C. Energy can neither be created nor destroyed, only transformed
D. Energy can be created but not destroyed
Answer: Option C
Solution (By JKSSB Mock Tests)
First law of thermodynamics: Energy is conserved in an isolated system. It changes forms (kinetic to potential etc.) but total constant. Perpetual motion machine impossible.
Explanation:
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:
Constant velocity implies zero acceleration, net force = 0 by Newton's first law. Engine force balances friction and air resistance. Net force zero, not necessarily zero forces.
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