Inelastic & Perfectly Inelastic Collisions — IISER Physics MCQs with Solutions
Free IISER Physics Inelastic & Perfectly Inelastic Collisions MCQs with step-by-step solutions (8 questions). Part of Centre of Mass, Momentum & Collisions. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Inelastic & Perfectly Inelastic Collisions · easy · theory
In a perfectly INELASTIC collision, the two bodies:
A. Exchange velocities
B. Both stop always
C. Stick together and move with a common velocity; KE is lost but momentum is conserved ✓ Correct
D. Conserve kinetic energy
Solution: Sticking = maximum possible KE loss consistent with momentum conservation. Momentum is still exactly conserved.
Q2 — Inelastic & Perfectly Inelastic Collisions · medium · theory
In ANY collision between two bodies (no external force), which is ALWAYS conserved?
A. Total linear momentum ✓ Correct
B. The velocity of the heavier body
C. Total kinetic energy
D. The speed of each body
Solution: Momentum conservation needs only F_ext = 0. KE survives only the elastic case — the single most-tested distinction.
Q3 — Inelastic & Perfectly Inelastic Collisions · easy · numerical
A 2 kg body at 6 m/s hits a 4 kg body at rest and sticks to it. Their common velocity is:
A. 2 m/s ✓ Correct
B. 3 m/s
C. 1.5 m/s
D. 6 m/s
Solution: v = 2×6/6 = 2 m/s.
Q4 — Inelastic & Perfectly Inelastic Collisions · medium · numerical
For that same collision (2 kg at 6 m/s onto 4 kg at rest, perfectly inelastic), the kinetic energy LOST is:
A. 6 J
B. 24 J ✓ Correct
C. 36 J
D. 12 J
Solution: KE_i = 36 J; KE_f = ½×6×4 = 12 J; loss = 24 J (two-thirds!). Shortcut: ΔKE = ½μu² = ½×(8/6)×36 = 24 J.
Q5 — Inelastic & Perfectly Inelastic Collisions · hard · numerical
A 10 g bullet at 400 m/s embeds into a 1.99 kg block hanging as a ballistic pendulum. The height to which the block+bullet swings is:
A. 0.2 m ✓ Correct
B. 0.1 m
C. 2 m
D. 0.4 m
Solution: v_common = 0.01×400/2 = 2 m/s; h = v²/2g = 4/20 = 0.2 m. Trap: never equate bullet KE to mgh — most KE is lost in embedding.
Q6 — Inelastic & Perfectly Inelastic Collisions · medium · numerical
Two identical cars moving at equal speeds v collide head-on and lock together. The wreckage moves at:
A. v
B. v/2
C. Zero ✓ Correct
D. 2v
Solution: Equal and opposite momenta cancel: the combined wreck stops; ALL the kinetic energy is dissipated.
Q7 — Inelastic & Perfectly Inelastic Collisions · hard · theory
The kinetic energy lost in a perfectly inelastic collision equals the kinetic energy in the C-frame because:
A. Sticking kills all relative motion, leaving only the (unchangeable) COM motion ✓ Correct
B. Energy is created in the C-frame
C. The C-frame violates momentum
D. The lab KE is always zero after
Solution: KE_lab = ½Mv_cm² + KE_rel; sticking sets KE_rel → 0 while ½Mv_cm² is protected by momentum conservation — so the loss is exactly KE_rel = ½μv_rel².
Q8 — Inelastic & Perfectly Inelastic Collisions · hard · numerical
Two balls, 2 kg at 6 m/s and 4 kg at 3 m/s, move towards each other and stick together. The energy dissipated in the collision is:
A. 54 J (they stop dead) ✓ Correct
B. 27 J
C. 9 J
D. 13.5 J
Solution: Momenta: 2×6 = 12 and 4×3 = 12, opposite ⇒ total zero ⇒ the pair stops. All the KE (36 + 18 = 54 J) is dissipated. Check via C-frame: ½μv_rel² = ½×(4/3)×9² = 54 J ✓.