Moment double pivot - What are the normal contact forces at P and Q?

Answer: Option A

Firstly, you need to know that at both pivot P and Q, there are normal contact forces acting upwards on the plank.

2 conditions for the plank to be in equilibrium:

1) Sum of clockwise moment = Sum of anticlockwise moment (POM)

2) Net force = 0N (i.e. total upward forces = total downward forces)

To find the value of the normal force, you can choose to take pivot about either P or Q. If you take moment about P, the normal force at P will not be in the Principle of Moments equation Ma = Mc as the line of action of the normal contact forces at P passes through the pivot P, there is no perperdicular distance, hence no moment created. We can determine the value of the normal contact forces at Q.

Once normal force at Q is found, you can use the concept net force = 0N to find the normal force at P straight away, which is faster.