A horizontal water jet of speed V strikes a vertical plate normally. Force F is required to hold the plate stationary. If the plate moves toward the incoming jet with speed V/2, what force is required? Image
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Question

A horizontal water jet of speed V strikes a vertical plate normally. Force F is required to hold the plate stationary. If the plate moves toward the incoming jet with speed V/2, what force is required? Image

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G1 Physics · Accepted Answer

Step 1: Force on a stationary plate is proportional to mass-flow rate times velocity, F=ρA V². Step 2: When the plate moves toward the jet at V/2, relative impact speed is 3V/2 and mass striking per second is ρA(3V/2), so force scales as (3V/2)² = 2.25V². Required force = 2.25F. Final answer: D.