JEE Main Physics question
A point mass of 1 kg collides elastically with a stationary point mass of 5 kg. After their collision, the 1 kg mass reverses its direction and moves with a speed of 2 ms⁻¹. Which of the following statement(s) is (are) correct for the system of these two masses? [IIT JEE 2010]
Answer: (A),(C)
Solution
Since the collision is elastic, the relative speed of approach equals the relative speed of separation (e=1): u = v + 2 (taking the 1 kg's initial speed as u, final as −2, the 5 kg's final as v). By momentum conservation: 1(u) = 5v − 1(2) ⟹ u = 5v − 2. Combining with u = v+2: v+2 = 5v−2 ⟹ 4 = 4v ⟹ v = 1 m/s, u = 3
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