Tennis Serve Activity

Construction Context
tennisserveTake an empty soda can, and pressing your thumbs into the middle, bend the top and bottom of the can towards each other. What happens when you do this? Bend forward and try touching your toes. What do you feel in your tummy? Both the can and your abdomen are experiencing the bending force. Now take the same can you bent and try twisting it in your hands. What happens to the can now? Bend forward and now touch your right hand to your left foot. Now bend over and touch your left hand to your right foot. The can and your abdomen are now experiencing the force of torsion. 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. Watch this video of a tennis serve to observe these concepts in motion:

Materials
Tennis racket
Tennis balls

Activity Description
Take a tennis racket and tennis ball. Bend your knees and bring the racket up behind your head. Throw the ball above your head while simultaneously swinging the racket. Hit the ball with your racket. Feel the way your body twists and bends while you are doing these motions. The twisting of your torso is like the force of torsion a bridge or building may experience in a high wind. As you bend forward while serving, your body experiences a bending force.

If a support in a bridge or building bent the same amount that your torso bends in the tennis serve, the effect would be devastating and could possibly cause the structure to fail. Have you ever stood on the top floor of a very tall building? If you have, you may have noticed that you can feel the entire building sway. If one stands on the observation deck of the John Hancock building in Chicago, one can feel the building sway up to 10 feet in any direction in high winds. When you stand on a bridge with a semi-truck driving over it, do you feel the bridge bend very slightly from the truck’s immense weight? Buildings and bridges need to be able to bend and twist slightly so that they can withstand the forces of bending and torsion. If a building could not bend and twist a little, then it would be too brittle and could break in high winds. If a bridge is not able to bend and twist a little, it would also be too brittle and could similarly break when carrying heavy loads.

Watch the Bending and Torsion video to help better understand how the forces of bending and torsion affect construction materials.

Science Concepts
Bending and Torsion

Next Generation Science Education Standards
Scientific and Engineering Practices:
-Planning and carrying out investigations
– Developing and using models

Crosscutting Concepts:
-Cause and effect
-Stability and change

Reflection Questions
1. Discuss with a friend how buildings and bridges have to be designed to resist the forces of bending and torsion. What effect would bending and torsion forces have on bridges or buildings?

2. Ask a skilled construction worker or a civil engineer how bending and torsion forces can deform or destroy a support beam or column. What construction and/or engineering practices are used to work against these bending and torsion forces?

3. Think about doing a sit up. Try doing a sit up and bending to the right as you come forward. Now try bending to the left as you come forward. What is happening to your insides when you twist during a sit up, instead of just doing a regular sit up? What effect would this twisting (torsion) force have if your stomach was a metal beam?

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