Law of Conservation of Mechanical Energy — JEE Main Physics MCQs with Solutions
Free JEE Main 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. Friction acts on it
C. The system is at rest
D. Any force 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 KE increases, PE decreases, and their sum stays constant ✓ Correct
B. Its total mechanical energy increases
C. Both KE and PE increase
D. Its PE stays constant
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. 30 m/s ✓ Correct
B. 90 m/s
C. 21 m/s
D. 45 m/s
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. 10 m
B. 25 m
C. 30 m
D. 20 m ✓ Correct
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. 14.1√2 m/s
B. 10 m/s
C. 20 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. 0.4 m/s
B. 4 m/s
C. 2 m/s ✓ Correct
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 · medium · numerical
A block slides from rest down a SMOOTH incline from a vertical height of 5 m. Its speed at the bottom:
A. Is 50 m/s
B. Is 10 m/s, independent of the incline angle ✓ Correct
C. Is 5 m/s
D. Depends on the incline angle
Solution: Energy conservation: v = √(2gh) = √100 = 10 m/s. The angle changes the time and path length, never the final speed (smooth incline).
Q8 — Law of Conservation of Mechanical Energy · hard · numerical
A simple pendulum of length 1 m is released from the horizontal position. The speed of the bob at the lowest point is:
A. 10 m/s
B. √10 m/s
C. 2√5 ≈ 4.5 m/s — same as √20
D. √20 ≈ 4.5 m/s ✓ Correct
Solution: Fall height = L = 1 m ⇒ v = √(2gL) = √20 = 2√5 ≈ 4.5 m/s.