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Homework Help: Taking the AP b and recently i did this lab on collision

  1. Dec 29, 2009 #1
    1. The problem statement, all variables and given/known data

    well this is just a over all question. im currently taking the AP b and recently i did this lab on collision. there were 3 collisions, a) one stationary while another one hits it b) both masses crash into each other c) both has velocity but one has higher velocity to collide with the lower velocity car. So far i did (a) and found that it is elastic collsion since both momentum and kinetic energy is conserved, however when i did (b) the kinetic energies were kinda off, but when i looked it up online, MANY SITES tell me that if 2 cars crash into each other, it is elastic. so idk what i shuld put for this

    2. Relevant equations
    m1v1^2/2 = m2v2^2/2

    3. The attempt at a solution

    ^ all up there
  2. jcsd
  3. Dec 29, 2009 #2


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    Science Advisor

    Re: Collisons

    I'm not sure what you are saying here. Two cars colliding with each other can be elastic or inelastic. It does not depend upon whether one or both cars are moving but on what happens in the collision. Kinetic energy is conserved if and only if the collision is elastic. That's pretty much the definition of "elastic collision".
  4. Dec 29, 2009 #3
    Re: Collisons

    No idea where you got those sites from, but NO, that is not true at all. Elastic collisions in reality are really ideal cases. Believe your experimental results; after all your conclusion should follow from the results in a lab right?
  5. Dec 29, 2009 #4
    Re: Collisons

    wow that was fast lol. ok so for

    a) One car is at rest and another car has velocity.

    for this part i put collision is elastic because one of my trails came out showing that momentum and kinetic energy is conserved

    b) Two cars crash into each other

    for this part, i know that momentum has to be the same but my results show me other wise. and from my kinetic energy data results, it shows me that the kinetic energies arent the same, so would this still be an elastic or an inelastic?

    and from my observation, the two cars DID recoil after they collided.

    c) one car has small velocity while another car with a higher velocity hits it from the rear ( rear end collision )

    didnt get up to here yet but i think it would be inelastic collision since both cars will still have velocity after collision?
  6. Dec 29, 2009 #5
    Re: Collisons

    That momentum is not conserved suggests friction being non-negligible in this particular experiment. However, since your first experiment managed to achieve COLM to a reasonable extent (ie negligible friction), then I would recommend that you repeat this particular experiment.
    There is no "I think it would be inelastic". It can be (almost) elastic or inelastic depending on experimental conditions.
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