Penny on a turntable
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.
a Fix smooth paper on the turntable. Place a penny about 7 cm from the centre.
b Gradually increase the speed to something over 1 revolution per second - to, say, 78 rev / min - so that the penny slides off.
c 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].
1 In step b, 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.
2 After step b, 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.
3 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.