Rotational Equilibrium and Normal Forces

AI Thread Summary
The discussion centers on understanding why the normal force exerted by a plank on a person is not considered a torque-producing force in certain scenarios. It highlights that while the gravitational force acting on the person is crucial for both translational and rotational equilibrium, the normal force is often distributed between supports, which affects its role in torque calculations. The conversation also clarifies that when analyzing forces, the perspective of whether the person is treated as a point object or part of a system influences the consideration of normal forces. Ultimately, the distinction between different diagrams helps clarify the application of these forces in rotational equilibrium contexts. This understanding is essential for accurately analyzing dynamics situations involving torque.
chemica1mage
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Hi,

The question I have is not for a numerical answer but for clarification.
Torque Normal Force Question.JPG

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!
 
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welcome to pf!

hi chemica1mage! welcome to pf! :smile:
chemica1mage said:
… 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:
 
Maybe that picture wasn't the best to demonstrate my problem. Here's another.
Torque Question.jpg

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!
 
chemica1mage said:
Maybe that picture wasn't the best to demonstrate my problem. Here's another.
View 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.
 
Thanks! That really clears things up for me. Seems so obvious, now that you say that.
 

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