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Homework Help: Car crash: Can it be an elastic collision?

  1. Apr 8, 2009 #1
    If two cars are going towards each other, collide, and then are separated (have separate masses and separate velocities).... that is an elastic collision, correct?
  2. jcsd
  3. Apr 8, 2009 #2
    yup i think so, inelastic is when the objects hit and then stop dead
  4. Apr 8, 2009 #3


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    An elastic collision requires conservation of kinetic energy and momentum. If there is so much as a single dent caused by the collision some kinetic energy will have changed in for example heat. In reality car crashes are always inelastic.
  5. Apr 8, 2009 #4
    There are two extremes.
    Perfectly inelastic when the objects stop dead and there is a total conversion of K.E.
    Perfectly elastic when there is zero loss of K.E.These collisions can happen with atomic scale objects such as gas molecules.
    The collision referred to lies between the extremes but is more inelastic.
  6. Apr 8, 2009 #5
    That's why I am confused... let me paraphrase the problem (I don't remember it verbatim- these numbers won't work but you'll get the idea):

    Car 1 is 1.45kg and is going 4 m/s in the positive x direction. Car 2 is 0.65kg and is going -0.9 m/s. They collide, and after car 2 is going 2 m/s... find the new velocity of car 1.

    There's where I got confused. I thought car crashes were inelastic, and momentum would be conserved by:
    MiVi (car 1) + MiVi (car 2) = (Mcar1+car2)(Vf)

    Since it gave a new velocity for car 2, I assumed it meant that they separated:
    MiVi (car 1) + MiVi (car 2) = MfVf (car 1) + MfVf (car 2)

    I plugged in the numbers and found car 1's new Vf.

    Does that sound right to you?
  7. Apr 8, 2009 #6
  8. Apr 8, 2009 #7


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    Your intuition is correct, you won't see two cars bounce off each other during a head on collision in real life. However with these kind of text book problems it's important to just look at the data given regardless of how realistic it is. Your answer is correct.
  9. Apr 8, 2009 #8
    Yeah, impossible to have elastic collision. Elastic collision = bad because KE is conserved and we would be bouncing back pretty fast!

    That's why car manufacturers have crush zones to help reduce KE into other forms of energy including heat.
  10. Apr 8, 2009 #9

    Thank you.
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