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-   -   Rotational Equilibrium and Normal Forces (http://www.physicsforums.com/showthread.php?t=669519)

 chemica1mage Feb5-13 12:41 AM

Rotational Equilibrium and Normal Forces

1 Attachment(s)
Hi,

The question I have is not for a numerical answer but for clarification.
Attachment 55386
Some of the questions involving torque/rotational equilibrium describe a person standing on a plank. I know that the gravitational force of the person on the plank needs to be considered for translational and rotational equilibrium. My question is, why don't I consider the normal force exerted by the board on the person as a torque-producing force? (Your typical dynamics situation where F(normal) = F(gravity).) Or is it because the "normal force" is distributed between the two supports.

Any help on this question is greatly appreciated!

 tiny-tim Feb5-13 05:47 PM

welcome to pf!

hi chemica1mage! welcome to pf! :smile:
Quote:
 Quote by chemica1mage (Post 4257753) … why don't I consider the normal force exerted by the board on the person as a torque-producing force? (Your typical dynamics situation where F(normal) = F(gravity).) Or is it because the "normal force" is distributed between the two supports.
by the board on the person? but the person is effectively a point object … where does torque come into it? :confused:

 chemica1mage Feb5-13 09:26 PM

Re: Rotational Equilibrium and Normal Forces

1 Attachment(s)
Maybe that picture wasn't the best to demonstrate my problem. Here's another.
Attachment 55411
Why is it that for the diagram on the right (person standing on the floor), you can consider the normal force of the person, but with the diagram on the left (person on a see-saw), you don't consider the "normal force" of board pushing up on the person? Or is it because that support force gets lumped into the support on the fulcrum?
I hope that it makes a little more sense, what I'm trying to ask. Thanks in advance!

 AlephZero Feb5-13 09:37 PM

Re: Rotational Equilibrium and Normal Forces

Quote:
 Quote by chemica1mage (Post 4258842) Maybe that picture wasn't the best to demonstrate my problem. Here's another. Attachment 55411
The left hand diagram shows all the forces acting on the board.

The right hand diagram shows all the forces acting on the person.

 chemica1mage Feb5-13 11:21 PM

Re: Rotational Equilibrium and Normal Forces

Thanks! That really clears things up for me. Seems so obvious, now that you say that.

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