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Collision easy and quick question

  1. Dec 6, 2014 #1
    1. The problem statement, all variables and given/known data

    Two objects of equal mass traveling toward each other with equal speeds undergo a head on collision. Which one of the following statements concerning their velocities after the collision is necessarily true?

    (a) They will exchange velocities. (d) Their velocities will be zero.

    (b) Their velocities will be reduced. (e) Their velocities may be zero.

    (c) Their velocities will be unchanged.
    2. Relevant equations

    I know there is elastic and inelastic coallison
    elastic keeps KE
    inelastic loses KE


    3. The attempt at a solution

    I would say that since both cars have SAME MASS AND SAME VELOCITIES they they will cancel each other right?
    and therefore their velocities will be zero

    so answer is D

    but right answer is E, my question is why do they say "may be" zero?????
     
  2. jcsd
  3. Dec 6, 2014 #2

    Bystander

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    Have you been told what type of collision occurs?
     
  4. Dec 7, 2014 #3
    no, what you are reading is all they have given me
     
  5. Dec 7, 2014 #4

    Bystander

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    So, you have no idea, and that is part of the problem statement. You are to consider the outcome of both possibilities, and select your answer to agree with both those outcomes.
     
  6. Dec 7, 2014 #5

    right and my thought is that both velocities will cancel each other and be zero but its wrong, can u help me? my test is on Monday
     
  7. Dec 7, 2014 #6

    haruspex

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    What physical law are you basing that on? What, exactly, will cancel out?
     
  8. Dec 7, 2014 #7
    because they are both at different directions, the force going to the right will cancel the force going to the left. The velocities will cancel, leaving the vehicles at zero speed
     
  9. Dec 7, 2014 #8
    oh shoot

    should i use the third law of Newton? action/reaction?
     
  10. Dec 7, 2014 #9

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    "BOTH." What is the outcome of an elastic collision? What is the outcome of an inelastic collision? Which answer is consistent with both outcomes?
     
  11. Dec 7, 2014 #10

    haruspex

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    There is no law that says velocities cancel.
    Forces can cancel, yes, but that doesn't tell you how the vehicles will move after collision.
    Think "conservation laws".
     
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