Principle of swing

Swing setshas two cases:
Sitting in the swing, to push the other one, if only a push, and then just as a simple pendulum swinging back and forth, but because of air resistance, amplitude will be more and more small, but at the same time constant, this kind of situation with the vibration of the pendulum when cannot ignore air resistance; If pushed, swing when swing swinging back to swing a thrust, the amplitude of the swing may be the same or increase, then the vibration of the forced vibration.
The second case is that people usually stand on the swing, squat down and up to swing a driving force, make the Swing setswing, this also is forced vibration. As long as people squat down and set up a perfect balance of time, will swing swing higher and higher, on the contrary, the time to grasp well, you might swing.