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View the entire bridge as the system, and find the torque about a pivot chosen to eliminate the torque from one of the unknowns
Find the sum of the torques ∑ τ Q about the right pier Q of the bridge. Remember that counterclockwise torque is positive
∑τQ - 2MgL− 3FpL
The sum of the torques is zero
FP = 2/3 Mg