The Pumping of a Swing (Conservation of Angular Momentum)

Nwr explanation is much more complete because it says how the engery you are spending transfers to the system and increases the angular momentum.Your explanation with energy is right, but it's form of ''you are spending energy and this energy goes to the system'' does not explain how it goes to the system.As for the OP, I'll just add (since I agree with nwr) that saying there is no outside force is incorrect, since the force of gravity is correct. And it is the force of gravity that adds angular momentum when you change the position of the center of mass.Remove the force of gravity, and you are correct that you will never be able to add angular momentum (you won't have an oscillatory system at all in fact)Edited by slevesque, : No reason given.

we are talking about the situation where the kid by itself pumps his own momentum. Of course he someone is pushing him it is pretty straightforward and easy, but your explanation doesn't explain why a kid, byt itself, without touching the ground, can swing.Just including the earth into the system doesn't necessarily do it, since the kid goes back and forth.

Yeah Ok sorry I didn't understand it that way (or misread)