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Math Help - Displacement and velocity

  1. #1
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    Displacement and velocity

    Suppose that the velocity (in meters per second) for a particle moving along a line is given by the function v(t)=t^2-4t-12 on interval 1<t<7

    a) Find the displacement
    b) find the displacement traveled by the particle during the given time interval


    ok for part a i know you have to take the anti-derivative and you get t^3/3 -4t^2/2 -12t and interval 1 to 7

    [7^3-((4(7)^2)/2)-12(7) ] - [1^3/3-((4(1)^2)/2)-12(1)]

    i got -194/3 meters


    I'm not sure what to do about part b. And is my answer and method for part a correct?
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  2. #2
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    Re: Displacement and velocity

    In part (a), you didn't divide 7^3 by 3, so your answer should be -54 meters.

    Part (b) probably should have "distance" instead of "displacement". What they want is this: the particle is moving to the left (v is negative) from t=1 to t=6. At t=6, it changes direction and moves to the right. They want the total distance traveled, which would be:

    \int_1^7|v(t)|\,dt=\int_1^6(-v(t))\,dt+\int_6^7v(t)\,dt

    - Hollywood
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    Re: Displacement and velocity

    Quote Originally Posted by hollywood View Post
    In part (a), you didn't divide 7^3 by 3, so your answer should be -54 meters.

    Part (b) probably should have "distance" instead of "displacement". What they want is this: the particle is moving to the left (v is negative) from t=1 to t=6. At t=6, it changes direction and moves to the right. They want the total distance traveled, which would be:

    \int_1^7|v(t)|\,dt=\int_1^6(-v(t))\,dt+\int_6^7v(t)\,dt

    - Hollywood
    thanks! these little mathematical errors are killing me. Do we use the numbers from part a for the velocity? I really appreciate your help!


    edit: i think I would do this right? :


    6 t2 +4t + 12dt
    1
    +
    7 t2 - 4t12 dt
    6
    Last edited by Steelers72; February 24th 2013 at 01:05 PM.
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  4. #4
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    Re: Displacement and velocity

    Yes, that's correct.

    - Hollywood
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    Re: Displacement and velocity

    After plugging in everything, I got 187/3 for the integral from 1 to 6. and I got and I got 13/3 for 6 to 7 integral. Added together, gives 200/3 and that is wrong to the system. I did it multiple times and checked on two different calculators..
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  6. #6
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    Re: Displacement and velocity

    I get 175/3 for the integral from 1 to 6.

    - Hollywood
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    Re: Displacement and velocity

    Quote Originally Posted by hollywood View Post
    In part (a), you didn't divide 7^3 by 3, so your answer should be -54 meters.
    Yes I'm annoying!

    At some point a positive direction should be chosen. Here you are tacitly stating that the initial velocity is in the "negative" direction, or to the left side of the paper. This is what most would choose so no real problem arises here, but in a more complex problem it might lead to confusion unless you state it.

    -Dan
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    Re: Displacement and velocity

    Quote Originally Posted by hollywood View Post
    I get 175/3 for the integral from 1 to 6.

    - Hollywood
    thanks. For the final answer which was correct to the system I calculated to be 188/3 when you add 1 to 6 and 6 to 7
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  9. #9
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    Re: Displacement and velocity

    Yes, 188/3 is correct.

    And yes, topsquark, part of defining our variables would be to state that positive velocity is to the right.

    - Hollywood
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