Whirling a rubber bung on a string

Class practical

To begin to explore circular motion in more detail.

Rubber bung

Health & Safety and Technical notes

Ensure there is sufficient area around each student when whirling. Keep away from windows, etc.

Securely attach the rubber bungs to the string. Ensure that the string is held tightly as it is whirled around the head.

Observers should wear safety spectacles.

A rubber bung is convenient for this experiment and safer than whirling stones or steel washers.

Procedure

a Whirl the bung fairly gently in a horizontal plane above the head.

b Allow the string to wrap round a finger. If the bung is whirling slowly at first, the consequent increase in speed will be apparent.

Teaching notes

1 Ask: Is the force on the bung a push or a pull? (Strings never push.) The same string which pulls the bung inwards also pulls the hand holding it outwards.

Step b is a model of a satellite about 200 km from the Earth where there is still some air resistance and so energy is transferred to the atmosphere and the satellite descends. As that happens, its time to orbit the Earth grows smaller and the satellite speeds up.

In this experiment a vertical gravitational force acts on the bung. A vertical component of the string's tension balances this force: note that the horizontal plane in which the bung orbits is slightly below the height at which your finger holds the string.

(Some students will maintain that there is a centrifugal force acting on the bung. They would certainly infer the presence of such an outward force if they were in a rotating frame of reference riding on the bung. See guidance note Centrifugal motion at a fun fair

2 Offer another example, a thought experiment: Sit on a slippery bus seat, with eyes closed. When the bus turns round a sharp left corner, in which direction do you appear to move? This may be a difficult question to answer because of our intuition. There is nothing to stop your motion (friction on the bus seat or bracing yourself not to move) continuing straight on (Newton’s first law). However, the bus turns underneath you and you hit the wall (or person) on your right.