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Homework Help: Conservation of Momentum and linear momentum

  1. Mar 7, 2010 #1
    1. The problem statement, all variables and given/known data
    In which one of the following situations is linear momentum not conserved?
    A) A bomb suspended by a string explodes into one hundred fragments.
    B) A bowling ball collides with ten pins.
    C) A golf ball is struck by a club.
    D) An astronaut floating in space throws a hammer away and subsequently moves in the opposite direction.
    E) A tree limb is struck by lightning and falls to the ground.

    2. Relevant equations

    Momentum=mass(velocity)

    3. The attempt at a solution

    I have no idea... I thought momentum was always conserved? Can someone help explain this please? Thanks!!!
     
  2. jcsd
  3. Mar 7, 2010 #2

    lightgrav

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    do you know how to compute the momentum in the lightning bolt? (prolly not yet)
    can you figure out a way to measure the Earth's speed moving up to meet the branch?
    (Nobel prize if you do it)
     
  4. Mar 7, 2010 #3

    vela

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    Say you are standing on the the ground. You're at rest, so your momentum is zero. If you now jump, you rise through the air, and your momentum is no longer zero. Clearly, your momentum isn't conserved. However, look at the system consisting of you and the Earth. When you jump up, the Earth recoils, and the momentum of the system as a whole is conserved. Two cars colliding is another example. The momentum of an individual car is obviously not conserved, but the momentum of the two-car system is conserved. So the question of whether momentum is conserved or not depends on what you're talking about, on what is included in the system.
     
  5. Mar 7, 2010 #4
    I think the question is faulty.We may not be able to prove by experiment that momentum is conserved in some of the situations described but theory based on other experiments suggests that within closed systems momentum is always conserved.
     
  6. Mar 7, 2010 #5
    Thanks for the responses! So, when you say that it must remain in a closed environment, are you saying the answer would be the one dealing with lightning or the one dealing with outer space.. sorry I'm still a bit confused. :confused:
     
  7. Mar 7, 2010 #6

    vela

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    Well, determining that is the whole point of the problem. I agree with Dadface, though. The question isn't worded very well. You're just going to have to use some common sense in deciding what you think they were asking and therefore what the correct answer is.
     
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