# Astronaut Backflip

#### jaldenpage

This isn't a homework problem or anything, it's just something I'm in a debate with someone about. Theoretically, if an astronaut were left stranded in the center of a room without the ability to push off of anything and without any means of propulsion -- would the astronaut be able to do a 360 degree backflip (or forward flip)?

Does this movement require gravity, momentum, or the ability to push off of another mass?

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#### cjl

I would say that they absolutely could do a flip. They would just need to windmill their arms, and due to conservation of angular momentum, their body would rotate in the opposite direction from their arms.

#### jaldenpage

If a human can do this rotation, does this mean that in theory a spaceship that is moving but not accelerating could choose to start rotating simply by moving its parts correctly? I'm talking about a rotation with little to no effect on direction and without using thrust.

#### Andy Resnick

This isn't a homework problem or anything, it's just something I'm in a debate with someone about. Theoretically, if an astronaut were left stranded in the center of a room without the ability to push off of anything and without any means of propulsion -- would the astronaut be able to do a 360 degree backflip (or forward flip)?

Does this movement require gravity, momentum, or the ability to push off of another mass?
I'm also going to say yes, because cats are able to right themselves in free-fall. Whether or not an astronaut is flexible enough to copy the motion, I can't say.

http://helix.gatech.edu/Classes/ME3760/1998Q3/Projects/Nguyen/

The site specifically mentions that humans have so far been unable to copy the motion.

http://web.archive.org/web/19980528234354/http://www.usa-gymnastics.org/publications/technique/1997/2/twisting.html

Seems to indicate there is a difference between twisting and flipping as well.

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#### Drakkith

Staff Emeritus
2018 Award

If a human can do this rotation, does this mean that in theory a spaceship that is moving but not accelerating could choose to start rotating simply by moving its parts correctly? I'm talking about a rotation with little to no effect on direction and without using thrust.
Yes if the spaceship were designed to be able to do this. If the ship was one ring inside another ring and could rotate, then the act of rotating would cause the outer ring to spin opposite of the inner ring.

#### cjl

If a human can do this rotation, does this mean that in theory a spaceship that is moving but not accelerating could choose to start rotating simply by moving its parts correctly? I'm talking about a rotation with little to no effect on direction and without using thrust.
Yep. See "reaction wheels" and "control moment gyroscopes" for details. Many satellites use these techniques for attitude control, since they don't need any fuel.

#### pallidin

I wonder if a cat, already knowing it is fully or partially inverted prior to fall, does not start the twist prior to drop/release, thus using the static holder as a twist initiation.

#### Dr Lots-o'watts

A very good video of a falling cat. It's clear there is no need to push off the static holder.

It's all in the way it alternatively adjusts the lengths of its legs:

1. Bend forward legs + extend backward legs simultaneously,
2. Twist waist half-way : this right-sides up the torso and head,
3. Bend rear legs + extend forward legs,
4. Twist waist half-way : this right-sides up the butt.

It's all there is to it.

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#### Andy Resnick

Excellent video! the cat looks like it's counter-rotating about the spine... too cool.

#### Borek

Mentor
If a human can do this rotation, does this mean that in theory a spaceship that is moving but not accelerating could choose to start rotating simply by moving its parts correctly? I'm talking about a rotation with little to no effect on direction and without using thrust.
That's what Hubble does if memory serves me well.

Whether or not an astronaut is flexible enough to copy the motion, I can't say.
I don't think you need to be specially flexible, seems to me that just rotating your arms should do the trick.

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