A car moves from X to Y with a uniform speed vu and returns to X with a uniform speed vd. The average speed for this round trip is

Question

Dr. Abubacker · Accepted Answer

[Solution from source] For equal distances d each way, total time = d/vu + d/vd, so average speed = 2d/(d/vu + d/vd) = 2vu vd/(vu + vd), the harmonic mean. The arithmetic mean (vu + vd)/2 is wrong because more time is spent at the lower speed. Answer (4).

Q. A car moves from X to Y with a uniform speed vu and returns to X with a uniform speed vd. The average speed for this round trip is

Explanation

Reviewed by Dr. Abubacker

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