Book: #2

     Having examined Newton's first and second laws, we now consider the third law which tells us that the forces which two bodies exert on each other are equal and opposite. We test this principle by allowing two gliders, initially stationary on a horizontal air track, to exert forces on each other. 

The forces applied to the two gliders are provided by a long "reaction spring" attached to one of them. The gliders are cocked by pushing them together and compressing the spring. They are held together by opening the air supply bypass so that they rest directly on the track. When the bypass is suddenly closed, they are abruptly lifted and are free to accelerate in the opposite directions. After the gliders have traveled convenient distances, the bypass is quickly opened, and they drop on the track and stop. (Skidding introduces only very small errors.) In the final section of this film, spring are attached to both cars.