Moment of specified axis

1. Sep 17, 2015

goldfish9776

1. The problem statement, all variables and given/known data
I cant understand why the M_0 ( M_0 = r x F)is pointing in the direction out of the paper ?

2. Relevant equations

3. The attempt at a solution

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2. Sep 17, 2015

tommyxu3

Based on the definition if torque:
$$M=r\times F$$
you can tell tell direction of torque by that of your position and the force.
The reference point the figure decided is important, which will influence the direction of the position hence torque also.

3. Sep 17, 2015

goldfish9776

do you mean the direction of moment can be anywhere as long as it is perpendicular to the plane formed by r and F ?

4. Sep 20, 2015

tommyxu3

I think that is correct, for the choice of the origin's position depends on you and the direction of the vector of the position $\vec{r}$ depends on the observer.