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Homework Help: What is the mass of the cart?

  1. May 4, 2006 #1
    Thank you--

    A 50 kg boy runs at a speed of 10 m/s and jumps onto a cart. The cart is initially at rest. If the speed of the cart with the boy on it is 2.50 m/s, what is the mass of the cart?

    I don't know what to do. This probelem seems very simple but I don't know where to start.

    These I know are the knowns:
    vo(cart) = 0 m/s
    vf(cart) = 2.50 m/s
    vo(boy) = 10 m/s
    vf(boy) = 2.5 m/s

    So I know velocities and I know the mass of the boy.
    Should I start off by finding the acceleration from F=ma, since I know a mass? But then I don't know how to find F. I know Fn = mg in the y component, but I don't know what I would use Fn next for. Am I supposed to use kinematics equations? But I don't know a, x, or time. Direction is needed, thanks!!
  2. jcsd
  3. May 4, 2006 #2

    Doc Al

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    Staff: Mentor

    It's much simpler than you think. Hint: What's conserved when the boy "collides" with the cart?
  4. May 4, 2006 #3
    Sum of external forces = 0! thank you!!!
  5. May 5, 2006 #4
    SO yeah, the net force is zero. This clue leads us to what principle pf physics, ya know, with its own set of silly equations and all.
  6. May 5, 2006 #5


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    Staff Emeritus
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    Gold Member

    Care to expand / explain your comment DaMasta? Edit. perhaps this is best discussed in the pf lounge actually.

    Last edited: May 5, 2006
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