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Why can't I jump or fly if I pull my hair upwards, while I can jump using my legs?

The way I see it, when jumping someone lifts its body using the muscles in the legs (while the feet are standing still). If this lift generates enough speed, it continues even after the body is stretched. The fact that one cannot fly by pulling up his own hair I think it has something to do with the energy of an isolated system (jumping can be seen as pushing the earth away from you, while pulling hair doesn't interact directly with the earth).

There are also similar situations when there is some energy that seems to appear from nowhere, like when swinging alone. Using feet, one can accumulate potential energy and release it quickly, but why does this make the swing move?

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  • $\begingroup$ When you jump, you push against the ground. When you pull your hair, you push against your own body. $\endgroup$ – Hot Licks Feb 1 '15 at 4:02
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Why can't I jump or fly if I pull my hair upwards, while I can jump using my legs?

Thats come from Newtons 1st law.

which is: When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.

Note that the defination says ''external force''.

When you pull your hair upward thats not an ''external force'' on you since ''you'' are applying that force on ''yourself''.

What happens when you jump using your legs ?

In this case you actually apply a force on earth,in return the earth also apply a force on you according to Newtons 3rd law.

Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.''

Also note that the force applied by the earth on you is now external (since its not a force on yourself by you).

So this external force pushes you upward. Read Newton's laws:

http://en.m.wikipedia.org/wiki/Newton%27s_laws_of_motion

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