I About gymnastic spins (moment of inertia related)

1. Dec 22, 2017

Sundown444

Now, if I am not mistaken, a gymnast can rotate around his or her axis while spinning, and they can speed up and slow down their speed of rotation by changing the radius part of their moment of inertia; you know, extending legs and arms outward or bringing them inward. I want to know, however: can a gymnast, or any athlete for this matter, change the speed and direction of their rotation in mid air above the ground without changing their moment of inertia by bringing their body parts in and out? Is it even possible at all? Why or why not?

Last edited: Dec 22, 2017
2. Dec 22, 2017

Staff: Mentor

Also, the last part of your question is hard to parse for me. Have you seen any of the demonstrations of this from the International Space Station? Google should turn up some of those if they help your understanding.

3. Dec 22, 2017

Sundown444

We'll focus on the direction part for now. Say the gymnast jumps into the air through some way and is spinning from top to bottom, like a figure skating spin, as shown here.

http://www.bbc.co.uk/staticarchive/4b53636889498862801a86d82980edbb7fbbb2e1.jpg

. Then the gymnast wants to change direction of rotation by either going counterclockwise from clockwise or going from a vertical axis spin to a cartwheel or somersault. Like these.

http://www.bbc.co.uk/staticarchive/b32276d3c848ccda25101cde595f695360310520.jpg

http://www.bbc.co.uk/staticarchive/a7fbcb49640964157fbd3fe2f2199510374f8926.jpg

My question, would that be possible to do without expanding or contracting moment of inertia of the body (by changing the radius of the MOI)?

EDIT: This is all in mid-air, of course.

EDIT 2: Is it possible at all, I mean?

Last edited: Dec 22, 2017
4. Dec 22, 2017

Sundown444

• Poster has been reminded to wait at least 24 hours before a "bump" post
I don't mean to double post, but is anyone going to answer?

5. Dec 22, 2017

jbriggs444

Is angular momentum a conserved quantity?

6. Dec 22, 2017

Sundown444

Yeah, why? It's always conserved, right?

7. Dec 22, 2017

jbriggs444

So doesn't that say that you cannot change the axis about which a body rotates?

Note that this may not answer your question -- it depends on exactly what your question is.

8. Dec 22, 2017

Sundown444

So you're saying the axis of rotation can't be changed in mid air at all? No going from a spin to a cartwheel and such?

9. Dec 23, 2017

jbriggs444

The axis of rotation can't change. But that does not say that you cannot re-orient your body along a new direction relative to it.

10. Dec 23, 2017

CWatters

You might want to look up the method by which a falling cat turns and lands on their feet. In short... By rotating one part of your body (such as an arm) in one direction you can rotate the rest of your body in the other direction. Spring board divers do it in different planes to perform somersaults and twists.

11. Dec 23, 2017

Sundown444

And this is all in mid-air, right?

12. Dec 23, 2017

jbriggs444

Right. Just as @CWatters suggested in #10.

13. Dec 23, 2017

Sundown444

So, just to make sure, why is it not possible to change the axis of rotation in midair?

14. Dec 23, 2017

jbriggs444

Angular momentum is conserved.

A non-rigid object may not always have a defined axis of rotation. But its angular momentum will be constant in the absence of any external torques.

15. Dec 24, 2017

Sundown444

Okay then. Say, can you explain this bolded part a bit better, please.

16. Dec 24, 2017

jbriggs444

Suppose for instance, you are facing south and spinning in the east/west/up/down plane. Like doing cartwheels. There is nothing that prevents you from contorting yourself and re-orienting your body so that you are facing [momentarily] east spinning head over heals with your right arm pointed south and your left arm pointed north. Like tumbling in a forward roll.

Your axis of rotation has not changed. You are still rotating about a north/south axis. But the orientation of your body has changed so that instead of your front side continually facing south, your right arm continually points south.