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Homework Help: Momentum,impulse, force question

  1. Mar 28, 2013 #1

    i have a question iv been working on for 4 hours and im soo confused.

    My example is a bumper car in a elastic collision.

    bumper car A travels at 3ms-1 east with a mass of 180kg
    Bumper car B travels at 4ms-1 west witha mass of 200kgs.
    the momentum before the collision is 1340kgms-1..this will be equal to momentum after the collision.

    Car A travels at 1.5ms-1 after the collision int he opposite direction west
    while Car B travels at 5.35ms-1 to the east.

    what i need help with is change in momentum.

    what is the change in momentum for both cars.

    Δmomentum= F* T ( how am i supposed to get the average force for both cars)
    the contact time was 2 secs but what is the change in momentum?

    i dont get if its change in momentum for the whole system or just 1 car?

    please help me out..

    feel free to substitue any values for F or T etc..keep it simple as well (im only 16)

    Thank you
  2. jcsd
  3. Mar 28, 2013 #2
    If there is no other force acting on cars A and B during the collision, then the total momentum of the A&B system does not change. This is "conservation of momentum". What may and usually does change is the momentum of each car.
  4. Mar 28, 2013 #3
    hi thanks for that.
    yes momentum for car a and b does change..
    so can i get change in momentum for car A?
    will it be final momentum (270Kgms-1 to west) - Inital momentum ( 540kgms-1 to the east)
  5. Mar 28, 2013 #4
    what i dont get is how to calculate impulse when both Bumper car A and b collide...?
    what force do i use?
    what change in momentum do i use?
    I know the Time?

    Thank you
  6. Mar 28, 2013 #5
    In a fully elastic collision you have conservation of momentum and you have conservation of energy. Together they are a system of two equations for two unknowns that you then solve. Let ## m_A ## and ## m_B ## be the masses of the cars, and ## v_{iA}, \ v_{iB}, \ v_{fA}, \ v_{fB} ## be their initial and final velocities. Conservation of momentum: $$ m_A v_{iA} + m_B v_{iB} = m_A v_{fA} + m_B v_{fB} $$ Conservation of energy: $$ \frac {m_A v_{iA}^2} {2} + \frac {m_B v_{iB}^2} {2} = \frac {m_A v_{fA}^2} {2} + \frac {m_B v_{fB}^2} {2} $$
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