# Kepler's Second Law using a dry ice puck

##### Demonstration

The speed of the puck increases as the radius of orbit of the puck decreases.

#### Apparatus and materials

Glass tube and fishing swivel, or freely swivelling pulley

Block of solid CO2,

Or

Glass plate and gantry

Wedges, 4

Brass ring pucks, 2

#### Health & Safety and Technical notes

Dry ice is very cold. Wear thermal gloves to handle it, and wear safety spectacles.

Take care when handling the glass plate.

The simplest technique is to use a swivelling furniture castor for the pulley. Drill a hole through the shank and cut a groove around the periphery of the wheel (see diagram). Support the pulley centrally just above the large glass plate so that the wheel can turn freely about the two axes. Use oil, if necessary, to allow the system to move freely.

Fix a cup hook, or other device, to a double puck, so that a length of light string can be fastened to its side. Place the puck on the glass plate, supported on a block of solid carbon dioxide about 1 cm thick and of external diameter about 2 cm larger than the metal block. Pass the string over the groove and through the pulley shank, so that it can be held by the hand.

An alternative arrangement uses the bar across the gantry to support a glass tube, which acts as the flexible pulley. Connect one end of the twine to a specially shaped puck, and the other end to a fishing swivel, which enables the twine to rotate freely. Connect this in turn to a spring balance which is held by hand. The puck must be specially shaped to give it a wider base, but the same mass as the usual CO2 pucks.

The glass plate should be carefully cleaned with ethanol before it is used.
Keep the carbon dioxide puck in motion before starting the experiment, so that it does not freeze to the plate.

#### Procedure

Start the puck moving in a circular orbit on the glass plate, applying the centrally directed force through the string. Increase this force and observe the effect.

#### Teaching notes

As the force on the puck is increased, its orbit gets smaller and its speed increases. Kepler's Second Law states that a planet in its orbit sweeps out equal areas in equal times.