Newton's Second Law of Motion
Newton's Second Law of Motion tells us that
Force equals mass times acceleration.
You can tell how hard a moving object is going to hit by knowing how big it is (how much mass it has) and how fast it is speeding up (its acceleration). For instance, if the pitcher throws the ball to the catcher, how hard it hits the catcher's glove depends on how much mass the ball has and whether it is speeding up or slowing down when it gets to the glove. You can make something hit harder either by making it bigger or by throwing it faster.
To find out more about Newton's laws, check out these books from Amazon.com or from your library: