Segment V5.5: Rotating Lawn Sprinkler

(Related to Textbook Section 5.2.4 - Application of the Moment-of-Momentum Equation)

The net rate of flow of moment-ofmomentum through a control surface equals the net torque acting on the contents of the control volume.

Water enters the rotating arm of a lawn sprinkler along the axis of rotation with no angular momentum about the axis. Thus, with negligible frictional torque on the rotating arm, the absolute velocity of the water exiting at the end of the arm must be in the radial direction (i.e., with zero angular momentum also). Since the sprinkler arms are angled "backwards", the arms must therefore rotate so that the circumferential velocity of the exit nozzle (radius times angular velocity) equals the oppositely directed circumferential water velocity.