Question

Is there a torque acting on it, and if so in what direction?

A. Yes, vertically
B. Yes, horizontally
C. Yes, at $20^o$ to the horizontal
D. no

Answer 1

Torque (sometimes referred to as Moment of Force in questions) is force which causes a body to rotate about an internal or external axis. Torque is given by the formula:
\vec \tau = \overset{\lower0.5em\hbox{\smash{\scriptscriptstyle\rightharpoonup}}} {r} \times \vec F
Where,
$\tau$ is Torque acting on the body
F is the Force applied on the body
r is the distance of the acting Force from its axis of rotation

Complete step by step solution:
Torque is given by the formula:
\vec \tau = \overset{\lower0.5em\hbox{\smash{\scriptscriptstyle\rightharpoonup}}} {r} \times \vec F
Where,
$\tau$ is Torque acting on the body
F is the Force applied on the body
r is the distance of the acting Force from its axis of rotation
Here we need to just concentrate on the rotating disk. The shaft tilted at an angle of 20o degrees is given in the diagram just to confuse us.

In the above diagram, we have removed the shaft to eliminate confusion. Now, we can see that torque is acting in the horizontal direction because the disk is seen rotating in the horizontal axis (Since the axis of rotation is perpendicular to the X – Y Plane (i.e. the Z – Axis)).
Hence Option (B) is correct.

Note:
In such types of questions, we must only concentrate on the relevant parts of the question. We can see clearly in the diagram that the disk is rotating in the x – y plane. Yet the figure had a tilted shaft to trick students into believing that the disk is rotating in at an angle of $20^o$ degrees from the horizontal.