As the reservoir behind a dam is filled with water, the pressure that the water exerts on the dam increases. eventually, the force on the dam becomes substantial, and it could cause the dam to collapse. there are two significant issues to be considered: first, the base of the dam should be able to withstand the pressure ρgh, where ρ is the density of the water behind the dam, h is its depth, and g is the magnitude of the acceleration due to gravity. this means that the material of which the dam is made needs to be strong enough so that it doesn't crack (compressive strength).
(a) what is the total force that the water in the reservoir exerts on the dam? the second issue has to do with the strength of the foundation of the dam. the force of the water produces a torque on the dam. in a simple model, if the torque due to the water were enough to cause the dam to break free from its foundation, the dam would pivot about its base (point p). the foundation of the dam should be strong enough so that the dam does not topple, which means that the material has to be strong enough that the dam does not snap (shear strength).
(b) what is the magnitude of the torque about the point p due to the water in the reservoir?