Laws of Motion

Slide a block of wood across a level uncarpeted floor and notice its behavior. The block continues to move as long as you apply a force. When the force stops, the block stops moving. The block will continue to slide for a while after you stop applying a force. Your pushing is not the only force acting on the block. There is also a frictional force opposing the motion. The block sliding across the floor stops because this frictional force acts on it. The block on an icy surface takes longer to stop because there is less frictional force. If you could slide the block across a surface with absolutely no friction, it would never stop. The block would keep moving until some outside force, such as the wall of the room, stopped it. A block on a level surface, without application of forces will not move unless something applies an outside force; it will remain there at rest forever.

The first of Newton's laws states an object will continue its motion at a constant velocity until an outside force acts on it. The block has a tendency to continue in its state of motion, whatever that state might be, until some force changes that state of motion. This tendency to continue in a state of motion is called the object's inertia. An object at rest simply has a constant velocity of zero, so it needs an outside force to start moving. The physicist's definition of velocity includes both speed and direction, so any deviation from straight line motion is a change in velocity and will require an outside force. The inertia of any object will cause it to continue to move at a constant (in a straight line) velocity (or stay at rest) until an outside force acts on it.

A block will slide more easily than, for instance, a refrigerator because it has less mass. Newton's first law says that a force is needed to change the velocity of an object; the second law tells us how much force. Any change in velocity (speed up, slow down, or change direction) is an acceleration. For the common case where the mass does not change, Newton's second law states that the force required is equal to the accelerated mass times the acceleration (Force = Mass × Acceleration). It's harder to slide the refrigerator than the block across the floor because the greater mass requires a greater force to accelerate it from rest. A force in the same direction as the velocity increases the velocity; in the opposite direction, decreases it. A force perpendicular to the velocity changes the direction of motion. Occasionally the accelerated mass changes. In this case, the force is equal to the rate at which the momentum changes with time.

Newton's third law states that for every action there is an equal and opposite reaction. The action and reaction are equal and opposite forces forming an action reaction pair. If you are sitting in a chair, Earth's gravity pulls you down. The reaction is that you pull Earth up with exactly the same amount of force. The action reaction pair is: you on Earth, Earth on you. The reaction is NOT as is often thought the floor or chair holding you up. Not all equal and opposite forces form an action reaction pair.

Newton's three laws of motion revolutionized physics. For the first time the same simple set of laws explained a wide variety of apparently unrelated types of motion both on Earth and in the heavens. Not until the twentieth century were these laws surpassed by quantum mechanics and relativity for the special cases of subatomic particles, motion near the speed of light and strong gravitational fields.