Applications of definite integrals: Fluid Force and Fluid Pressure

The vertical end of a tank containing water weighing 62.4lb/ft^3 has the given shape.

a)Explain how to approximate the force against the end of the tank by a Riemann Sum.

b)Find the force as an integral and evaluate it.

The shape is a semi-circle with a radius of 3ft. (a bowl pretty much) and its completely filled with water.

I don't know how to find the force of a round surface, just a flat one. Please explain to me how to do this thank you.