Suppose you are watching an ice-hockey game with some classmates. As the puck is sliding across the ice at a relatively constant speed, it passes close to a player, who hits it (with his stick) in the same direction as it is already moving. After the hit, you notice that the puck is moving much faster than it was before the hit. Your classmates propose the following three explanations for why the puck is moving faster after the hit than before. Whose reasoning do you agree with? Student A: "The force of the stick was transferred to the puck during the hit. After the hit, the puck has more force, so it moves faster."
Student B: "The puck still has the force that started it moving, which is what keeps it moving at a constant speed. When the player hit it, he adds to this force, which makes the speed of the puck increase. After the hit, this extra force is gone, so the puck stops increasing its speed, but the original force is still there so now it moves at a faster constant speed."
Student C: "While the stick is in contact, it applies a force to the puck that makes its speed increase. As soon is contact is lost, this force disappears, so the speed of the puck stops increasing."