Hydraulic Machines MCQ with answer and solution

Concept

A lawn sprinkler rotates because water rushes out of its nozzles. This creates a rotating force or torque using the moment of momentum principle.

Step-by-step solution

1. Understand how a lawn sprinkler works. 2. Water flows through the rotating arms and comes out of the nozzles at high speed. 3. The escaping water creates a reactive force (torque) on the sprinkler arm. 4. This is governed by Newton's second law of motion applied to rotational flow, which is known as the moment of momentum principle (or angular momentum principle). Answer: (C) Moment of momentum principle

Explanation

A lawn sprinkler works on the turbine principle where the change in angular momentum of water produces rotation. This is based on the moment of momentum principle.

Why the other options are wrong

A: Energy equation deals with pressure, kinetic, and potential energy conservation along a streamline. B: Continuity equation is based on conservation of mass. D: Impulse-momentum principle is for linear forces like jet impact on stationary or moving flat plates.

Shortcut

Sprinklers and turbines rotate due to angular momentum change, so always link them to the moment of momentum principle.

Common mistake

Students often choose the energy equation, thinking about pressure and head loss, but rotation is caused by angular forces.