The swing
A swing does the same trade as the bicycle, but it does it over and over, and at one point in every swing the exchange is complete.
Figure to be added
From A to B
A is the lowest level the child reaches and B is the highest. As the child moves from A towards B, the kinetic energy gradually falls — but the child is rising above the level of A, so the potential energy increases.
At B, the exchange is complete
On reaching B the velocity is zero, so all of the kinetic energy has been converted into potential energy.
That is the moment the swing appears to hang still before starting back — the one point in the whole arc where no kinetic energy is left.
From B back to A
As the child then moves from B back towards A, the potential energy contained in them is converted back into kinetic energy, and the cycle begins again.
| Point of the swing | Kinetic energy | Potential energy |
|---|---|---|
| A — the lowest point | Maximum | Minimum |
| B — the highest point | Zero | Maximum |
Fastest at the bottom, highest at the top
Kinetic energy is greatest where the swing is lowest and fastest; potential energy is greatest where it is highest and momentarily still. One is always growing at the other's expense.