Real life physics question - gymnastics

[calcbandit]

I am a junior mechanical engineering major and consider myself decent at understanding forces and moments and why things move.

I also used to do gymnastics, competitively, for 10 years.

I do not understand the physics behind twisting. I could do it myself, gymnasts are instructed to pull their hands upwards and to the side during a flip to acheive a twist.

Since the gymnast starts the flip with no angular momentum about the twisting axis (from bottom of feet through head), and end up twisting, some moment must have been applied to them to provide this angular momentum.

At first, I thought that this moment was applied from the floor mat as they took off for the flip, or perhaps from the high bar as they let go. However, this does not make sense, since it is possible and commonly done to do 2 flips in the air, the first one with no twist and the second one with twist (this is called a back in, full out). How did they suddently attain angular momentum while in the air between the first and second flips?

[alphatmw]

i would think whatever muscles are contracting for them to start the turn applies the necessary force to do so.

[Borodog]

It's called a zero angular momentum turn. It involves changing the moment of inertia. Imagine sitting in a frictionless office chair, holding a barbell in each hand.

Extend the right barbell out on front of you at arm's length, the left, straight out to your left. Rotate both to the right, so that the left is straight out in front of you, and the right is straight out to your right. Because of conservation of angular momentum, this will cause your body to rotate. Bring both barbells straight back in towards your body until they are tight against you, and then extend them back in the original directions. Repeat. You can keep turning indefinitely without ever changing the total angular momentum of the system.

[Skidoo]

Maybe you are converting pitch to roll through a torque of some sort.

[calcbandit]

[ QUOTE ]
It's called a zero angular momentum turn. It involves changing the moment of inertia. Imagine sitting in a frictionless office chair, holding a barbell in each hand.

Extend the right barbell out on front of you at arm's length, the left, straight out to your left. Rotate both to the right, so that the left is straight out in front of you, and the right is straight out to your right. Because of conservation of angular momentum, this will cause your body to rotate. Bring both barbells straight back in towards your body until they are tight against you, and then extend them back in the original directions. Repeat. You can keep turning indefinitely without ever changing the total angular momentum of the system.

[/ QUOTE ]

Interesting, thanks.

But, I don't think this is quite it for gymnasts. The way you described, moving the weights countercclockwise while they are extended will twist your body clockwise. Moving the weights back to the initial position will rotate your body counterclockwise, but to a lesser extent than it twisted clockwise, since you arent holding the weights out as far. Thus it is analgous to taking a large step forward followed by a small step backward.

Not only does this small step phenomenon not occur during gymnastics, but twisting gymnasts twist pull their hands up and to the left and twist in that same direction, not the opposite direction as in the barbell example

Does anybody think it has to do with drag forces from the air?

[Borodog]

No, it has absolutely nothing to do with drag forces from the air. It really is a zero angular momentum turn, I promise. You said the gymnast throws their arms up and to the side, yes? That changes the body's moment of inertia. Remember that while the total angular momentum cannot change, since there are no external forces to exert torques, that does not mean that internal forces cannot exert internal torques, such that one portion of the body gains angular momentum in one direction while another portion gains angular momentum in the opposite direction. My guess, without knowing anything about it, is that to "stick" the landing, the gymnast must somehow "undo" the move, by pulling the arms down and back to center.

[calcbandit]

[ QUOTE ]
No, it has absolutely nothing to do with drag forces from the air. It really is a zero angular momentum turn, I promise. You said the gymnast throws their arms up and to the side, yes? That changes the body's moment of inertia. Remember that while the total angular momentum cannot change, since there are no external forces to exert torques, that does not mean that internal forces cannot exert internal torques, such that one portion of the body gains angular momentum in one direction while another portion gains angular momentum in the opposite direction. My guess, without knowing anything about it, is that to "stick" the landing, the gymnast must somehow "undo" the move, by pulling the arms down and back to center.

[/ QUOTE ]

I see what you are saying, but the entire body rotates during a twist, not just most of it, with a small part rotating the other way to keep angular momentum constant.

I agree that the drag forces thing is kind of ridiculous.

EDIT
=====

Thinking back to my gymnastics days, what you say almost certainly has something to do with it. However, there is still the problem that the gymnasts rotate in the direction of the arm throw, not in the opposite direction as your theory suggests.

Right before twisting begins, gymnasts almost always have their hands straight to the side, and then pull them in and to the side.

[Skidoo]

It could be that when you throw your arms to one side it creates a lever arm which leads to a torque under gravity to cause rotation about your main axis.

[arahant]

I'm no Boro, but I am handy with google.
This excellent article (http://www.usa-gymnastics.org/publications/technique/1997/2/twisting.html) should address your question nicely.

Edit: Good question. Very educational..thanks

[calcbandit]

[ QUOTE ]
I'm no Boro, but I am handy with google.
This excellent article (http://www.usa-gymnastics.org/publications/technique/1997/2/twisting.html) should address your question nicely.

Edit: Good question. Very educational..thanks

[/ QUOTE ]

Thank you! I'm glad I'm not retarded for having been confused by this.

[Borodog]

Exactly! I was going to post some excerpts from a book I have called Physics and the Art of Dance, which addresses mid-air zero angular momentum rotations, although not particularly twists, but that article is fantastic.