When is net torque equal to 0?

[jaded18]

when is net torque equal to 0??

Each of the four bars shown can rotate freely in the horizontal plane about its left end. For which diagrams is the net torque equal to zero?
http://session.masteringphysics.com/problemAsset/1003642/26/170580B.jpg
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i know that for torque to equal O angular momentum = I(omega) is constant and conserved... but I'm having trouble applying this to the diagram ... i thought D would be part of the answer because the Newtons all cancel out ... help??

 

[Doc Al]

Whether the force cancels out doesn't matter. Just add up all the torques from each force to get the total.

 

[jaded18]

anyone else can help me? i need more to go on...

 

[Doc Al]

Do you know how to calculate the torque from each force? Since the forces are all perpendicular to the distance (r) it's simply T = rF. (Be sure to give clockwise and counterclockwise torques different signs.)

 

[jaded18]

let's do it this way. look at diagram a. if torque = rF then torque for the four forces from left to right is 0, 6(L/4), 1(3L/4), and 7(L)?? maybe if i know how to solve for one i can do it on my own

 

[Doc Al]

I don't know where you got the 6 N force, I see 8 N. Maybe a typo?

Here's how I would do it. First, I'll call counter-clockwise torques positive and clockwise torques negative. Here's what I get for each force in A:
0 + 8(L/4) - 1(3L/4) - 7(L) = 2L -3L/4 - 7L = -5.75L (which is sure not zero. Next!)

 

[jaded18]

thanks :)