Tennis balls bounce from the pressurized air pumped into the center of each rubber core. As the ball compresses against the ground, the air inside the core launches the ball off the ground. Tennis ball makers inject a specific amount of air into the rubber core center.
These reasons include kinetic energy, shape, materials and the gas manipulated into the ball.
Kinetic Energy
When you throw a tennis ball against the floor, it gains kinetic energy. The molecules in the tennis ball begin to stretch apart. Then, they squeeze back together. As this process occurs, the ball gains more energy and is able to bounce off the ground.
Shape
As a tennis ball gains kinetic energy, it transforms shape from oval to round. With the transformation, the molecules cause the tennis ball to bounce up. And the ball bounces because it must transform itself back into the round shape.
Material
Rubber balls contain more bounce than other types. Rubber consists of a strand of polymer molecules. These molecules form a mass of molecular structure that entwines itself.
When a tennis ball is thrown to the floor, the molecules unwind for an instant. Then, the rubber condenses them back together causing upward motion to occur.
Furthermore, the fuzz on the outside of the tennis ball plays a role in its ability to bounce. According to NASA, the fuzz adds additional friction and increases the bounce velocity.