Final answer:
The interaction between a racquet and a tennis ball primarily changes the ball's momentum, which can start, stop, or alter the ball's direction.
Step-by-step explanation:
The effect of the force between the racquet and the tennis ball serves multiple purposes, including to start the ball moving, stop the ball moving, or change the direction the ball is moving in. When a tennis player strikes the ball with a racquet, the location of the impact on the racquet, the part of the stroke during which the impact occurs, and the motion of the racquet all contribute to the effect on the ball. Hitting the 'sweet spot' minimizes vibration and maximizes the velocity of the ball. The force applied by the racquet changes the ball's momentum (Δp), which is a product of the net force exerted on the ball and the duration of that force's application.