# Homework Help: M=rFsinθ Cannot find θ

1. Oct 17, 2015

### Fischercr

• Thread moved to HH from the technical forums, so no HH Template is shown
I failed this quiz, and I'm trying to understand b. I need to calculate the magnitude of force P which is necessary to create the same moment (I think it's torque in English?) about point B, knowing that alpha is 10 degrees. The moment in part a was -80Nm.

I now understand that I should have used the following equation:
M (about point B) = P * r * sin θ
80 Nm = P (0.5m) sin θ

However, I don't understand where theta is. How do I find theta in general for this equation? My TA wrote 120°, but I don't understand how to get this number. My FBD is below, and I wrote my guess at where θ is, but my guess can't be correct. Help me find theta, please.

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2. Oct 17, 2015

### Staff: Mentor

It doesn't matter if you use 60° (which you indicated in the sketch as angle between AB and the force) or 180°-60° = 120°, the sine of the angle is the same in both cases.

3. Oct 17, 2015

### Fischercr

So in the equation
Moment (in French) = F * r * sin θ
θ is the angle between F and r?

4. Oct 17, 2015

### Staff: Mentor

Sure.

Torque is the English word.