Penny on a turntable
Practical Activity for 14-16
A further example of the motion of an object travelling in a circle.
Apparatus and Materials
Health & Safety and Technical Notes
A record turntable would be suitable for this demonstration.
- Fix smooth paper on the turntable. Place a penny about 7 cm from the centre.
- Gradually increase the speed to something over 1 revolution per second - to, say, 78 rev / min - so that the penny slides off.
- Replace the paper with a polystyrene ceiling tile fixed to the turntable. Stick a drawing pin into the tile near to the penny to stop the penny sliding, and increase the turntable speed [the pin should not be used to hold down the penny and prevent it from moving].
- In step 2, once limiting friction is exceeded then the penny moves. A little ink under the penny leaves a track.
- Ask students to describe what happens from the point of view of an outside observer at rest (the penny moves along a tangent) and then to speculate what that would look like from the point of view of an observer standing on the turntable (the initial motion of the penny is radial).
- When the penny slides, initially it does so at a tangent but friction with the rotating table makes its motion turn into a spiral.
- After step 2, ask: "Where should the pin be placed, in order to prevent the coin sliding? (At A, B, C, D or elsewhere?)" The force of the pin on the penny plus the frictional force between the penny and the tile provide a centripetal force pulling the penny into an orbit. (Correct answer is B.)
- With the extra force from the pin, the table can rotate faster without the penny sliding.
- Students may find it helpful to imagine a toy truck moving along in a straight line. To get it to turn left, then it could be nudged sideways to the left; in other words towards the centre, of which the left turning arc is a part. To enable it to continue along that arc, the truck will need periodic nudges. And the nudges must change direction so that they are always at right angles to the truck.