Cartesian diver

HOW

1. Press and hold down the lever.
2. Observe the behaviour of the diver who is partly filled with liquid and partly with air. Vary the force of the pressure. Note how the air volume in the diver changes. Try to immobilise it halfway up the tube.

WHY

There are two forces acting on the diver:

• downward force of gravity

• uplift force.

The difference in the values of these forces determines the direction of the diver’s movement. By pressing the lever, the pressure in the tube is increased. This compresses the air in the diver to a smaller volume. As a result, more water is poured into it. It then becomes heavier and sinks.