Any time a ball bounces, it loses momentum because it transfers some of that momentum to the ground. This makes its first bounce height less than its starting point, and each subsequent bounce height will be less than the preceding one. However, when two balls of different masses are dropped one on top of the other, the results are dramatically different. If a basketball is set on a tennis ball and both dropped, the basketball will bounce off the tennis ball, leaving it on the ground. However, if the tennis ball is set on top of the basketball and both dropped, the basketball will appear to bounce normally but the tennis ball will fly off of it at a much faster speed to a larger height. This results from a transfer of momentum and energy between the balls. When the much smaller tennis ball is on the bottom, almost all of its momentum is transferred to the basketball, leaving itself on the ground. When the much larger basketball is on the bottom, it transfers its momentum to the tennis ball, sending it away at a much higher speed.
Temperature can affect the ability of a tennis ball to bounce. This results from gas laws, which state that pressure rises when a gas is heated. Greater pressure inside a warmer tennis ball will result in a greater ability to bounce. Conversely, lower pressure inside a cooler tennis ball will result in a reduced ability to bounce. Students can experiment with this concept by warming tennis balls in the sun or under a heating pad, then testing the height of their bounce. They can then cool the tennis balls in a freezer or ice bath, after which they can test the height of their bounce. Students can also test these bounce heights against tennis balls at room temperature, which serves as a control.
Tennis balls that have lost their bounce are said to be "dead" balls. They lose their bounce because the air trapped inside the balls has slowly seeped out, equalizing the pressure with the outside air. Students can compare the bounce height of tennis balls that have come fresh out of a new package against the bounce height of tennis balls that have been used in a large number of matches. Other factors of tennis ball age, such as the loss of the fuzz on its surface and the hardening of the elastic shell, can also be factors in the loss of its bounce.
Because the bounce of a tennis ball relies on the pressure inside it, students can design an experiment to evaluate how well a tennis ball retains its bounce after losing pressure. Begin with a brand new tennis ball and measure its bounce height. Then use a pin to poke one or several holes in the ball's shell, after which students can measure the bounce height. Finally, use a sharp, thick tool such as a large nail, ice pick or screwdriver to poke a significant hole in the tennis ball's shell. Students can measure that bounce height and compare it in a data table with the other bounce tests.