A 4 kg trolley moving at 9 m/s collides with a stationary 2 kg trolley and they stick together. Find their common speed.

Answer and explanation

Correct answer: B. 6 m/s

  1. Step 1: Momentum is conserved in the collision: m1 v1 = (m1 + m2) v.
  2. Step 2: v = 4 x 9 / (4 + 2) = 6 m/s.
Answer
6 m/s.
Why not the others: '4.5 m/s' averages the speeds instead of conserving momentum; '36 m/s' gives the initial momentum, not the speed; '3 m/s' uses m2 in the numerator instead of m1.

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