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Homework Help: Impulse, help!

  1. Nov 3, 2006 #1
    I am stuck on a part of my physics hw, please help if you can

    "Object A has mass mA = 7 kg and initial momentum vector pA,i = < 17, -8, 0 > kg · m/s, just before it strikes object B, which has mass mB = 11 kg. Just before the collision object B has initial momentum vector pB,i = < 3, 5, 0 > kg · m/s.
    The forces that A and B exert on each other are very large but last for a very short time. If we choose a time interval from just before to just after the collision, what is the approximate value of the impulse applied to the two-object system due to forces exerted on the system by objects outside the system?"

    i already calculated the total momentum of the system to be <20, -3, 0> kg m/s but i just cant figure out how to calculate the impulse without a given force or time.
    Last edited: Nov 3, 2006
  2. jcsd
  3. Nov 3, 2006 #2


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    There is no way to tell from the information given. Are you sure you stated the problem correctly?
  4. Jun 24, 2009 #3


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    First of all, there's no time, so impulse cannot be found.

    Also, you cannot analyze a collision of any sort withou knowledge of what kind of collision it is. Therefore the total momentum of the system is a useless value, because you don't know if the collision is elastic, inelastic, or something in between.

    Finally, it asks for the force exerted by an external force, but it isn't stated what happens to the system. If the time is immediately after the collision, there is no external force on the system at all yet.

    Actually, that might be the answer. No force or impulse at all from the outside.
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