The impact which the tennis ball will make on the racket after having hit it, is calculated by its mass times its acceleration. We can alter the impact or force the object has on the object being hit, by making the object smaller, bigger, slower or faster. In conclusion force is measured by mass times acceleration.
Helpful Definitions
Mass: The amount of matter in an object.
Acceleration: Acceleration is an increase in speed or velocity.
Force: A power exerted upon an object, most commonly a pull, push or twist.