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Collision- I don't think conservation of linear momentum can be established

  1. Nov 17, 2008 #1


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    1. The problem statement, all variables and given/known data
    The system is a table with 2 particles on it. A particle of mass 3kg is going at West with a speed of [tex]\frac{1m}{s}[/tex]. Another particle of 4 kg is going to North with a speed of [tex]\frac{5m}{s}[/tex]. When they are close enough they interact during 2 seconds and the first particle leaves out with an angle of about 53° ([tex]\arctan \left( \frac{4}{3}\right)[/tex]° ) measured from East to North, with a speed of [tex]\frac{5m}{s}[/tex].
    a)Find the magnitude and the direction of the second particle after the interaction.
    b).... (I'll do it alone)
    c)Tell whether the collision is an explosive one or a plastic one.

    2. The attempt at a solution I never faced such a problem before and I was going to ask to a helper at my University tomorrow but I just can't wait for that...
    I'm more than 100% sure that the linear momentum is not conserved (2s is a lot for an interaction, furthermore that's why they ask if it was an explosive or a plastic collision. That means that the mechanical energy is not conserved hence nor is the linear momentum).
    My guess : The second particle will leave out following an opposite direction with respect to the first particle but I'm not sure at all. If so, I don't know why it is like this. And much less I can say about its speed.
    How can I tackle this problem?
    Thanks in advance!
  2. jcsd
  3. Nov 17, 2008 #2

    Doc Al

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    Staff: Mentor

    Just because mechanical energy might not be conserved has nothing to do with whether momentum is conserved. As long as they only interact with each other, linear momentum will be conserved.
  4. Nov 17, 2008 #3


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    Thank you Doc Al for your very fast reply. I am... :eek: but now I remember a university helper saying that if the interaction lasts quite long the linear momentum wouldn't be conserved if there is an external force acting on the particles... ok here there is no force that influence the system.
    But still it's strange that the first particle gets such a direction. Maybe an explosion or something like that. (I'll try to find out).
    Thanks again...
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