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Calculate the final velocity of the cart

  1. Nov 19, 2003 #1
    Three students, each having a mass of 60 kg, climb onto a large flatbed cart that has a mass of 120 kg. Standing at one end and taking turns they run to the opposite end and jump off, one immediately following the other, each with a velocity of 10 m/s with respect to the cart. Calculate the final velocity of the cart and students with resepect to the earth.

    I know that we have to use:
    m1*v1 + m2*v2 = m1*v1 + m2*v2

    the left side is before and the right side is after
    m1 = mass of object A
    v1 = velocity of object A
    m2 = mass of objectB
    v2 = velocity of object B

    Any help will be appreciated.
  2. jcsd
  3. Nov 19, 2003 #2
    well i asssume the system starts at rest, so the left side of your equation would = 0 and your equation would reduce to:
    m1*v1 + m2*v2 = 0

    m1 = mass of the cart
    v1 = final velocity of the cart
    m2 = mass of a child
    v2 = final velocity of a child

    you need another equation to solve for 2 variables:

    V1 - v2 = 10

    solve it for v1:
    v1 = 10 + v2
    and plug it in to your first equation:

    120*(10+v2) + (60)(v2) = 0
    v2 = -20/3
    plug into the second equation: v1 = 10/3

    this is just the first step. you have to do it three times, one for each child, using the result from previous one as the initial for the second.
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