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. 48 km/h
B. 50 km/h
C. 45 km/h
D. 52 km/h
Answer: Option A
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:
Law of reflection: angle of incidence = angle of reflection, both measured from normal. So 30°. If angle with mirror is given, convert to normal. Always equal for smooth surfaces.
Explanation:
The Stefan-Boltzmann law dictates that the total energy radiated per unit surface area of a black body across all wavelengths per unit time (Emissive Power, E) is strictly directly proportional to the fourth power of the black body's thermodynamic (absolute) temperature. E = sigma * T^4, where sigma is the Stefan-Boltzmann constant.
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