The force required to hit a tennis ball depends on how fast the ball is coming at you and how fast you want to hit it back. For example, imagine a professional tennis player is serving the ball at you. Serena Williams can serve at over 120 miles per hour, but due to air resistance (the drag on a moving object as it pushes through the air) the ball will have slowed down to about 60 miles per hour by the time it reaches your racket on the other side of the court. If you want to hit it back, and you swing your racket at 60 miles per hour (assuming you are also a professional tennis player), the ball will leave your racket between 60-75 miles per hour, depending on the type of racket you have. To hit your 75 mile per hour return will require about 2000 Newtons of force (a Newton is a unit of force, just like a pound, is a unit of mass and a second is a unit of time).

However, if you are not quite a professional tennis player, and you want to return a 60 mile per hour serve which reaches you at 30 miles per hour, and you swing at 30 miles per hour, you will need to deliver about 500 Newtons of force. So, when professionals serve twice as fast, they have to return the ball with four times the force!

In order for balls to be acceptable for use in tennis tournaments, they must be approved by the International Tennis Federation. When testing, the balls are compressed to 80 Newtons, and the deformation of the ball, or how much it is squished, is measured to make sure that the balls behave the same. Then, the ball is further compressed, and then allowed to relax before measuring the deformation at 80 Newtons again. If the deformation changes after the hard compression, the ball is not good for tournament play. Therefore, you probably won’t find any defective balls. If you do find a defective ball though, the amount of force needed to break it will depend on what the defect is. If the defect is bad enough, the ball might fall apart without any force on it at all!