A 0.2 kg hockey puck moving at 10 m/s to the right collides head-on with a stationary 0.3 kg puck. After the collision, the 0.2 kg puck moves to the left at 2 m/s. What is the velocity of the 0.3 kg puck immediately after the collision? (Assume the collision is isolated and frictionless.)
Show the answer and explanation
Correct answer
C. 8 m/s to the right
Principle or equation
Conservation of momentum: total momentum before collision equals total momentum after collision.
Why this answer is correct
Initial momentum = 0.2*10 + 0.3*0 = 2 kg·m/s to the right. After collision, momentum of 0.2 kg puck = 0.2*(-2) = -0.4 kg·m/s (left). Let v be velocity of 0.3 kg puck: 0.3v - 0.4 = 2, so 0.3v = 2.4, v = 8 m/s to the right.
Example
If a 1 kg ball at 5 m/s hits a 2 kg stationary ball and stops, the 2 kg ball moves at 2.5 m/s.
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