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Homework Help: Fast Car Momentum questions

  1. Feb 6, 2008 #1
    The problem statement, all variables and given/known data
    A fast-moving massive car has a head-on collision with a slow-moving light car. Which car has a greater impulse?

    The attempt at a solution

    I think more information is needed to answer the question. But could the answer be that the impulse is the same?

    The problem statement, all variables and given/known data
    A cannon with a long barrel fires a shell faster because the shell experiences a greater:
    a) acceleration
    c)change in inertia

    The attempt at a solution
    I say impulse but then i want to say none of the above. is it impulse because of the increase intime?

    The problem statement, all variables and given/known data
    If a fast-moving object strikes you, you'll experience less force of impact if you can:
    a)shorten the time the momentum decreases.
    b)increase the time the momentum decreases.
    c)either shorten or increase the time, for the result is the same.

    The attempt at a solution
    the wording of the problem just confuses me, because at first i thought it was b but then thought a and went back to b. i just dont get the "the momentum" decreases part of the two answers.
  2. jcsd
  3. Feb 6, 2008 #2
    For your first problem, I think you are correct in thinking the magnitude of the impulses are the same. One way to explain it is with conservation of momentum, because momentum lost by the first object (its impulse) must be momentum gained by the second object (its impulse).

    For your second question, you are told that the shell is fired faster if the barrel is longer. The shell's velocity is greater, so its momentum is greater, so its change in momentum is greater. I think you're right in thinking it is impulse. Why did you second guess yourself?

    Perhaps the answer choices would be better phrased as "increase/shorten the time it takes for the object to lose its momentum." In either case, the greater the force, the greater the force of impact on both you and the object (you would be hurt more).

    You have the right idea; don't let the wording confuse you. Think about how force is related to momentum and impulse. Think about real world cases: what's the difference between falling onto a bed and falling onto concrete?
  4. Feb 6, 2008 #3

    Shooting Star

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    Homework Helper

    The force exerted on one car is equal and opp to the force exerted on the other car, and the time is the same. So, by definition, the magnitude of the impulse is the same. You can also think in terms of change of momentum, as mentioned in post #2.

    The impulse is more because it stays in the barrel for longer time under an (almost) constant force, so F*t is more.

    The change in momentum which is equal to F_avg*t is same. What would make the force lesser?
  5. Feb 7, 2008 #4
    thanks for the clearing up these questions for me
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