PURPOSE: To illustrate forces in an accelerating system.

DESCRIPTION: A rigid frame hangs from a spring scale as photographed. In the frame, a mass hangs from a spring. The mass is pulled down and attached to a hook at the bottom of the frame by a short thread loop. (Ask your students how this affects the weight shown by the spring scale.) In this position the spring scale reads about 8 Newtons. Q: When the string is burned, releasing the mass, will the reading on the spring scale immediately after the string breaks (a) increase, (b) decrease, or (c) stay the same? A: It will increase, as seen on the accompanying mpeg video.

The photograph below shows details of the lower connection of the weight to the hook.

SUGGESTIONS: Ask your students what will happen, and why, before doing the experiment.

See Question of the Week #35 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1G30.20)

EQUIPMENT: Mass hanging from spring in frame hanging from spring scale, as photographed.

SETUP TIME: None.