# Net Torque

1. Apr 16, 2005

### physicsCU

I have a circle, multiple forces applied, I need to calculate net torque. I have tried it 4 times, no idea on what to do now.

http://session.masteringphysics.com/problemAsset/1000963/4/knight_Figure_13_14.jpg

That is the picture of the problem. Let me know if it doesn't work.

Thanks.

I think the answer might be -.06, but I am not sure.

Last edited: Apr 16, 2005
2. Apr 16, 2005

### quasar987

I have -0.94, provided the z axis points outward the computer screen.

By the way, what is the unit of torque?

$$\vec{\tau} = \vec{r}\times \vec{F}$$

So it's N*m = J !!

I realize that the N*m of torque is not the same kind of N*m as that the one involved in work, for exemple. The first speaks of Force exerted AT a distance, the second of a force exerted ON a distance. But the units remain N*m, and that's Joules! I'm thrown down my chair.

Last edited: Apr 17, 2005
3. Apr 16, 2005

### physicsCU

Torque is in N*m

4. Apr 17, 2005

### quasar987

But N*m is Joules. Don't you find that strange? I find that troubling. :grumpy:

I simply calculated all the torques and added them together:

-.10*30 + .5*30*sin(45°) + .5*20

5. Apr 17, 2005

### physicsCU

I have accepted it, no worries.

But what about that force that is off the circle in the third quadrant? Do you ignore that, or what?

6. Apr 17, 2005

### whozum

That last vector isnt doing any work to turn the circle, so its got no effect. Remember Torque is a cross prodcut between Force and Radius, the force is parallel to the radius, in which case sin(0)=0 and the vector applies no torque.

7. Apr 17, 2005

### Theelectricchild

Quasar makes an interesting point however--- the concept of torque being measured in units of Nm ---> Joules? Energy???

It's best to keep your units of moments around a point in Nm, just as when one calculates the complex power for ckt elements under an sinusoidal steady state voltage or current... the real power (avg power) is measured in Watts, whereas the imaginary reactive power stays as VA (volt amps), rather than watts.

8. Apr 17, 2005

### whozum

I read an article about that. I dont remember what the explanation was, but they said it made sense.

9. Apr 17, 2005

### Theelectricchild

you might be correct.

10. Apr 18, 2005

### al_201314

Anyway just to clarify, Torque is the turning effect, although it has the same units as Joules it's not energy, so it msut never be written in J, but N m.