Two balls that look and generally feel the same, but when you bounce one it springs up like it drank two cups of coffee and an energy drink, while the other thuds to the ground as if exhausted. The balls are a trick favored by stage magicians, who switch them out to amuse and astound audience members; but you’ll know better. To uncover what’s happening, we’ll take a look at why rubber balls bounce, and how a few extra atoms in each molecule cause these two to bounce differently from each other.
When a dropped ball hits the ground, the force of the impact causes it to slightly deform; a moment later the material of the ball snaps back, at which point it exerts a restoring force on the ground as it springs back into its equilibrium shape. That force acts in the opposite direction of the way the ball was deformed, pushing back down and causing the ball to spring up into the air.
How high the ball springs up depends on how much the material absorbs kinetic energy, and how fast the material returns to its original shape after the deformation. Famously, steel balls will bounce higher than rubber ones, because the steel springs back to its original shape much more quickly than the rubber. “Elasticity” is the measure of how much something can return to its original shape during the collision, and the process of it occurring is called “elastic deformation” (“plastic deformation” is when something changes shape but doesn’t change back).
But you’ll notice that even the springiest superball doesn’t 100% bounce back up to the height from which it was dropped. Why not? When a ball (or anything else) falls, it converts gravitational potential energy into kinetic energy. After it hits the ground and the ball is deformed, that kinetic energy is mostly turned into elastic potential energy, and when the ball springs back up, the potential energy is converted back into kinetic energy.
But some of that kinetic energy is also absorbed in the deformation and gets lost, so it isn’t available to spring the ball up to its original height. Where does it go?When the rubber is deformed, most of the “lost” energy is transferred to the molecules of the ball and dissipated by internal friction, causing heating. So each time you drop a ball, you’re actually heating it up a bit, though generally not enough that you can feel the difference.
So a key part in why balls bounce has to do with that deformation. And the reason the two rubber balls act so differently is because of the way rubber reacts when it hits the ground and gets deformed. Rubber is a polymer which, generally speaking, means its molecules are arranged in long chains. Those chains can rotate around the chemical bonds that link them together, allowing them to change shape, at least for a moment. The key, as you probably guessed, is that each of the balls are made of a different kind of rubber. To see why one of your balls springs back up and the other doesn’t, take a look at the molecules:
The “bounce ball” is made of natural rubber in the top image, and the “no bounce” of butyl rubber, in the bottom image. The key difference is those bulky CH3 sections, called methyl groups, which make it harder for the molecule to change shape. You’ll see that the natural rubber molecule has just one methyl group, but the butyl rubber molecule has three. That means the natural rubber will deform and spring back much more easily, as the molecule rotates smoothly around its chemical bonds. But because it doesn’t move as freely, the butyl rubber ball absorbs much more of that kinetic energy, and won’t bounce as high. It also technically gets much warmer than the bouncing ball.
The bounce/no-bounce ball set is great for illustrating how small molecular differences can have big effects in the macroscopic world, for demonstrating kinetic and potential energy – and for just having fun!