Impact of Jets MCQ with answer and solution

Concept

Impact of Jets

Formula

F = ρA(V - u)²

Step-by-step solution

1. Identify the given values: fluid density (ρ), cross-sectional area of jet (A), jet velocity (V), and plate velocity (u). 2. Calculate the relative velocity of the jet with respect to the moving plate: V\_rel = V - u. 3. Substitute the values into the formula F = ρ \* A \* (V - u)² to calculate the force exerted by the jet on the moving plate.

Explanation

When a plate moves away in the direction of the jet, the water strikes it with a reduced relative velocity (V - u) instead of the full jet velocity V. Since the force depends on the rate of momentum change from this relative velocity, the resulting impact force is less than it would be on a stationary plate.

Shortcut

Remember "Moving away = Less impact": A plate moving in the direction of the jet reduces the relative speed of impact from V to (V - u), which always results in LESS force than on a stationary plate.

Common mistake

Many students mistakenly select Option B (Equal to that on a stationary plate) by forgetting to account for the relative velocity (V - u) between the jet and the moving plate, assuming the impact force depends solely on the absolute jet velocity.