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Homework Help: Elastic collision problem help

  1. May 10, 2009 #1
    1. The problem statement, all variables and given/known data

    10. A red convertible of momentum mv travelling north on the Murdoch Drive collides, at the intersection of Murdoch Dr and South St, with a green sedan having the same magnitude of momentum, but travelling west on South St.

    a. What is the magnitude and direction of the resulting momentum of the vehicles if it is an inelastic collision?

    b. Calculate the resulting momentum if the mass of each vehicle is 2200kg and their initial speeds were both 14ms-1

    2. Relevant equations
    m1u1 +m2u2 = m1v1 +m2v2

    P1i + P2i = P1f + P2f

    3. The attempt at a solution

    red car goes west

    green cars goes north


    2200.14 + 2200.14 = m1v1 +m2v2
    er that is as far as i got cuz i am confused???
  2. jcsd
  3. May 10, 2009 #2
    Re: momentum

    This isn't a 1D problem. If you take a better look at it you will see that it's in 2D, so try to draw yourself a picture. And one more thing. If it's an inelastic collision then the cars will continue to move as one with one speed which will be common for both of them.
  4. May 10, 2009 #3
    Re: momentum

    do you mean that they will both go in the same direction cuz i thought that was a perfactly elastic collision...does that mean i am also using the wrong equation?
  5. May 10, 2009 #4
    Re: momentum

    By definition, if there is an elastic collision, both objects will bounce one of each other and each will go in their own direction with their own speed.

    You are given a inelastic collision, so there will be no bouncing and the 2 cars could be observers as a single object which goes in a direction different from the initial directions of both cars.

    The general idea you have is OK, but you need to draw the picture to realize that the speeds are not on the same line.
  6. May 11, 2009 #5
    Re: momentum

    i have drawn a picture and i know they don't crash head on but i don't see how you can predict the final directions there only way i know how to work out which direction they going is when you know which car as more force??? Is equation i am using right?

    if so how come there is not enough data in the question to complete it?
  7. May 11, 2009 #6
    Re: momentum

    Hint: Both velocity and momentum are vectors and should be dealt with accordingly.
    You have one car moving north with speed 14m/s, so his speed in 2D is 14j m/s
    The other car is moving west, so his speed is -14i m/s.
  8. May 12, 2009 #7
    Re: momentum

    ok if two cars are going in any direction not matter what directiion as long as they are not the same then one speed is negative
  9. May 12, 2009 #8
    Re: momentum

    Ok, here's the picture before collision. You should be able to visualize what I was trying to say to you. You need to look at this matter in 2D, because the cars are not on the same track.

    http://img8.imageshack.us/img8/2686/beforeead.jpg [Broken]
    Last edited by a moderator: May 4, 2017
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