Bouncing balls and elasticity
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Bernard Nebel.
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All kids experience that some balls bounce and others, e.g. bean bags, don’t. Invite kids to analyze and explain why the one bounces and the other doesn’t. For their explanations, guide them in reviewing and utilizing their Volume I lesson, C-3 regarding exchanges between kinetic and potential energy and Lesson C-6 friction. The following videos provide may help:
Bouncy balls: https://www.youtube.com/watch?v=QfRXBAgIzwY
Dead balls: https://www.youtube.com/watch?v=ewDXEpyT-tw
Give and discuss the meaning of elasticity, the ability of a material–when stretched, bent, compressed, or otherwise deformed–to return to its original shape. What sorts of material are elastic? In addition to rubber, consider a steel rod or spring. Also, drop a glass marble on a stone/tile surface. (It will bounce with surprising efficiency.)
Draw kids to consider two aspects of elasticity: 1) the amount that a material can be deformed (without breaking or becoming permanently bent) and still return to its original shape, and 2) the efficiency of energy transfer between kinetic and potential forms. For example, glass can be deformed very little before breaking, but short of breaking the transfer from kinetic to potential energy and back is very efficient (little loss to friction). The surface an object strikes invariably plays a like role.
Rubber balls and inflated balls (e.g. soccer ball)
Have kids consider the distinction between a solid rubber ball and an inflated soccer ball. Both bounce. Conduct discussion to reveal: In the rubber ball, rubber itself is the elastic material. A deflated soccer ball will not bounce: it is the air (its compression and decompression) that provides elasticity to the inflated soccer ball. Yes, air exhibits the properties of elasticity.
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