Steam turbines are used to generate mechanical power from steam to drive generators in steam power plants. Early steam power was generated using reciprocating engines. Turbines use the impulse principle where the change in momentum equals the impulse force. For flat plates, the maximum power occurs when the velocity of the blade is half the initial jet velocity. For cylindrical plates, the jet reverses direction by 180 degrees, resulting in an exit velocity of 2VB - Vs. The impulse force is the change in momentum and power can be calculated from the impulse force and velocity.