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A railroad flatcar of weight W can roll without friction along astraight horizontal track. Initially, a man of weight w is standing on the car, which is moving to the right with speed v_0. What is the change in velocity of the car if the man runs to the left so that his speed relative to the car is v_rel?

I know that the man and the flatcar are moving inopposite directions. Would the answer be: v_f-v_m= mv/(m_flatcar - m_man)?

The mans change in momentum (-mv) relative to the original moving axis (speed v0) added to the change in car momentum is zero.

W(deltaV)+ w(-v)= 0

deltaV= w/W (v)

  • Physics -

    Let final velocity be v_1
    Conservation of momentum implies that:
    ==> (m_car + m_man)v_0 = (m_man * v_rel) + (m_car + m_man)v_1
    Solve this equation for v_1 - v_0 to get:
    v_1 - v_0 = (m_man * v_rel)/(m_man + m_car)
    so the change in velocity is:
    (w * v_rel)/(w + W)

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