The physics behind a swing; How do we make it go higher?

Question

After recently seeing a video of a man doing a loop on a swing, it occurred to me that swinging in that manner is pretty counterintuitive to the things I learned in physics class.

Basically, how do we make it go higher without an external force to push us, and without pushing ourselves off of anything? I’m stuck thinking that you can’t lift a chair that you’re sitting on, so a swing should work the same.

My first thought was it must be something to do with how you generally swing your legs, but in the video I saw, the guy on the swing didnt use his legs in that way at all as he was standing on the seat of the swing.

So, in simple terms what actually is happening there, and where does that momentum come from?

In: 9

Answer 1

*You* are the “external” force. Even standing on the swing, you are throwing your weight against the swing, leaning forward or backward. This shifts the center of gravity of the swing so that gravity will cause it to shift. Once the swing becomes balanced again, you shift your weight the other direction so that gravity is pulling the swing in the other direction.

Because of the mass of the swing and you on it, the swing has some momentum. It doesn’t immediately stop when it should be balanced: it will move a little bit past the balance point and *then* slow down, and then stop. If you weren’t shifting your balance each time, you would slow down from energy lost due to friction and things. Eventually you would come to a full stop.

The other important thing is the *resonant frequency* of the swing. The length of the ropes/chains and the mass determine the period of the swing – how often it naturally wants to go back and forth. No matter how high the swing goes, the period stays the same. When you swing your mass properly, you’re matching the period so that the frequency of the swing moving and the frequency of you shifting your weight *constructively interfere*. That preserves the energy of the swinging motion and adds to it so that you keep swinging higher.

But again to be clear: *you* are adding energy into the system by shifting your weight against the natural swing, which would otherwise destructively interfere with the swing and slow you down.