A 70.0-kg ice hockey defender, originally at rest near the goal, has a 0.110-kg

Question

A 70.0-kg ice hockey defender, originally at rest near the goal, has a 0.110-kg hockey puck slapped at him at a velocity of 45.5 m/s. Suppose the defender and the puck have an elastic collision, and the puck is reflected back in the direction from which it came. What would the final velocities of the defender and the puck be in this case? Assume that the collision is completely elastic. To practice the center-of-mass frame content from Lesson III 8, tackle this problem using a CM frame approach (That means you won't directly employ conservation of energy, nor relative velocity __________ principle.) Note that, for this 1-D problem, there won't be any angles to worry about.

Explanation / Answer

the final velocity of the defender is,

vd = [m1 - m2/m1+ m2]v1i + [2m2 / m1 + m2]v2i

= 0 + [2*0.11/ 70+.11]*45.5 = 0.143 m/s

The final velocity of the puck is,

vp = [2m1/m1+m2]v1i + [m2-m1/m1+m2]v2i

= [0 + 0.11 - 70 /0.11 + 70]*45.5

= -45.357 m/s

= -45.4 m/s

Magnitude: 45.4 m/s

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