Friction Toys 2 - Leap Frog
‘Mr Wiggle’s Leap Frog’ is a rarely seen toy. If you come across one, buy it! Given a steady pull on the string the hind quarters proceed in a series of short hops as the spring stretches and contracts.
The Physics
This toy makes use of the difference between Static and Sliding
friction.
Once an object starts to slide the force needed to maintain movement is less
than the force needed to get it started.
With the hind quarters at rest the spring extends, gradually increasing the
force it exerts, until static friction is overcome.
By the time the rear end starts to slide, the tension in the spring is more
than sufficient to overcome the sliding friction. The rear end then
accelerates towards the front end.
As the spring goes slack the tension becomes insufficient to overcome the
sliding friction and the rear end comes to a momentary halt. Whereupon static
friction again comes into play and the process is repeated.
Note that the spring used is a flat cut helical spring - see under ‘Slinky’.