• Think of a jet of water striking a vertical plate in the middle which is hinged at O.
• Due to the force exercised by the jet toward the plate, that plate will swing through some angle on the hinge.

Let,

x = distance of the centre of a jet from hinge O,

θ = angle of swing about a hinge,

W = weight of plate acting at C.G. of the plate.

• The dotted lines show the position of the plate before the jet strikes the plate.
• Point A on the plate will be at A’ after the jet strikes the plate.
• The distance OA = OA’ = x. 
• Let the weight of the plate is acting at A’.
• If the plate is in equilibrium after the jet strikes the plate, the moment of all the forces about the hinge must be zero.
• Two forces are acting on the plate. They are….!

Force due to a jet of water, normal to the plane, 

Fn = ρav^2 sinθ1

where,

 θ1 = Angle between jet and plate

Moment of force Fn about hinge

= Fn x OB

= ρav^2 sin(90o – θ) x OB

= ρav^2 cosθ x OB

= pav^2 cosθ x (OA/cosθ)

= ρav^2 x OA

Moment of weight W about hinge

= W x O A’ sinθ

= W x x x sinθ

ρav^2 x x = W x x x sinθ