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
What you’re looking at is not momentum (at least not in the physics sense of the word). Gravity act on you all the time, so your momentum keep changing. Your momentum are never preserved.
The word you are looking for is energy. Essentially, your body+swing system keep exchanging between kinetic energy and potential energy. Total energy is loss through friction, and max potential energy determine the highest point you can reach. With that argument it looks like you shouldn’t be able to swing higher than when you start….but that’s wrong, because energy can come from many sources. In particular, chemical energy from within your body get converted into kinetic energy, so you can actually add energy to this kinetic+potential total energy.
So that should explain your misconception away. Momentum is not preserved, energy is also not preserved either. Both of them can increase.
So, the next question, is how to actually do it?
When you move your body on the swing without pushing on anything, you can’t change the angular momentum of the entire body+swing system. However, what you can change is your mass distribution, which change your center of mass. Because the angular momentum does not change (as a result of center of mass change), changing the center of mass can increase or decrease your speed: the closer the center of mass from the pivot (ie. your center of mass is raised), the faster your speed. And increase speed means increase kinetic energy, and hence total energy. For maximum gain, raise your center of mass at maximum angular momentum.
Of course, just raise your center of mass won’t be a sustainable method, because you can’t raise forever. So you do need to move the center of mass back. So what happened when you do so? Well, the extra energy can’t just go back into your body, so you just need to ensure it does not go to the wrong place to keep all that extra energy to yourself. When you raise your center of mass, you also done work against gravity, so your potential energy increase too. This gain in potential energy pay back the loss of potential energy when you lower your center of mass. To avoid huge loss of kinetic energy, lower your center of mass at minimum angular momentum.
In summary, you use gravity to increase your maximum angular momentum, and you use your body’s chemical energy to increase your maximum kinetic+potential energy, so neither of these can limit you.
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