Newton’s three laws of motion form the basis for analysis of many types of motion. The First Law states that a body in motion tends to remain in motion, while a body at rest tends to remain at rest. To change motion to rest or vice versa requires force. For example, an ice hockey player who steps onto the ice with his skates will not begin to move across the ice unless he applies force with his legs or arms to push himself. Likewise, if he is speeding across the ice after the puck, he will not stop without another force – friction, producing a “hockey stop” with his skates, or the imposition of another player in his path. The Second Law of motion states that the acceleration – the change of motion, increase or decrease, in either speed or direction – is dependent upon the forces acting on an object and the mass of the object. This law is often described as “force equals mass times acceleration.” This equation results in several concepts. If forces are equivalent, a hockey player with greater mass will experience less acceleration. When two players collide, the player with the lower mass will accelerate more than the other player. Another way to look at the second law is that a more massive player will need to expend more force to accelerate to a given speed, but also more force will be required to stop her.
Newton’s Third Law of motion states that for every force, there is an equal and opposite force. This may sound contradictory to the second law, but it isn’t. This is part of the reason that when two players collide, the less massive player has more change in motion. When a collision occurs, Newton’s third law says that forces are equal. Therefore, the force on the smaller player is the same as the force on the larger player. Mass times acceleration (smaller) is the same amount as mass times acceleration (larger). We know that the masses are different. Therefore, the accelerations must be different if the forces are equal. Another hockey example of Newton’s three laws of motion is the movement of the puck. Ice is a low-friction surface, such that the puck will tend to keep moving. However, when the players crowd around the puck, trying to hit it with their sticks, there may be several forces acting on the puck at one time. Newton’s second law says that the net force will equal the mass of the puck times its acceleration. Since the third law says that there is an equal and opposite reaction to every action, the hockey players will experience complex forces as well, based on their actions toward the puck and the other players.