A jet of water moving at 10 m/s with a cross-sectional area of 2 cm² strikes a flat plate at right angles, after which the water spreads out parallel to the plate. Determine the force exerted on the plate. (Density of water = 1000 kg/m³.)

20 N
10 N
40 N
2 N

Correct answer: 20 N

Solution

The water hits perpendicularly then moves parallel, so its normal momentum is fully destroyed. Mass per second = rho A v. Force = (rho A v) v = rho A v² = 1000 x 2e-4 x 100 = 20 N.

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