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-   -   Person/object spinning (http://www.physicsforums.com/showthread.php?t=669823)

nos Feb6-13 11:12 AM

Person/object spinning
 
Hi all,

I was wondering if one on you could answer this question.

Imagine a person standing on a spinning merry go round. The friction exerted by the floor is the centrifugal force that keeps the person spinning in a circle. As the speed is building up, the person needs to lean back more and more because i guess the friction force on his feet would tend to rotate the person about his centre of mass and this effect needs to be canceled by gravity. But the gravity exerts a force in the centre of mass so it cant offset the moment created by the friction. So why does the person need to lean back?

Thanks

nos Feb6-13 11:30 AM

Re: Person/object spinning
 
I just realized, there is a normal force acting on the shoes of the person which could cancel the moment of the friction force

jbriggs444 Feb6-13 11:41 AM

Re: Person/object spinning
 
Quote:

Quote by nos (Post 4259534)
Hi all,

I was wondering if one on you could answer this question.

Imagine a person standing on a spinning merry go round. The friction exerted by the floor is the centrifugal force that keeps the person spinning in a circle. As the speed is building up, the person needs to lean back more and more because i guess the friction force on his feet would tend to rotate the person about his centre of mass and this effect needs to be canceled by gravity. But the gravity exerts a force in the centre of mass so it cant offset the moment created by the friction. So why does the person need to lean back?

The frictional force exerted by the floor on the person's feet is a centripetal force. But that's just terminology.

You are looking at this problem in terms of balancing torques. That is a valid way to proceed. You have chosen to use an axis of rotation at the person's center of mass. That is also a valid way to proceed. You have correctly identified the horizontal frictional force of the merry go round on the person's feet is one torque about this axis. You have correctly realized that gravity does not produce any torque about this axis.

But there is another force being applied to the person. Can you identify that force?

Edit. I see you have realized this yourself.


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