A car travels from point A to B with speed 40 km/h and returns from B to A with speed 60 km/h. The average speed for the entire journey is: MCQ with Answer and Explanation
A car travels from point A to B with speed 40 km/h and returns from B to A with speed 60 km/h. The average speed for the entire journey is:
A. 45 km/h
B. 50 km/h
C. 48 km/h
D. 52 km/h
Answer: Option C
Solution (By JKSSB Mock Tests)
Average speed = Total distance / Total time. Let distance AB = d km. Time for AB = d/40 h, time for BA = d/60 h. Total distance = 2d km. Total time = d/40 + d/60 = d(3+2)/120 = 5d/120 = d/24 h. Average speed = 2d / (d/24) = 48 km/h. Note: Average speed is not arithmetic mean of speeds (which would be 50 km/h) because time intervals differ. This harmonic mean concept is crucial for motion problems. Exam tip: For equal distances, average speed = 2v₁v₂/(v₁+v₂). Frequently tested in competitive exams.
Explanation:
The thermal efficiency (η) of a reversible Carnot engine depends exclusively on the absolute temperatures of the hot source (T1) and the cold sink (T2). The formula is η = 1 - (T2/T1), or (T1 - T2) / T1. Temperatures must be in Kelvin. It shows no engine can be 100% efficient unless the sink is at absolute zero.
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