Why is the direction of M_0 (r x F) out of the paper?

[goldfish9776]

Homework Statement

I can't understand why the M_0 ( M_0 = r x F)is pointing in the direction out of the paper ?

Homework Equations

The Attempt at a Solution

 

[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.

 

[goldfish9776]

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.

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

 

[tommyxu3]

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

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.