Isaac Newton's first law of motion is inertia. Inertia holds that objects at rest remain at rest and objects moving at a constant velocity continue to move at that velocity unless they are acted upon by an external force. Inertia is directly related to mass. The more massive an object, the more it will resist changes to its state of motion. Pushing a real car, for example, is harder than pushing a toy car.

Yet, behavior occurs that seems to contradict the law of inertia every day. When a hockey puck is given a push so that it slides across the floor, it stops after traveling only a short distance. No one has thrown a paper airplane that did not eventually slow down and crash. There may appear to be no forces acting on boxes and airplanes after they are pushed or thrown, but there is one, friction...