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Homework Help: Energy:Momentum relationship

  1. Oct 26, 2008 #1
    1. The problem statement, all variables and given/known data

    Car A has a greater mass than car B. Let:

    - KA = the kinetic energy of car A
    - KB = the kinetic energy of car B
    - pA = the magnitude of the momentum of car A
    - pB = the magnitude of the momentum of car B
    - vA = the speed of car A
    - vB = the speed of car B

    a) If car A and car B have the same kinetic energy (KA = KB), then

    2. Relevant equations
    Ke= .5(m)(v^2)

    3. The attempt at a solution
    I tried to set up a situation:
    Car A- 10 kg Car B- 5kg Ke=20 J
    .5(10)(v^2) = 20 .5(5)(v^2)=20
    v= 2 m/s v= 2.828 THEREFORE Vb > Va
    Pa=10(2) = 20 Pb=5(2.828) THEREFORE Pb < Pa

    But the selection Pb<Pa and Vb>Va is wrong
    Is there a flaw in my reasoning
    thank you for your help
    Last edited: Oct 26, 2008
  2. jcsd
  3. Oct 26, 2008 #2
    with Ma=2 and Mb=1 and Ke=10
    im getting



    Vb>Va and MVb<MVa

    same result as earlier and its wrong
  4. Oct 26, 2008 #3


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    Gold Member

    I deleted my previous post because you edited your post after I had replied and therefore my previous post didn't make sense.
  5. Oct 26, 2008 #4
    sorry about that it was a typo
  6. Oct 26, 2008 #5
    Your answers look fine. You should, though, try to prove the relations in general, not just for particular values of mass, speed, etc.
  7. Oct 26, 2008 #6
    the answer itself is wrong.
    But i tried graphing the m: ke function and it shows that when the mass increases the Velocity decreases
    and when the mass increases the momentum increases....
    im confuzed haha what am i doing wrong here

    pB < pA and vB > vA

    was the answer i selected and it came out to be wrong
  8. Oct 26, 2008 #7
    pB<pA and vB>vA are the correct answers. Who says it's wrong?
  9. Oct 26, 2008 #8
    the professor... thanks for your help now that i know that i have someone to confirm it i guess i can email him
  10. Oct 26, 2008 #9
    good luck :-)
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