Force Couple System: Explained

[noname1]

This is not really a homework question just a review but i wanted someone to explain to me why the forces of the moments negative on Fb, Fc and Fd

thanks in advanced

 

[ehild]

The component of torque of a force F acting at a point r is equal to the vector product of the vector r and the force.
M_x=y F_z -z F_y,
M_y=z F_x -x F_z,
M_z=x F_y -y F_x.

You can also decide the sign from the direction of rotation the force would cause along an axis. Seeing from the positive x, y, or z axis, the moment is positive if it rotates anti-clockwise, and negative, if the rotation is clockwise.

ehild