Check my static equilibrium diagrams

[bananan]

Hi physics fans,

I've made four diagrams for inclusion in a physics lab report. They depict situations in which a meter stick is in static equilibrium (no net torque, no net force). My professor mentioned to me that my analysis of the forces on the stick is not correct, but was not more specific than that.

Can you take a look at my diagrams and help me figure out where my error is? Do I need to show a force vector pointing towards the Earth for each weight attached to the meter stick? I thought it was correct to consolidate all of them into one (since the center of mass of the system has to be over the pivot point for static equilibrium to exist) Links to my diagrams are below:

http://img325.imageshack.us/img325/9105/physlab8apparatus1sn5.th.png
http://img261.imageshack.us/img261/3782/physlab8apparatus2fk8.th.png

 

[dragginwagon406]

To me, the diagrams are confusing for the reason you stated.

"Do I need to show a force vector pointing towards the Earth for each weight attached to the meter stick?"

So, yes. The diagrams look easy on the eyes, but is the professor looking for more of a plain free body diagram?

Maybe add to the pivot point that sum of the moments about the pivot point is equal to zero. EMpivot=0

 

[bananan]

Well, the trouble with plain free body diagrams is that they don't show both torques and forces. I think the professor is ok with the format of the diagrams, but your advice about adding individual force vectors to the weights is well taken.

One other question: in diagram four, does there need to be a torque vector pointing "out of the page" on the left side of the pivot point? I believe I need to add this, since (W-left side of ruler) * (moment arm from pivot to left side of ruler center of mass) is > 0.

 

[dragginwagon406]

I would also show the mass and location of the ruler independently from the weights because a condition of equilibium cannot be seen as depicted.