# Homework Help: Calculating net torque on a plate

1. Mar 18, 2015

### tdashrom

1. The problem statement, all variables and given/known data

I'm supposed to be finding the net torque on a square plate. The sides are 1.7 m in length, with a force acting on each corner (30 N top left pointed right, 60 N top right pointed down, 20 N bottom left pointed up, and 40 N bottom right at an angle of 37 degrees). Just for simplicity I have included the picture of the question, and WebAssign has given the answer.

2. Relevant equations

τ=Frsin(Θ)

3. The attempt at a solution

I understand how to get part b from the solution in part a. However, I'm completely stuck on how the answer in part a is reached.

The radius in this case should be 1/2 of the side length, so 0.85 m. The 20, 30, and 60 N forces should all have negative torque values because they are turning clockwise, and the 40 N force will be positive. I have tried -.85*(20+30+60)+(40*sin(37)*.85), which does not give the correct answer. If you multiply the three CW forces by sin(45) you get -45.6, which while close is not accepted by the system. I've reworked this 10 times multiple different ways, but still to no avail. Where am I going wrong?

2. Mar 19, 2015

### haruspex

Always double check whether it should be sin or cos.

Btw, the question should make clear it is the torque about the plate's centre that is required. Since there is a net linear force, the torque depends on the axis.

3. Mar 19, 2015

### SteamKing

Staff Emeritus
You have assumed that the force of 40N @ 37° is resolved into only one component which produces a torque around the center of the plate. Is this what really happens?

4. Mar 19, 2015

### haruspex

Yes, that's a better response, thankyou.

5. Mar 19, 2015

### tdashrom

Got it, thanks!