PURPOSE: To illustrate dramatically the concept of buoyancy.

DESCRIPTION: A float is at rest in a water vessel suspended by a spring from a fixed point. The vessel is lifted up and released from rest, so that it oscillates vertically on the spring. In the picture above a band around the floater lies between the two bands around the larger vessel when the system is at rest. Q: How will the float move inside the water vessel as the vessel executes simple harmonic motion? A: Surprisingly, the float will remain at rest in the water vessel as it oscillates (click here for video). It will even remain at rest when the vessel is stopped suddenly with your hand! Click here for video. As the vessel oscillates, the weight density of both the floater and the water bath vary together, as the acceleration of the vessel varies, so the ratio of their densities remains the same, and they will continue to float with the same geometrical relationship!!

SUGGESTIONS: See Question of the Week #69 and Question of the Week #70for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for further description and discussion of this demonstration. I have been told that this idea was proposed early in the NASA space program as a way to reduce injury to astronauts if a crash were to occur to re-entering space vehicles. If anyone has any information on this I would be interested.

A relevant paper from the European Journal of Physics, A demonstration of Einstein’s equivalence of gravity and acceleration, by Ronald Newburgh has been reproduced here.

EQUIPMENT: Floater in water vessel hanging from spring on stand.

SETUP TIME: 5 min.