A 100\text{ g} sphere is moving at a speed of 20\text{ m/s} and collides with another sphere of mass 50\text{ g}. If the second sphere was at rest prior to the collision and the first sphere comes at rest immediately after the collision, considering the collision to be elastic, the speed of the second sphere would be

  1. A. 10\text{ m/s}
  2. B. 20\text{ m/s}
  3. C. 30\text{ m/s}
  4. D. 40\text{ m/s} ✓

Correct Answer: D. 40\text{ m/s}

Explanation

By the principle of conservation of momentum: m_{1}u_{1} + m_{2}u_{2} = m_{1}v_{1} + m_{2}v_{2}. Since the first sphere comes to rest (v_{1} = 0), 100 \times 20 + 0 = 0 + 50 \times v_{2}, which gives v_{2} = 40\text{ m/s}.

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