Rotational Equilibrium and Normal Forces

[chemica1mage]

Hi,

The question I have is not for a numerical answer but for clarification.

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]

welcome to pf!

hi chemica1mage! welcome to pf!

… 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?

 

[chemica1mage]

Maybe that picture wasn't the best to demonstrate my problem. Here's another.

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]

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.

 

[chemica1mage]

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