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I love opening a new canister of tennis balls. That popping sound, the new tennis ball smell and the nice bounce you get when you play with them. However, I’ve never given much thought as to why tennis balls bounce (or any ball for that matter). In this blog post, let’s take a look at why this happens as well as various factors which can affect the bounce of a tennis ball. Why Tennis Balls Bounce Tennis balls bounce due to 3 factors which all work seamlessly together: 1. Kinetic energy and ball shape As the ball falls to the surface it has kinetic energy. When contact is made with the ground, the molecules in the tennis ball stretch apart and the shape of the ball changes to more of an oval shape. Some of the kinetic energy is lost as heat or sound, but a lot of it is retained and the molecules squeeze back together (a bit like a coiled spring when you let it go) pushing back on the ground, causing the ball to bounce and the shape returns to a sphere. 2. Pressurized air Tennis balls also have a hollow centre which is full of pressurized air. When the ball hits the ground, the pressurized air is forced inwards and then pushes back, causing the ball to expand and rebound off the surface. 3. Rubber material Another reason which helps tennis balls bounce is due to the rubber inner core. This rubber core gets compressed upon impact with the ground stretching the molecules apart, due to its high elasticity. It quickly snaps back into shape, pushing back on the outer core of the tennis ball, which pushes on the ground, shooting the ball back up in the air. How does temperature affect a tennis ball bounce? The tennis ball will bounce higher with a warm temperature compared to a cooler temperature. With a higher temperature the gas molecules move around inside the ball more and expand. Therefore the ball has more energy and a higher internal pressure, thus a higher bounce. If you play tennis outside all year round, you’ve probably noticed that if you leave tennis balls in the car overnight in the winter months, they can feel dead when you use them on the court. I certainly notice my shots have a bit less zip on them when I play on a cold day. I get less kick on serves and less bounce on my topspin forehand shots. This is because the low temperature causes the pressure inside the ball to drop, so the molecules move around less and have less energy. This results in a lower bounce. How does water affect the bounce of a tennis ball? Although playing tennis in the rain is not the safest activity, as you can fall on the slippery surface, water can affect the tennis ball too. When the tennis ball is wet, it will bounce less. This is because the fur of the ball soaks up water, and when it comes into contact with the ground some of the kinetic energy is absorbed by the water molecules. This means the ball has less energy to bounce back up off the surface. Does the age of the tennis ball affect the bounce? Yes. Tennis balls will bounce less over time even if you don’t use them. That’s because the pressurized air inside the ball leaks out over time causing a drop in pressure. Consequently, the ball has less kinetic energy after impacting the surface resulting in a lower bounce. However, if you purchase some new tennis balls in a can, the balls won’t lose any of their internal pressure for several months. That’s because the air inside the can is also pressurized. It’s only after you’ve opened the can that the balls lose their pressure more noticeably, whether it’s a week, a month, or several months later. What surface does a tennis ball bounce the highest? A tennis ball will bounce the highest on a hard surface. If you were to test the bounce on concrete, grass, wood, carpet or tile, it would bounce the highest on concrete (assuming controlled conditions were in place). This is because less kinetic energy is absorbed from the ball to a hard surface during contact. A softer surface, like carpet, will absorb more energy and the bounce of the ball will be less. Why do tennis balls lose their bounce? I wish tennis balls would last longer. However, over time they lose their bounce due to the pressurized air escaping from the interior of the ball. This can happen over time naturally even if you don’t play with them, but during a match the pressure drops even more due to the ball contacting the racket and the surface at great speed. How quickly do they lose their bounce? There isn’t a straightforward answer to this. The biggest factor is how hard you and your opponent hit the ball. At the pro level they change balls every 9 games but at the recreational you should be fine for a best of 3 set match.