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Question on momentum, impulse, and collisions

  1. Jun 26, 2011 #1
    1. The problem statement, all variables and given/known data
    Question:A 2,800kg car is traveling at a velocity of 22m/s. The car hits the wall and comes to a complete stop. The collision took 0.3 seconds.

    a. What is the cars impulse?
    b. What force did the wall exert on the car?
    c. What force did the car exert on the wall?
    d. Calculate the acceleration rate and the distance the car traveled during its acceleration?

    m=2,800kg
    Vi=22m/s
    Vf=0m/s
    t=0.3 seconds
    F=?

    Am I missing any other variables?
    2. Relevant equations
    FΔt=mVf-mVi

    There must be other equations but I have no idea what they are.


    3. The attempt at a solution
    So for part a this is what I tried
    FΔt=mVf-mVi
    F=(2800)(0)-(2800)(22)/.3
    F=-205333
    This seems wrong to me just because its so big. So what am I doing wrong?

    I don't really know what to do for b, c, and d so if someone could get me started on those that would be great. Thanks!
     
  2. jcsd
  3. Jun 26, 2011 #2

    Doc Al

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    Staff: Mentor

    You're doing nothing wrong. That's the answer to part b. (Be sure to give the correct units.) It seems big because it is. You have a briskly moving heavy vehicle being stopped quickly.

    What about part a?

    For the rest, apply Newton's laws.
     
  4. Jun 26, 2011 #3
    oh I thought I was doing part a. Then how do you find impulse? Is there another equation for that?
    The for part c how is that any different than part b. Is it just the opposite. So 205333. (not negative)
    and finally for part d I understand how to find acceleration (F=ma i think) but how do I find the distance? Do I have to you use kinematics?
     
  5. Jun 26, 2011 #4

    Doc Al

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    You don't need another equation. What's the definition of impulse?
    Right. What law tells you that?
    Yes, you'll need to use kinematics.
     
  6. Jun 26, 2011 #5
    In my book it says the definition of impulse is "the product of the force and the time over which the force acts on an object"
    The wording is what really confuses me. What does "the product of the force mean"?

    Newton's third law?
     
  7. Jun 26, 2011 #6

    Doc Al

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    "Product" just means multiply. The product of A and B is AxB.

    With that in mind, look at the equation you already used.
    Right!
     
  8. Jun 26, 2011 #7
    Okay so for impulse I think it is FΔt=ΔmV
    So I get the same answer for both a and b?
     
  9. Jun 26, 2011 #8

    Doc Al

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    Yes: The left side of that equation is the definition of impulse.
    No. For a you want the impulse; for b you want the force (which is just one factor of the impulse).
     
  10. Jun 26, 2011 #9
    So what equation do I use for b. if I use FΔt=mVf-mVi I get the same answer as I do in a. I thought about using F=ma but I don't have the acceleration.
     
  11. Jun 26, 2011 #10

    Doc Al

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    Staff: Mentor

    You already solved b! Anyway, it's the same equation, but you're solving for F instead of impulse.
     
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