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Homework Help: Basic Impulse Problem

  1. Mar 13, 2007 #1
    1. The problem statement, all variables and given/known data

    A 50. kg cart is moving across a frictionless floor at 2.0 m/s. A 70. kg boy, riding in the cart,
    jumps off so that he hits the floor with zero velocity.

    a. What impulse did the boy give to the cart?
    b. What was the velocity of the cart after the boy jumped?

    2. Relevant equations

    I= mv-mv(initial)

    3. The attempt at a solution

    (50kg)(2.0m/s)-(120kg)(2.0m/s)=-40 kgm/s

    I really have no idea how to get the velocity after he jumped out so I'm pretty sure the impulse is incorrect as well. Any help would greatly be appreciated. Thanks.
  2. jcsd
  3. Mar 13, 2007 #2


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    Science Advisor
    Homework Helper

    Answer these questions:

    1) What was the total momentum of cart and boy before the boy jumped? What was the momentum of the cart separately?

    2) What was the momentum of the boy after he jumped?

    3) Calling the velocity of the cart after he jumped 'v', what was the momentum of the cart after he jumped?

    4) How does the total momentum in 1) compare with the sum of the momenta in 2) and 3)? Can you solve for v?

    5) How much did the momentum of the cart change? That's the impulse.

    Then you're done.
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