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How does the astronaut in the following video do what he is doing ? https://youtu.be/7ZPVg3qD07g?t=145

This question is kind of a follow up to Why don't we ever see astronauts on space walks adjusting their orientation by rotating their arms?

I was under the impression that linear motion and rotational motion were analogous with displacement, velocity, acceleration, mass, momentum, force being analogous with angular displacement, angular velocity, angular acceleration, moment of inertia, angular momentum and torque respectively.

We all know that an astronaut in space cannot undergo any linear displacement without having some external force ( the most he can do is throw objects, or gases in one direction and move in opposite direction , but even then the Center of mass of the man + object system will stay right where it was ) . In short, the center of mass should wind up back where it started.

So, how come in case of rotational motion, the astronaut is able to undergo angular displacement without any external torque ? He is facing one way and at the end he has rotated and is facing another way, without there being any external torque during the process. What is the physics i am getting wrong here ? Whatever he does, should he not wind up back where he started ?

silverrahul
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When the astronaut first begins spinning his arms around he applies force to them to begin their spin. So according to Newton's third law of motion the arms apply an equal and opposite force on his body spinning it in the opposite direction. As long as he keeps rotating his arms in one direction his body keeps rotating in the opposite direction. When he stops rotating his arms the force used to stop their spin causes an equal and opposite force to stop the spin of his body. If he stops spinning his arms when he is facing away from where he was when he started, that is where he will stop. However his center of mass will not have translated except for possibly from some air resistance from the motions.

Adrian Howard
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