A car travels first half distance at 40 km/h and second half at 60 km/h. Average speed is
A. 52 km/h
B. 48 km/h
C. 45 km/h
D. 50 km/h
Answer: Option B
Solution (By JKSSB Mock Tests)
Average speed = 2v₁v₂/(v₁+v₂) = 2×40×60/(40+60) = 4800/100 = 48 km/h. Not arithmetic mean (50). For equal distances, harmonic mean of speeds. If times equal, arithmetic mean applies.
Explanation:
By conservation of angular momentum: I1w1 = I2w2. Moment of inertia of a solid sphere is 2/5 MR^2. So, (R1^2) * (2pi/T1) = (R2^2) * (2pi/T2). If radius halves, R2 = R1/2. R1^2 / T1 = (R1/2)^2 / T2. T2 = T1 / 4. A day is 24 hours, so 24/4 = exactly 6 hours.
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