Friction toys 1 - the woodpecker
A toy which never seems to be out of production is the bird, monkey, etc which slides down a vertical pole when given a nudge.
This one came from Germany and is called Oscar, der Specht (woodpecker).
It remains at rest on the pole, supported by friction until it is set in motion. Then it moves down the pole in a series of short movements as the bird bounces up and down on the spring. It also makes a woodpecker-like sound as the beak strikes the pole. An endless source of entertainment – very satisfying!
The Physics
The frictional force between two surfaces depends on how hard they are pressed together – the “normal reaction” in physics jargon. When the bird is at rest its weight at the end of the spring causes the sliding collar to be pressed into the pole, providing sufficient frictional force to prevent movement.
When the bird bounces upwards on the end of the spring the load momentarily comes off the sliding collar and it is free to start sliding down the pole – only to be stopped short when the bird bounces downwards and increases the ‘normal reaction’ again.
Thus a series of small movements of the collar accompanies the oscillations of the bird on the spring. Moreover the loss of potential energy as the system moves downwards is fed back through the spring so maintaining the oscillations, which would otherwise quickly die away.