I'm having trouble understanding how a child can seemingly increase the angular momentum of a swing (i.e, swing higher and higher). You can visualize the swing as a pendulum. The pumping would be the changing of the position of the bob.
There are two ways to "pump up a swing."
1.) Stand on the swing in a crouching position and start off with a little push. As you rotate through the lowest point (directly underneath the pivot) you stand up. This changest your center of gravity (the bob), and therefore you and swing move faster and therefore higher. At the apex, you crouch back down. Do this enough times and you'll increase the amplitude significantly.
2.) Sit in the swing. At the forward peak, swing your legs forward and your upperbody backwards. I think this works because your adding momentum to the bottom of the pendulum, i.e., your feet. Your upper body gets thrusted back in the process, so this would decrease your momentum. But since your feet are further away from the pivot, this change in momentum would increase the angular momentum more than the decrease in angular momentum caused by the head moving backwards.
I'm not sure this is exactly right, but that's my understanding of the process. If its not, then I would gladly be corrected.
Assuming this is right, it would seem that the angular momentum of the system has changed without any outside force. How is this possible.
Any help would be greatly appreciated.
Justin
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