. In order to design a bridge or building that is safe for people to use, engineers must consider and design against many different forces that will act on the structure. When you ride a skateboard, your body “feels” the effect of forces because of acceleration in a curve or on a hill. The skateboard also “feels” these forces and reacts by bending or twisting. The mountings for the trucks on the skateboard also experience the force of torsion or torque as well as sheer forces.
Knee and elbow pads
Have one person who normally rides a skateboard stand on the skateboard and lightly bounce up and down without their feet coming off the board. Notice how the board itself bends with the movement of the riders body. Think about how this works as the rider is turning or jumping. Have the rider lean side to side and watch the skateboard tilt creating a twisting motion on the mountings of the tucks. Look at the skateboard at ground level to see how the trucks are mounted so as to take this twisting force. Turn the skateboard over and look at the mountings. Try to move them with your hands in all directions. Describe the motion of the skateboard that would bend the truck in a particular direction.
Examine how the wood on the skateboard is made. It is not one solid piece but is rather built in layers. Why is it made this way? A skateboard is a good example of “mechanics” and “materials”. The mechanics have to do with the bending and twisting the board goes through. The materials concern how the skateboard is made so that it can withstand the mechanics. Look up “laminated beams” on the Internet to learn more about the structure of skateboards. Where else do you see laminated beams in buildings?
Have the rider execute a series of turns in front of a group of people. Observe how bending and torsion forces act on the trucks and wheels when a turn is executed. Ask the person riding the skateboard if they can feel the increased effect of gravity while they are making turns. Think about what is happening in the trucks of the skateboard while these turns are occurring. Watch the Bending and Torque video to further understand the forces of bending and torsion.
Finally, have the rider stand on the skateboard as if he/she were riding on a flat surface. Mark with tape where the heel and toe of each foot is located on the skateboard. Now balance the skateboard on a broom handle or any round piece of wood. Mark where the center of gravity is on the top of the skateboard. Describe how the feet are positioned relative to the center of gravity. Explain why the feet are positioned in this way.
Bending and Torsion
Next Generation Science Education Standards
Scientific and Engineering Practices:
-Asking questions (for science) and defining problems (for engineering)
-Planning and carrying out investigations
-Structure and function
-Stability and change
1. How could the forces that the trucks and wheels of the skateboard are experiencing lead to the failure of something like a bridge? Think about how bending force and torsion force could twist or deform a bridge support beam. What kind of event could cause a sturdy bridge to fail from being bent and twisted?
2. Imagine that the trucks of the skateboard were made of brittle clay. What would happen to these clay trucks if a sharp turn were made? How can making the trucks from a stronger material like aluminum help keep the trucks from breaking or deforming when they are under extreme stress?
3. Stand in place and extend your arms straight out from your body. Now twist like a windmill and try to touch your left foot with your right hand, keeping your arm straight. Return to standing position, and then try to touch your right foot with your left hand, still keeping your arm straight. Feel the way your abdomen twists when you do these toe-touches. What effect would this “torsion force” have on a steel beam if it twisted as much as your body just twisted?
1. From Google Science Fair 2012, professional skateboarder Steve Caballero talks about what inspired him to start skateboarding and where ideas come from. Science doesn’t have to be all about Bunsen burners and test tubes. What is the science behind skateboarding?
2. The Science of Skateboarding – X Games
A mobile museum built out of skateboards teaches people about the science and art of skateboarding.