A single force acts on a particle situated on the positive x axis.The torque about the origin is in the negative z direction.The force might be:
A)in the positive y direction
B)in the negative y direction
C)in the positive x direction
D)in the negative x direction
E)in the positive z direction

Question

Quizplus · Accepted Answer

The torque about the origin is given by the cross product of the position vector and the force: 
$\vec{	au} = \vec{r} 	imes \vec{F}$ 
Since the torque is in the negative z direction, this means that the force and position vector must be perpendicular to the z-axis. The only force that satisfies this condition is a force in the negative y direction, since the cross product of any vector in the positive x direction with a vector in the negative y direction will be in the negative z direction. Therefore, the force must be in the negative y direction.

A Single Force Acts on a Particle Situated on the Positive