A uniform plank of wood of mass 32 kg and length 4.0 m is used to cross a ditch. In the ditch is a rock, which is used to support the plank horizontally 0.80 m from one end. The other end is supported by the bank. Calculate the rock's supporting force.

We need to calculate moments about a point through which one of the forces is acting. I choose the bank support point. At this point, 

moments are caused by the weight of the plank acting at its centre of mass (middle of the plank) which is 2m away, and by the rock reaction force F which is 4 - 0.8 = 3.2m away. These two moments are equal for a still plank:

Using the moment formula (force*moment arm):

F3.2 = 329.81*2   =>  F = 196.2