Physics
posted by Steve .
A sign is supported at its top right corner, point P. The sign is a square with 40 cm on each side and has 8.0 kg mass. What is the magnitude of the horizontal force that P experiences?

If the sign is supported only at P, it requires a moment connection and there is no horizontal force.
Assume that the sign is supported at P and the bottom of the same side (40 cm down) is butted against the support.
Take moments:
mg * (40/2) = P*40
Solve for P. 
Where did the 40/2 come from?

40/2 Came from Steve's answer, get with it.