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Homework Help: Momentum and Such question

  1. Nov 15, 2004 #1
    I'm a bit stumped on the following, as I'm not sure how to distinguish between the following. Thanks to all those who shed some light.

    A bug and a windshield of a fast moving car collide. Indicate and explain whether each statement is true or false.

    a. The forces of impact on the bug and the windshield are the same size.
    True? I suppose because of Newton's third law the force would be indentical.

    b. The impulse on the bug and the car are the same size.
    I'm a bit confused. I know FxT=Impulse=Change In Momentum. I would suppose that the big's momentum loss would be greater, but I'm only speaking from a practical/obvious POV.

    c. The change in speed of the bug and the car is the same
    Falso. Perhaps their related somehow, but I know thats false.

    d. The changes in momentum of the bug and the car are the same size?
    I would say true, because wouldn't the system after the collision have to somehow equal the momentum of the two seperate systems before it? I assume this could only be achieved if the momentum's were related.

    Thank you to all
  2. jcsd
  3. Nov 15, 2004 #2


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    Homework Helper

    a) I think Newton's Third Law is a good way to argue that. The force of the bug on the windshield must be the same magnitude as the force of the windshield on the bug.

    b) Impulse is change in momentum. Since linear momentum is absolutely conserved, and since we are considering only the bug and the car, whatever momentum one of them loses must be gained by the other one - otherwise, there'd be a net gain or loss.

    c) We know the change in momentum is the same for the two, so the change in the product of the mass times the velocity must be the same for the two. Since the mass of the windshield and the bug are both constant (a good assumption for the windshield, probably not for the bug, but let's not get disgusting here...), the difference must come in the change in velocity. Since the mass of the bug is (presumably, unless you live in Florida) much less than the mass of the windshield, the change in velocity of the bug must be correspondingly greater.

    The question, however, doesn't ask about velocity but speed, and this could make a difference - I'll leave you to puzzle out why it doesn't.

    d) Already answered in part b.
  4. Nov 16, 2004 #3
    thanks so much
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