Water emerges horizontally from a tube with speed v and strikes a nearby vertical wall normally, then falls vertically after impact. If the water speed is increased to 2v, which statement is correct?
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Question

Water emerges horizontally from a tube with speed v and strikes a nearby vertical wall normally, then falls vertically after impact. If the water speed is increased to 2v, which statement is correct?

G1 Physics · Accepted Answer

Step 1: For a fixed tube area, mass-flow rate ṁ = ρAv, so the thrust on the wall is F = ṁv = ρAv²; doubling v makes thrust four times, so A and B are false. Step 2: Kinetic energy lost per second is (1/2)ṁv² = (1/2)ρAv³, which becomes 2³ = 8 times. Correct answer: D.