Question

An extended body (not shown in the figure) has its center of mass at the origin of the reference frame. In the case below give the direction for the torque τ with respect to the center of mass of the body due to force F acting on the body at a location indicated by the vector r.

Options are:
-Y
x
Z
-Z

Answer 1

+ X axis.

The equation for torque is:
$\tau = r \times F$

Torque is the CROSS-PRODUCT of 'r' and 'F'.

We can use the right-hand rule to determine the direction of torque.

Looking at the diagram, with an open hand, point your fingers towards the top of the picture (Along the +Z axis in the direction of the 'r' vector.)

Then, curl your fingers in the direction of the force vector, (along the -Y axis).

Your thumb will stick out in the direction of the torque, or the +X axis.

You can also just eliminate any options that involve axes that have already been used for 'r' and 'F'. In this case, there are components in the Z and Y axis, so this leaves the 'X' axis to be considered.