# How can astronauts rotate their body in empty space ? How do they undergo "angular displacement" without any external torque? [duplicate]

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 ?

• Angular speed or angular displacement doesn't require any torque until angular speed is constant. Only change in angular speed requires torque. Apr 23 at 7:10
• @JohnRennie Thanks for that link. I would never have found that question. The question seems to be same as mine, and it goes to the root of the issue much more precisely and eloquently . I will look at the answers, hopefully they will have good answers Apr 23 at 7:18
• @VikashKumar I know it does not. My question was WHY it is the case, that linear displacement requires external force, but angular displacement does not require external torque Apr 23 at 7:20
• Again, linear displacement or linear velocity doesn't require any force until velocity is constant (both magnitude and direction). External forces requires only when change in velocity is needed. Apr 23 at 7:24
• @VikashKumar " External forces requires only when change in velocity is needed " This is not true. External forces are also required when linear displacement (or change in linear/translational position) is needed . However, external torque is not required when " angular displacement " is needed. This difference in linear displacement and angular displacement was the heart of my question Apr 23 at 7:27