Rotational Equilibrium Problem-Solving
* Choose one object that is in equilibrium to be the system.

* Draw the free-body force diagram for this system. You will need this to calculate the torque for each force since the torque is the product of a force perpendicular to the lever arm times the lever arm distance to the pivot point.

* Choose a pivot point for the axis of rotation. You will need this to determine the lever arm of each force acting on the system.

Since the object does not rotate any point can be the chosen. However, some pivot points make the problem simpler to solve. In particular, choosing the pivot point to be along the line of action of some unknown force will eliminate that force.


* Choose a rotational direction to represent positive rotations. This is arbitrary but need to figure out what sign to put in front of each torque (or to determine which torques are CW and which are CCW). Having the wrong sign on some torque term is one of the most common sources of error.

* Pick at coordinate system so that you can resolve the static force vectors into components. Typically, they might be horizontal and vertical directions.

* Write down the static equilibrium conditions for your choice of coordinate systems.

Net Force = 0
Sum of Force Up
= Sum of the Forces Down
Sun of Force to Right = Sum of Force to Left
or
S Fx= 0 S Fy= 0

* Write down the rotational equilibrium conditions.

Net Torque = 0
Sum of CW Torques = Sum of CCW Torques

or
S t = 0

* If all goes well, you should be able to solve the resulting expressions for any unknowns.