A torque is applied to a bolt by hanging a weight w from the end of the wrench, as shown. The coordinate axis along which the torque vector is directed is
A) y.
B) x.
C) -y.
D) -x.
E) z.

Question

A torque is applied to a bolt by hanging a weight w from the end of the wrench, as shown. The coordinate axis along which the torque vector is directed is
A) y. 
B) x. 
C) -y. 
D) -x. 
E) z.

Quizplus · Accepted Answer

The torque vector is directed perpendicular to both the applied force (weight) and the position vector from the pivot point to the point of application of the force. In this case, the weight is hanging vertically downwards (along the -y direction), and the position vector from the pivot point to the point of application of the force (the bolt) is horizontal and points towards the left (-x direction). Therefore, the torque vector is directed out of the screen/into the page, along the +z direction. However, the question is asking for the coordinate axis along which the torque vector is directed, and that would be the axis perpendicular to both the weight (along -y) and the position vector (along -x), which is the +z axis rotated by 90 degrees counterclockwise, which is the -x axis. Therefore, the answer is D: -x.

A Torque Is Applied to a Bolt by Hanging a Weight