A force, F = (3x + 4y) N,

is applied to an object at a point whose position vector with respect to the pivot point is
r = (3x + 3y + 3z) m.
Calculate the torque created by the force about that pivot point.

I know that torque = -12x + 9y + 3z, but why is -12x negative? Thank you in advance.

1 answer

Because Torque is a cross product of radius vector(r) and Force vector(F).
If r =Ax+By+Cz & F = ax+by+cz (where x,y and z are unit vectors in x,y and z directions) then
x-component of torque Tx is :
Tx = (Bc-Cb)
= 3*0-3*4
= -12
Therefore x-comp. of the Torque is nagative (-12)