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2d Inellastic collisions

  1. Aug 11, 2005 #1
    Hi all,

    I was wondering if someone can help me, its been a while since I did classical mechanics, I have a question I would appreciate a little help from anyone!

    2 objects with the same mass m and same velocity v have an inelastic collision . After the collision the 2 object system of mass 2*m moves with speed v/2. What is the angle "Theta" between the final line of motion and either of the initial velocities?

    All I need is a way to get started if anyone can help!!

    Thanks alot guys

    hhh79bigo :bugeye:
  2. jcsd
  3. Aug 11, 2005 #2


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    How about using conservation of momentum?
  4. Aug 11, 2005 #3
    So should I use

    2mv=2m(v/2)cos(x) where x is the angle described as theta.

    in which case theta is cos-1(2) which cannot be done

    or can it Im not sure

    someone enlighten me please!!!!!!


    hhh79bigo :surprised
  5. Aug 11, 2005 #4


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    HOW are to objects with the same VELOCITY going to collide? If they have the same velocity then they are both going at the same speed in the same direction and will always have a constant distance between them! Did you mean the same speed?

    "What is the angle "Theta" between the final line of motion and either of the initial velocities?"
    You need to make it clear which of the initial velocities theta is measured with respect to- and the angle between the initial velocities has to come into this somewhere. I would be inclined to set up a coordinate system with the x-axis along the initial line of motion of one of the bodies.
  6. Aug 11, 2005 #5
    The initial velocities are the same, the problem I have been given has said so. The only thought in my mind is that the 2 bodies collide from an angle but the question doesn't specify that.

    The first question was copied word for word from my script. The only other thought is that the 1st body is set off at an earlier time than that of the 2nd body. But again my question doesnt specify.
  7. Aug 11, 2005 #6


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    The two ball are moving with velocities of same magnitude v. This must be the sense of the question. The total momentum of the two balls will not be 2mv (in magnitude) then.
  8. Aug 11, 2005 #7
    hhhhhm i dont fully understand if the masses are the same and the velocity is the same and they are moving linearly, then surely the total momentum before the collision is 2mv

    correct me if Im wrong

    Im just trying to understand, because this part of mechanics has always confused me
  9. Aug 11, 2005 #8


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    They are not moving linearly. If you and your friend driving with same velocities on a straight track how one can catch the other, the distance between you two will remain constant, as Hallsofivy has already posted. For collision if they have same speed they must move at an angle.
  10. Aug 11, 2005 #9
    Ok its just that it doesnt specify that in my question from my proffessor it says that the "Velocities" are the same in which case both the direction and magnitude maybe its a type error. I appreciate your help though. I understand that the velocities cant be the same Im just confused :)
  11. Aug 11, 2005 #10


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    and with the symmetry we can immediatly guess that the final line of motion must bisect the angle between the initial velocities. Is it??????
  12. Aug 11, 2005 #11
    I presume so however there is no specifications to the initial angles.

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