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Homework Help: Momentum .Please help

  1. Oct 11, 2008 #1
    Momentum.....Please urgent help!!!

    My Question:
    A 1500 kg car moving east at 38 m/s collides with a 1780 kg car moving south at 15 m/s and the two cars stick together.
    (a) What is the velocity of the cars right after the collision? _______m/s ______o

    (b) How much kinetic energy was converted to another form during the collision? ______kJ

    My attempt:

    So i used the momentum formula and got the total momentum before collision and made it equal to the momentum after the collsion but didn't get the right answer.

    Before collision: car1:
    p=m1v1=(1500kg)(38m/s)=57000

    car2:
    p=m2v2=(1780kg)(-15m/s)=-26700

    p1+p2=P total
    57000+(-26700)=30300

    After collision: car1:
    p=1500v1

    car2:
    p=1780v2

    v1=v2 because of collision therefore

    30300=(1500+1780)v
    v=9.24m/s

    when i took out the negative i got a different answer (25.5m/s) which was also wrong.

    Part B i've no idea how to do:uhh:

    Please tell me what i'm doing wrong!!!!!
     
  2. jcsd
  3. Oct 11, 2008 #2
    Re: Momentum.....Please urgent help!!!

    Remember that velocity, and hence momentum, are vector quantities and need to be treated as such when adding them together. I think you attempted to do this with the minus sign, but these vectors are not opposite in direction. They are at 90 degrees to one another. Draw a diagram of the momenta vectors. It may help you see how to combine them for the total.

    Also, for part b, remember Kinetic energy is not a vector quantity. determine the total kinetic energy before the collision and compare it to thee total kinetic energy after the collision and determine the difference.
     
  4. Oct 12, 2008 #3
    Re: Momentum.....Please urgent help!!!

    First of all........thanks Galileo's Ghost for all your help

    Ok.....part a: Yes i understand what your saying....my first try was without the negative sign and i still got that wrong.....so it must be my method that's messed up

    And how am i supposed to find the angle?????

    Lastly as long as i know how to get part a then i can get part b.
     
  5. Oct 13, 2008 #4
    Re: Momentum.....Please urgent help!!!

    do you go to wayne state university?
     
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