Lindsay Bartlett of Dunedin asks :-
How can we keep a swing going without touching the ground or being pushed?
Blair Blakie, a physicist in the Jack Dodd Centre for Photonics and Ultra-Cold Atoms at the University of Otago Physics Department responded.
From an early age most of us are introduced to the playground swing by our parents who, standing firmly on the ground, provide the necessary pushes to set us into motion. The work being done by the parent is required to overcome the energy that is lost to air resistance and friction as we oscillate back and forth. Indeed, if the pushing ceases and we sit rigidly on the swing, the motion would rapidly slow until we eventually come to rest.
As we get older we realize that we can keep the swing going, without any assistance, by rhythmically changing our body in sync with the swing's motion. If we do this appropriately we can maintain or even accelerate our oscillation. This might seem to pose a challenge to the laws of physics: Where is the energy coming from?
If we look carefully we find that the energy came from the person on the swing: our muscles are being used as we pull in our legs at the low points and push them out at the high points of the swinging motion. The physics of this is related to an ice skater executing a spin: as she pulls her arms in her spin speeds up. Similarly, if we pull in our legs as the swing passes through the low point we speed up. The skillful trick is to extend our legs again at the high point so that we do not completely undo the energy we have added to the motion - this is why timing is so important in driving the swing.
One interesting point is where did the muscle energy come from? From the Sun, which supplies the energy to grow the food we eat which provides the chemical energy our blood supplies to the muscles. So in essence we are solar powered machines.