I guess when I'm moving my hands around, the electrostatic potential stored in the chemicals in my body is converted to motion of my arms?

But what about momentum conservation? I think the brain sends Electrical signals to my arm, which probably further use the potential energy stored in my muscles' chemicals to make my hands move.

If, at the fundamental level, electrostatic repulsion is all that's causing the movement, then the electrons in my body which apply repulsion force to my muscles (to make them move) must have gained momentum equal to my muscles in the opposite direction? Is this where the momentum goes? To the floating electrons inside my body? But still those tiny atoms inside my body, which gained this extra momentum in the opposite direction of the muscle movement, can't keep floating around in my with that extra momentum forever, right? I mean.. the body has a boundary. The atoms with this newly gained momentum will interact with the other organs, further passing on the momentum. And my body will be destined to hold this extra momentum forever?

The fact that living things can freely move their organs around around at free will really puzzles me. Can some explain living body physics to me? How can they just freely "create motion" while also adhering to physical laws like momentum conservation?

  • 2
    $\begingroup$ A clock is also moving its hands. Why do you only find 'living things' puzzling? Energy stored in it is programmed to be transformed to a different type of energy, just like in our bodies. $\endgroup$ Sep 2, 2020 at 11:08
  • $\begingroup$ Are you asking why if I move my arm then it arrives at a point in which it stops, although from a different perspective? Are you considering that our motions are dissipative processes? It seems you treat this as the collision of ideal rigid bodies. $\endgroup$
    – Alchimista
    Sep 2, 2020 at 11:35

2 Answers 2


But what about momentum conservation?

Momentum conservation doesn't apply to your body, because your body is not an isolated system. Your body is in contact with the ground, and therefore there are also forces acting from your body to the earth and vice versa. Therefore, when you move a part of your body, you also change the momentum of the earth.

Quoted from Momentum conservation:

In physics and chemistry, the law of conservation of momentum (or the law of conservation of linear momentum) states that the momentum of an isolated system remains constant.

Therefore momentum conservation applies only to whole system (earth + your body + other bodies). Well, this is still an approximation, because it neglects the moon and the sun. But you get the idea.

  • $\begingroup$ But astronauts are also able to freely move around. When they move their hands around, what part of their body gains the equal and opposite momentum? Is it the free atoms floating in their body? Are their bodies, as a whole, destined to carry this momentum forever? Wouldn't their bodies eventually rip apart when they move their hands because of momentum conservation? $\endgroup$
    – Ryder Rude
    Sep 2, 2020 at 12:30
  • 3
    $\begingroup$ @RyderRude, Nobody said the momentum of any single thing (e.g., your hand) has to be conserved. "Conservation" means that if the momentum of your hand changes, then the combined momentum of one or more other things must change by an equal and opposite amount. If you can account for the combined momentum of everything that possibly could be affected (i.e., of an isolated system) then the total momentum of the system will not have changed. An astronaut on a spacewalk is "isolated." They move their hand one way, and their body moves the opposite way. Their total momentum is conserved. $\endgroup$ Sep 2, 2020 at 12:44

In addition to Thomas' answer about the enlarged isolated system, one should include the infrared radiation (heat) produced by the friction induced by the moving muscles.

One gets very hot during muscle exercises . That radiation also takes away momentum.

  • $\begingroup$ There is indeed dissipation during muscle action, but it is opposing motion so the associated forces are of the same sign as inertial forces generated in reaction to muscle action. $\endgroup$ Sep 25, 2020 at 21:16

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