# What variables are needed to find a moment?

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1. Sep 5, 2016

### Alex Katko

1. The problem statement, all variables and given/known data
You wish to find the moment at the origin produced by a force, ̅F= {Fi}̂, that is applied at r= {̂ai+bj +ck ̂}. Which variables (F,a ,b,c , ) do we need to know to find a numerical answer? Explain using math and figures.
2. Relevant equations
Mo = r x F

3. The attempt at a solution
I know that the above equation is used to find a moment. Therefore, to me it makes sense that all variables (a,b,c,F) are needed unless you were given some sort of extra information (coordinate direction angles). Needing all the variables seems to be too simple. Any advice or hints are appreciated!

2. Sep 5, 2016

### haruspex

Perform the cross product algebra and see what happens.

3. Sep 5, 2016

### Alex Katko

Performing the cross product gives me: (bFz-cFy)i + (cFx - aFz)j + (aFy - bFx)k. All variables (the 3 components of F and a,b,c show up).

4. Sep 5, 2016

### haruspex

You are given $\vec F=F\hat i$. So what are Fx, Fy and Fz?

5. Sep 5, 2016

### Alex Katko

Ohh i think i am understanding. Fx = Fi. Fy = 0 Fz = 0

6. Sep 5, 2016

### haruspex

Almost. Fx=F, a scalar. There is no variable Fi.

7. Sep 5, 2016

### Alex Katko

If Fy and Fz are both zero, then the croos product becomes cFxi - bFxj. Therefore, the variables needed are F,c, and b

8. Sep 6, 2016

Yes.