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Velocity on a moving platform

  1. Jan 10, 2010 #1
    hello, I have a simple question.
    a long vehicle' s velocity is 26 m/s , a person on the vehicle runs at -6 m/s, what is his velocity ? I think it is 20 m/s from an inertial frame
    However, imagine this situation, an elevator is at free falling and just before it collide it has 26 m/s and a person in the elevator jump at 6 m/s just before the collision. would impulse effect decrease ?

    this question in my book solved like this; boy is 90kg

    26 m/s - (-6m/s) = 32 m/s
    impulse = final moment - initial moment
    = 0*90 - 32*90
    = -2880

    why did he subtract -6 velocity ? If the boy jump, wouldn't his velocity become 20 m/s from an inertial frame ?

    thank you for your helps
  2. jcsd
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