# Does bending your arm in space require any energy?

Since your are weightless in space, your arm has no weight, right? Does this mean that bending it in space requires no energy? Why or why not?

• I’m voting to close this question because it fit better in the biology SE site. Commented Aug 13, 2020 at 16:54

The short answer is yes, bending your arm in the weightlessness of space still requires energy. You are correct that bending your arm does not require us to overcome the weight of your arm, but we do have to overcome its inertia. Inertia refers to the sluggishness that massive objects have (even in the weightlessness of outer space) given how much matter they have.

A bit of explanation to clarify things: we could just as well ask does it take any energy to push a 100,000 lbs. asteroid a distance of five feet while in the weightlessness of space?

I'm sure you'd agree that the asteroid doesn't weigh anything at all in outer space. But that doesn't mean you can just float up and thump it with your finger and expect it to take off at near light-speed. You still have to push on it (and expend energy in doing so) to overcome its inertia.

The simple summary of all of this is that when we try to move objects in outer space, we don't have to fight against any gravitational weight, but we still have to overcome the basic sluggishness that massive objects have (i.e. inertia).

• "You still have to push on it (and expend energy in doing so) to overcome its inertia." I could just touch the asteroid to give a very small, but non zero acceleration, and then after years the asteroid might cover a 5 feet distance. So theoretically, I can do as less work as possible, which means I can even do infinitesimal work to move the asteroid.
– user258881
Commented May 10, 2020 at 6:07
• @FakeMod-Inactiveaccount - If we want to bend our arm in space before we run out of life support, we should try to do it with finite speed.
– Pere
Commented May 10, 2020 at 13:35
• So you need energy to make it move, and once it is moving, there is hardly any resistance. Commented May 10, 2020 at 17:01
• Then you need energy to make it stop. And also because your body is not solid, but made of moving muscles there is the energy expended by the different muscle groups elogating and contracting. And also the hydraulic stuff happening with your bodily fluids will have some effects. Commented May 10, 2020 at 17:03
• This will be a lot simpler if you used something that is not basically a balloon filled with fluids containing very complex hydraulic/mechanical/chemical and otherwise actuated clockwork. Commented May 10, 2020 at 17:09

Well, yes.

Movement of your body parts (hands, legs eyelids, etc.) occurs due to the contraction of muscle fibers. This process requires energy (from cleavage of ATP molecule to form ADP). This is the only way an astronaut can move his arm.

Transforming the internal energy (chemical) into mechanical energy requires the expenditure of ATP. So, the answer is yes.

But, you can make it smaller, I guess.

The arm has mass, even if it is weightless due to being in microgravity.

And force = mass • acceleration and work = force • distance.

For a mass to move, work has been done. Weight doesn’t play into this calculation at all.

1. An astronaut in a spaseship (air around):

Body joints and adjacent body tissues still have non-zero friction.

So do clothes.

Air has some viscosity.

Every body part has some inertia. You need to accelerate it and then decelerate it near the new position. Muscles are especially bad at deceleration - they use more or less the same energy as in acceleration ("static muscle work"). (A well-designed electric machine would get back a great deal of energy used for the acceleration when decelerate.)

Muscles themselves work in pairs at every joint. When one contracts, the other elongates and the one that elongates consumes some energy from the other that contracts (see "muscle tonus").

1. An astronaut in a spacesuit: all of the above + compressing or decompressing the air when moving the joints.

One can imagine a (very simplified) spacesuit as an inflated baloon that has its own form. Every movement of the astronaut changes the form and the baloon reacts rather forcefuly against these changes.

• "One can imagine a (very simplified) spacesuit as an inflated baloon" - if I understand correctly, modern spacesuits are more of skin-tight uniforms rather than air-pressurized enclosures. I do suppose the helmets will stay as pressurized bubbles, but they don't have to bend so no issue there either. Commented May 10, 2020 at 13:07
• That's why I wrote "very simplified". Commented May 10, 2020 at 13:14
• This should be the accepted answer as it mentions a lot of the processes where the energy goes that is needed to bend your arm in space. Commented May 10, 2020 at 15:47

There is friction between tissues in the inner structure of the it, so yes, even that you put a very small angular velocity (near to zero, whose energetic cost will approach zero), your arm will stop after this friction turns to inner energy of blood and tissues.

• Even if there's no internal friction some amount of energy needs to be spent. I think version 1 of your answer seems to suggest in the absence of friction we don't need any energy to move our hands in free space which is not true. Commented May 10, 2020 at 4:47
• You have to think on the grounds that, with no energy, no movement would happens, beginning from nothing. With very little energy (you can say E->0), the movement begins and continues ad eternum, but this is not true for the arm since there is friction. Commented May 11, 2020 at 5:05

When your arm is in motion it has rotational kinetic energy. Since it was, at a previous time, not in motion, some external energy source must have inputted energy, due to the conservation law.