Inelastic Collisions
A 2 kg lump of clay moving at 6 m/s strikes and sticks to a 4 kg block initially at rest on a frictionless floor. What fraction of the initial kinetic energy is lost in this collision?
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About This Question
- Subject
- physics
- Chapter
- work, energy and power
- Topic
- inelastic collisions
- Difficulty
- Hard
- Year
- 2025
Solution
Correct Answer:
Two-thirds
As outlined in NCERT Class 11, Chapter 6 (Work, Energy and Power), in a perfectly inelastic collision momentum is conserved but kinetic energy is not. Momentum conservation gives the common velocity: (2)(6) = (2 + 4)v, so v = 12/6 = 2 m/s. Initial kinetic energy = (1/2)(2)(6²) = 36 J. Final kinetic energy = (1/2)(6)(2²) = 12 J. Energy lost = 36 - 12 = 24 J, and the fraction lost is 24/36 = two-thirds. The option one-third gives the fraction retained rather than lost. The option one-half ignores the mass ratio in the calculation. The option one-quarter underestimates the loss. A consistency check: when bodies stick together, the lost fraction equals the ratio of the target mass to the total mass for a stationary target, here 4/6 = two-thirds, matching the energy calculation.
This hard difficulty physics question is from the chapter work, energy and power, covering the topic of inelastic collisions. It appeared in the 2025 exam.
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