Happy/Sad Balls
While this demo is a straightforward way to demonstrate elastic and inelastic collisions, this is possible due to something not so visible: hysteresis. Elastic hysteresis looks at how much of an object’s energy is dissipated from internal friction. The amount of energy dissipated depends on the material the object is made out of.[1] In the case of the happy/sad balls, the “happy” ball is made of neoprene and the “sad” ball is made of polynorbornene.[2]
The happy ball has a lower hysteresis so it will dissipate more energy in the collision with the table, causing it to bounce relatively high. Think of it as a transfer of energy; less of that energy is being opposed by the ball’s internal friction. The sad ball has a higher hysteresis so it will dissipate less energy in the collision with the table, causing it to barely bounce, if at all.[3]
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Happy/Sad balls can come after demonstrating the large/small balls collision so the students have seen how energy can transfer and how it remains conserved. I like to do the large metal ball collision after to demonstrate a different form of energy conservation (via heat and sound). This particular demo is a bit more abstract as the students will be missing the internal aspect with hysteresis, but you can explain this more in depth after showing the demo. I like to start by showing then asking the students if there’s any difference in the balls. They look exactly the same so most, if not all, will say no. I will then pass them around so each student can try to feel for any difference. Usually the student would have to squeeze pretty hard to have the rubber give in their hand. After everyone is given a chance to compare/contrast, I’ll drop both at the same height and time, making sure everyone can see. The happy ball will bounce significantly higher than the sad ball. I will then ask the audience which ball is the “happy” ball and which is the “sad” ball based on what they saw happened. The happy ball is, of course, the one that bounced and then I will explain what hysteresis is and how that affects the amount of energy stored in each ball. Since the sad ball stores its energy far more than the happy ball, that energy is not being transferred in the collision, which is why it appears to be an inelastic collision.