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Homework Help: Perfect inelastic collison

  1. Jun 24, 2010 #1
    1. The problem statement, all variables and given/known data
    A 6 kg object moving with a speed of 9.9 m/s collides with a 19 kg object moving with a velocity of 7.2 m/s in a direction 24* from the initial direction of motion of the 6 kg object. What is speed of the two objects after the collision if they remain stuck together?


    2. Relevant equations

    Px = m1v1icos[tex]\theta[/tex]+m2v2icos[tex]\theta[/tex] = m1v1fcos[tex]\theta[/tex] + m2v2fcos[tex]\theta[/tex]

    Py = m1v1isin[tex]\theta[/tex]+m2v2isin[tex]\theta[/tex] = m1v1fsin[tex]\theta[/tex] + m2v2fsin[tex]\theta[/tex]
    3. The attempt at a solution

    Edit I realized my original way was faulty in its logic.

    So what I did was I began by finding the initial and final momenta components in both the x and y directions for the two particles. Being that they stick together their final velocity ought to be the same.

    [tex]\Sigma[/tex]Pxi = m1v1cos[tex]\theta[/tex] + m2v2cos[tex]\theta[/tex]

    =184.37302

    [tex]\Sigma[/tex]Pyi = m1v1sin[tex]\theta[/tex] + m2v2sin[tex]\theta[/tex]

    =55.641573

    As I mentioned their final velocity should be equal since they are stuck together. Equating my initial momenta to my final momenta in the x direction I get the following:

    Vf = [tex]\frac{(m1v1cos\theta + m2v2cos\theta}{(m1+m2)cos\theta}[/tex]

    =8.072856 m/s

    When I checked it using the momenta of the y direction I come to a different value for the final velocity which doesn't make sense so I know I made a mistake somewhere... Thanks in advance.

    Joe



    Edit: I tried a new way, but
     
    Last edited: Jun 24, 2010
  2. jcsd
  3. Jun 24, 2010 #2

    Doc Al

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    Looks good to me.
     
  4. Jun 24, 2010 #3
    Thanks Doc Al, are you refering to the first way I solved the problem or refering to this edited version? The first way did not work, so thats why I tried a new method. Thanks for all your help in all of my posts.

    Joe
     
  5. Jun 24, 2010 #4

    vela

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    Actually, your first attempt was the correct solution. How do you know the answer you got is wrong?
     
  6. Jun 24, 2010 #5

    Doc Al

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    I was referring to your original post, which looked fine to me. (I haven't looked at your revision.)
     
  7. Jun 24, 2010 #6
    It is an online homework module, and this is not the first question that I have been correct about and it kicked back my answer as incorrect...Thanks for your assistance.

    Joe
     
  8. Jun 24, 2010 #7

    vela

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    Thought so. Could be you're entering too many significant figures.
     
  9. Jun 24, 2010 #8
    Do you know the direction (angle) after collision?
     
  10. Jun 24, 2010 #9

    Doc Al

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    Why would that matter?
     
  11. Jun 24, 2010 #10
    I mean after collision, two objects stuck together. So velocities are the same but problem didn't mention direction is angle theta. I think angles are not the same before and after collision.
     
  12. Jun 25, 2010 #11

    Doc Al

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    True, but the direction of the final velocity doesn't matter for this problem. All they ask for is the speed.
     
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