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Old Jan 9th 2018, 08:35 AM   #1
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Physics Homework Help

Hi, I need a solution for my homework task but I still need help and explanation. On an ice skating rink, a girl of mass 50 kg stands stationary, face to face with a boy of mass 80 kg. The children push off of one another, and the boy moves away with a velocity of +3 m/s. What is the final velocity of the girl?

–1.9 m/s

+1.9 m/s

–4.8 m/s

+4.8 m/s

Please, give me some explanation, not only the answer.

Last edited by topsquark; Jan 13th 2018 at 02:56 PM.
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Old Jan 9th 2018, 10:12 AM   #2
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The Total momentum (which is mass times velocity) remains constant.
Note that in this case the total momentum is zero.
This works out OK because the (final) velocities of the children are of opposite sign
(one is positive and the other negative)

so:
BoyMass * BoyVelocity = minus GirlMass * GirlVelocity

so:
80*3 = - 50*?

Another (equivalent) way of viewing this is that the final velocities will be apportioned according to the two masses
(the lighter mass ending up going faster)
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Last edited by Woody; Jan 9th 2018 at 10:16 AM.
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