FRAMES OF REFERENCE

Film Loop: A matter of relative motion.

Length: 3:40 min., Black and White, No sound

In this film, two carts of equal mass collide. Three sequences labeled Event A, Event B, and Event C, are shown. Describe these space-time events in words.

The three events are photographed by a camera on a cart which is on a second ramp parallel to the one on which the colliding carts move. The camera is our frame of reference, our coordinate system. This frame of reference may or may not be in motion with respect to the ramp. As photographed, the three events appear to be quite different. Do such concepts as position and velocity have a meaning independent of a frame of reference, or do they take on a precise meaning only when a frame of reference is specified? Are these three events really similar events, viewed from different frames of reference?

Even though Events A, B and C are visibly different to the observer,in each the carts interact similarly. The laws of motion apply for each case. Thus, these events could be the same event observed from different reference frames. After viewing the initial sequences of the film, it is evident that they are closely similar events photographed from different frames of reference.

You might think that the question of which cart is in motion is resolved by sequences at the end of the film in which an experimenter, Franklin Miller of Kenyon College, stands near the ramp to provide a reference object. Other visual clues may have already provided this information. The events may appear different when this reference object is present. But is this fixed frame of reference any more fundamental than one of the moving frames of reference? Fixed relative to what? Or is there a "completely" fixed frame of reference?

If you have studied the concept of momentum, you can also consider each of these three events from the standpoint of momentum conservation. Does the total momentum depend on the frame of reference? Does it seem reasonable to assume that the carts would have the same mass in all the frames of reference used in the film?