Book: #2

Newton's first law tells us that in the absence of external forces, a body would continue forever in its initial state of motion. This cannot be illustrated rigorously using a body at the earth's surface since such a body is always subject to the force of its own weight. However, we would overcome this objection if we could use a body moving along a horizontal, frictionless surface. Q273.1 Why? 

In the first sequence of this loop, an air-track glider is used to give some impression of what such a "persistent" motion would look like. the center half of the track is straight and horizontal; both ends are turned up slightly to reverse the velocity of the car. Unfortunately, we can only approximate a frictionless surface, and even a vary small force will soon slow a moving object noticeably. Watch how the transit-time of the glider across the level section gradually lengthens over a period of several minutes. 

Newton's Second Law. Here we exert a force on a body in a direction in which it is free to move. We apply the force to a heavy glider on a horizontal track by means of a thread (invisible in the film) which leads to a "constant force device" (out of frame at the right). This exerts the same force whether the glider is moving or not. The force's magnitude is indicated by the extension of a coil spring linking the thread to the glider. Note that in this way adopted one possible definition of force.