The force on the wheel due to the jet is equal to the rate of change of momentum of the jet in the direction of the cup move- ment. The absolute velocity in the plane of rotation of the wheel is known as the whirl velocity of the flow vw, as shown in Fig. 3. The force is therefore the product of the change in whirl velocity and the jet mass flow rate. The change in the whirl velocity can be calculated for the top Dvw and bottom Dv0w of the cup by:Þ