The two wheels weigh the same, yet one rolls faster than the other. Why is that?
Look closely at the nuts and bolts. The two wheels have exactly the same number, but they are in different places. On the fast wheel they are in the middle. On the slow wheel they are on the edge.
The two wheels have the same amount of energy at the starting point. But when you release the wheels, the nuts and bolts on the slow wheel have to travel further to complete one circle. That requires a lot of energy. The wheel has less energy left for moving forward and arrives at the bottom later. If the nuts and bolts are near to the centre, they only have to rotate in a small circle. This leaves more energy to gather speed.
With the same amount of energy, one wheel can therefore turn faster than the other because the mass is distributed differently. We call this ‘the law of conservation of angular momentum’. Keep those wheels turning!
The law of conservation of angular momentum in figure skaters
How to do a good pirouette.
1. Start spinning slowly with your arms or legs outstretched.
2. Make yourself small in order to reach top speed.
3. Extend your arms or legs back out to slow down and carry on skating.