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Math Help - Kinematics... please help

  1. #1
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    Question Kinematics... please help

    hey guys i got my maths exam in two days and need urgent help in the kinematics area.

    i get some of the basic stuff but anyways the question i'm struggling with is

    Particle moves in a straight line so its position x cm relative to O at time t seconds (t0)
    is given by x=t
    -7t+12
    FIND
    a. initial position
    t=0
    so therefore x=
    0-7(0)+12
    x=12 cm to right of O

    b. position at t=5
    so therefore
    x=5
    -7(5)+12
    x=25-35+12
    x=2 cm to the right of O when t=5
    c. initial velocity
    d. when and where velocity = 0
    e. average velocity in the first 5s
    f. average speed in the first 5s

    alright the ones in red are the ones im sturggling with

    anyone help would be greatly appreciated... thanks a lot guys and girls





    Last edited by jimzer; November 23rd 2007 at 10:20 PM. Reason: added my workings out
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by jimzer View Post
    hey guys i got my maths exam in two days and need urgent help in the kinematics area.

    i get some of the basic stuff but anyways the question i'm struggling with is

    Particle moves in a straight line so its position x cm relative to O at time t seconds (t0)
    is given by x=t-7t+12
    FIND
    a. initial position
    t=0
    so therefore x=0-7(0)+12
    x=12 cm to right of O
    b. position at t=5
    so therefore
    x=5-7(5)+12
    x=25-35+12
    x=2 cm to the right of O when t=5
    c. initial velocity
    The velocity is:

    \frac{d}{dt}x=2t-7

    So the initial velocity is:

    x_0 = -7

    d. when and where velocity = 0
    e. average velocity in the first 5s
    f. average speed in the first 5s
    You should be able to do d. yourself.

    e. The average velocity in the first 5 seconds is the change in position
    in those five seconds divided by 5, so:

     <br />
\bar{v}_{(0,5)} = \frac{x_{t=5}-x_{t=0}}{5}<br />

    f. The average speed is the average of |v| over the interval.

    RonL
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  3. #3
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    thanks a lot for the reply man! really appreciate it

    but what i dont get is how you got

    x=t-7t+12

    to become

    2t-7

    how does the

    (d/dt)x formula work?
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  4. #4
    Grand Panjandrum
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    Quote Originally Posted by jimzer View Post
    thanks a lot for the reply man! really appreciate it

    but what i dont get is how you got

    x=t-7t+12

    to become

    2t-7

    how does the

    (d/dt)x formula work?
    Calculus - differentiation of polynomials.

    RonL
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  5. #5
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by jimzer View Post
    thanks a lot for the reply man! really appreciate it

    but what i dont get is how you got

    x=t-7t+12

    to become

    2t-7

    how does the

    (d/dt)x formula work?
    If you don't know that you have no business to be trying to answer this question. There's no way to do this problem without Calculus.

    Edit: Well, I suppose you could do it using the basic kinematics equations for motion with constant acceleration, but to expect a student to come up with the solution on their own would be like expecting Aristotle to come up with Kepler's Laws: in principle he could have done it, but it wouldn't be very likely.

    -Dan
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