PURPOSE: Demonstrate buoyancy with a counterintuitive element.

DESCRIPTION: A ball floats in a beaker of water (at left in picture above). The ball sinks in a beaker of mineral spirit paint thinner (at right above). The mineral spirit paint thinner is immiscible with water, and will float above the water when poured slowly on top of water. Q: If the ball is floating on the water, what will happen to the ball when mineral spirits is slowly poured on top of the water? A: The ball will (a) move up, (b) move down, or (c) stay in the same position relative to the surface of the water. The picture below shows what actually happens.

SUGGESTIONS: Green food coloring has been added to the water so that you can distinguish it from the mineral spirits, which is clear.

See Question of the Week #3 for information on using this demonstration to enhance class involvement.

This demonstration was developed by Wolfgang Rueckner for Eric Mazur (Harvard University) as a demonstration question to be asked of students in the "Peer Instruction" tradition.

REFERENCES: (PIRA unknown)

EQUIPMENT: Ball with beakers of green water and mineral spirits. TV will be provided in the lecture halls.

SETUP TIME: 5 minutes