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laica
#1
Feb21-10, 04:08 PM
P: 1
1. The problem statement, all variables and given/known data

There are two important conditions for a rigid body to be in equilibrium. The first condition is that the net force on a body must be zero. The second condition is that the net torque on a body must also be zero.


2,3. Relevant equations; attempt at solution.

I apologize for the simple question but I am self-studying physics and having a hard time with this concept.

So my question is: how are these two conditions not redundant? My initial reaction is that net torque must be zero if net force is zero, since [tex]\tau[/tex]=Fr. Then I drew this situation (a door revolving around a central pivot point) that seems to show net torque without net force (the 2 forces add up to zero but there is still rotation). The img is attached to this post.

The problem is, I still think net torque would be zero.
[tex]\tau[/tex]=Fr
[tex]\tau[/tex]=-Fr
[tex]\Sigma\[/tex][tex]\tau[/tex]=(Fr)+(-Fr)=0

Where am I going wrong here? Is one radius -r maybe?

Thanks in advance!
Attached Thumbnails
Force.jpg  
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