A tennis ball reaches its maximum speed the moment it leaves the racquet – about five milliseconds after contact. During a pro player’s serve, the racquet tip is travelling at about 160 kilometres (100 miles) per hour when it makes contact, passing on its kinetic energy. The racquet’s strings stretch, conferring additional energy to the ball. Upon leaving the strings, the only forces acting on the ball are gravity and drag. Even if the ball has a downward trajectory, drag is likely to outweigh any acceleration caused by gravity, meaning that from this point onwards, the ball slows down. Andy Murray’s fastest serves clock in at a blistering 233 kilometres (145 miles) per hour – but by the time it reaches his opponent, the ball has usually slowed to half that speed.
Answered by Alex Cheung