A 50 kg roller-skater traveling 10 m/s east collides with a 75 kg hockey player

Question

A 50 kg roller-skater traveling 10 m/s east collides with a 75 kg hockey player going 5 m/s west. Which person has more momentum before the collision? Roller-skater hockey player both have the same Before the collision, the momentum of the roller-skater-hockey player system is To the east to the west zero After the collision, the momentum of the roller-skater-hockey player system is To the east to the west zero Immediately after the collision, the roller-skater is moving to the east at 4 m/s. What happened to the hockey player? It is moving east It stopped It is moving west

Explanation / Answer

1) pskater = mskatervskater = (50kg)(-10m/s) = -500 Ns (Note, I called the east direction negative)

pplayer = mplayervplayer = (75kg)(5m/s) = 375 Ns (Note, I called the west direction positive)

So, the skater has more momentum (a)

2) The momentum of the skater is to the east, and the momentum of the player is to the west. Since the skater has more momentum, the overall momentum of the system points to the east (a)

3) During a collision, the momentum of the system is conserved. Therefore, it will be the same (and point in the same direction) as before the collision, to the east (a)

4) If the skater is moving to the east at 4 m/s, then its momentum is

p'skater = (50kg)(-4m/s) = -200 Ns

Ptotal = -125 Ns = p'player + p'skater =  p'player -200 Ns

This implies that the momentum of the player must be positive (to increase -200 to -125), meaning the player moves to the west (c)

Hope this helps.

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