When a plate is placed in the path of a jet, the jet exerts a force on the plate. This force can be calculated from the impulse momentum equation. Momentum equation is based on Newton's second law of motion, which states that “The algebraic sum of external forces applied to control volume of fluid in any direction is equal to the rate of change of momentum in that direction." The external forces include the components of the weight of the fluid and of the forces exerted externally upon the boundary surface of the control volume.

If a vertical jet moving with velocity V is made to strike a target, which is free to move in the vertical direction, then a force will be exerted on the jet by the impact of jet. According to momentum equation, this force must be equal to the rate of change of momentum of the jet flow in the same direction.

Due to impact of the jet on the flat stationary plate, the entire velocity of the jet is destroyed and due to the rate of change of momentum, force acts on the plate. The jet after striking will move along the plate. But the plate is at right angles to the jet. Hence the components of the velocity of the jet in the direction of the jet after striking it will be zero. The force exerted by the jet on the flat plate in the direction of the jet.