Law of Conservation of Mechanical Energy — IISER Physics MCQs with Solutions
Free IISER Physics Law of Conservation of Mechanical Energy MCQs with step-by-step solutions (8 questions). Part of Work, Power and Energy. Practise online on Prepizo — no login needed.
▶ Practise Law of Conservation of Mechanical Energy online (free)
Questions with solutions
Q1 — Law of Conservation of Mechanical Energy · easy · theory
The total mechanical energy (KE + PE) of a system is conserved when:
A. Only conservative forces do work on it ✓ Correct
B. Any force acts on it
C. The system is at rest
D. Friction acts on it
Solution: If only conservative forces (gravity, spring…) do work, KE + PE = constant. Friction/air resistance drain mechanical energy into heat.
Q2 — Law of Conservation of Mechanical Energy · medium · theory
A ball falls freely from rest (no air resistance). During the fall:
A. Its PE stays constant
B. Both KE and PE increase
C. Its total mechanical energy increases
D. Its KE increases, PE decreases, and their sum stays constant ✓ Correct
Solution: Free fall converts PE to KE continuously; the sum mgh + ½mv² stays fixed at every instant.
Q3 — Law of Conservation of Mechanical Energy · easy · numerical
A ball is dropped from a height of 45 m. Its speed on reaching the ground is:
A. 90 m/s
B. 45 m/s
C. 21 m/s
D. 30 m/s ✓ Correct
Solution: mgh = ½mv² ⇒ v = √(2gh) = √(2×10×45) = √900 = 30 m/s.
Q4 — Law of Conservation of Mechanical Energy · medium · numerical
A stone is dropped from a height of 40 m. At what height above the ground are its kinetic and potential energies equal?
A. 30 m
B. 20 m ✓ Correct
C. 25 m
D. 10 m
Solution: KE = PE means each is half the initial PE ⇒ mgh = ½mg(40) ⇒ h = 20 m.
Q5 — Law of Conservation of Mechanical Energy · hard · numerical
A body is dropped from a height H = 20 m. Its speed when it has fallen through H/2 is:
A. 20 m/s
B. 14.1√2 m/s
C. 10 m/s
D. 10√2 ≈ 14.1 m/s ✓ Correct
Solution: v = √(2g·H/2) = √(gH) = √200 = 10√2 ≈ 14.1 m/s. Trap: half the height does NOT give half the final speed.
Q6 — Law of Conservation of Mechanical Energy · medium · numerical
A pendulum bob is released from a point 0.2 m above its lowest position. Its speed at the lowest point is:
A. 4 m/s
B. 2 m/s ✓ Correct
C. 0.4 m/s
D. 1 m/s
Solution: v = √(2gh) = √(2×10×0.2) = √4 = 2 m/s — the string tension does no work.
Q7 — Law of Conservation of Mechanical Energy · hard · theory
A block slides down a rough incline. Which statement is correct?
A. Mechanical energy is conserved
B. Total energy is not conserved
C. Kinetic energy equals the loss in potential energy
D. Mechanical energy decreases, but total energy (including heat) is conserved ✓ Correct
Solution: Friction converts part of the PE into internal (heat) energy: ΔKE = mgh − W_friction. Energy conservation always holds for the TOTAL energy.
Q8 — Law of Conservation of Mechanical Energy · hard · numerical
A 2 kg block slides from rest down a rough incline of vertical height 5 m and reaches the bottom at 8 m/s. The heat generated by friction is:
A. 64 J
B. 28 J
C. 100 J
D. 36 J ✓ Correct
Solution: Q = mgh − ½mv² = 2×10×5 − ½×2×64 = 100 − 64 = 36 J.