GRAVITATION

Film Loop: Moving system of orbiting bodies.

Length: 2:20 min., Black and White, No sound

Using computer generated animation this demonstration shows the behavior of two bodies attracted to each other by a force varying inversely as the square of the separation of the bodies (gravitational force field). The bodies are given arbitrary initial velocities and the path of each body is quite complicated (see Figure). The center of mass of the system is seen to move in a straight line. Then the two-body system is viewed from a frame of reference moving with the center of mass. From the point of view of the center of mass the motion of the bodies are simple closed elliptical orbits.

NOTES: The conventions used in the film are all shown in the figure: the size of the circle shows the mass (2:1 ratio), the center of mass of the two-body system is marked by a cross, and a line traces the path of each body (In the 2nd sequence the line traces the path of the center of mass); the forces are not indicated. The description of the two bodies (whether kinematic or dynamic) is completely valid in either of the reference frames shown: the center of mass was not accelerating in either frame (inertial frames). However, both the motions and the description of the energy of the bodies are simpler in the center of mass frame of reference. These ideas are valid for the analysis or description of any system of interacting bodies.