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Calculating final speed using momentum

by ilovedeathcab
Tags: final, momentum, speed
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Jan20-09, 08:10 PM
P: 11
1. A 73.8 kg girl runs at 4 m/s and jumps onto a 3.4 kg stationary skateboard. Ignore friction and calculate the final speed of the girl.

* DO NOT include units in your answer.
* Give your answer with one decimal.
* Answers will be accepted if they are within 0.2 of the correct answer.
2. I understood that this uses the law of conservation of momentum, but couldn't get past this basic equation without getting it wrong.
m1v1i + m2v2i = m1v1f + m2v2f

Assuming object 1 is the girl, and object 2 is the skateboard, v2i would equal 0 (stationary skateboard), v1i would equal 4m/s (girl), and you would plug in the masses (m1=73.8, m2=3.4kg). I'm not sure what to put in for v2f, becuse it doesn't specify in the problem the final velocity of the skateboard. I'm pretty sure I'm missing the big picture.
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Jan20-09, 08:28 PM
HW Helper
P: 6,202
The girl and the skateboard move with the same final velocity.

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