# Can an astronaut rotate herself?

#### physicstudent_B

1. Homework Statement
An astronaut has been left outside the space station, her back towards the station. The astronaut is not rotating (angular momentum ~ L = 0) and cannot reach anything. Is it possible for the astronaut to rotate herself into facing the space station?

2. Homework Equations
Conservation of angular momentum

3. The Attempt at a Solution
I think that the astronaut cannot rotate herself only by moving. So the answer would be no. She could maybe throw something from her backpack (if she has something) to change her angular moment.

Related Introductory Physics Homework Help News on Phys.org

#### gneill

Mentor
Do some research on the "Falling Cat Problem"

#### physicstudent_B

OK, of course the astronaut is not a rigid body ... so she can rotate herself. Thank you!

#### haruspex

Homework Helper
Gold Member
2018 Award
Nearly 50 years ago I failed to convince a visiting Russian physicist that this was possible. I was handicapped by being a mere maths undergrad in a slightly off-axis swivel chair and the worse for vodka. The two historians and the lawyer also present were no help.

#### physicstudent_B

And there was no Youtube 50 years ago.
(look at 5:45)

#### mpresic

Even if the astronaut were a rigid body, he or she could still rotate albeit inefficiently. For example, let the astronaut raised his hands over his (her) head, and consider this the initial orientation. Now if (s)he moved his arm forward, his legs would go backward. When the astronaut moved his arms over his head again, the astronaut would return to the initial orientation. Now if the astronaut moved his arms out to one side, the legs would go to the other side. When the astronaut returned his arms directly over his head, (s)he would again return to the initial orientation.

Now assume the astronaut moved his arms forward so his feet went backward. Now the astronaut moves both arms to one side, and his legs go the the other side. Now the astronaut moves his arms directly over his head again, (in one step; not the two steps (s)he took to get to the side). The astronaut would have his/her arms directly over his head, but (s)he would have rotated. This is because, as is well (should be) known to students of college freshman mechanics, finite rigid body rotations are non-commutative.

As a grad student, I briefly talked to some (other) grad students and they do not buy this argument, but at the same time they could not refute it.

#### haruspex

Homework Helper
Gold Member
2018 Award
let the astronaut raised his hands over his (her) head,
Bit of a struggle for a rigid body.

#### mpresic

to Hauspec, Granted. The astronaut would have to be a collection of coupled (assumed rigid) bodies rigid arms and legs. Maybe the analysis of the falling cat is the best idea.

"Can an astronaut rotate herself?"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving