A Levels Physics (9702)•9702/13/M/J/23
Explanation
Kinetic energy from given post-collision speed Steps:
- Identify mass of Y as 4m and its speed after collision as 5v/4.
- Use kinetic energy formula: KE = (1/2) × mass × (speed)².
- Substitute values: KE_Y = (1/2) × 4m × (5v/4)² = 2m × (25v²/16).
- Simplify: 2m × 25v²/16 = 50mv²/16 = 25/8 mv².
Why B is correct:
- Direct application of KE formula to given mass and speed yields exactly 25/8 mv².
Why the others are wrong:
- A: Underestimates by using incorrect speed or partial calculation (e.g., 1/2 × 4m × v²/2 or similar error).
- C: Overlooks full simplification, possibly stopping at intermediate 50/16 mv² ≈ 3.125 but misfractioning to 15/8.
- D: Far too small, perhaps confusing with velocity change or momentum term.
Final answer: B
Topic: Linear momentum and its conservation
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