A 0.40 kg object moving to the right at 5.0 m/s collides head-on with a 0.60 kg object moving to the left at 3.0 m/s. After the collision, the 0.40 kg object moves to the left at 2.0 m/s. What is the velocity of the 0.60 kg object after the collision? (Take right as positive.)
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Correct answer
B. 1.7 m/s to the right
Principle or equation
In an isolated system, total momentum is conserved: m1v1i + m2v2i = m1v1f + m2v2f.
Why this answer is correct
Initial total momentum = (0.40)(5.0) + (0.60)(-3.0) = 2.0 - 1.8 = 0.2 kg·m/s. After collision, momentum of first object = (0.40)(-2.0) = -0.8 kg·m/s. Let v be the final velocity of the second object: -0.8 + 0.60v = 0.2 → 0.60v = 1.0 → v = 1.67 m/s, positive, so to the right.
Example
If a 1 kg ball moving at 2 m/s hits a 2 kg ball at rest and stops, the 2 kg ball moves at 1 m/s.
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