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Math Help - displacement

  1. #1
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    displacement

    A car is moving along a straight road. It passes a point A. After t second its distance, s m, from A is given by s = 10t - tsquared

    a) find the time when the car passes through A again?

    Don't know

    b) find ds/dt? = 10 - 2t

    c) find the maximum distance of the car from A.

    We need to find ds/dt = 0
    10 - 2t = 0
    10 = 2t
    t = 5 seconds

    d) find the speed of the car 3 seconds after the passing of A.
    10 - 6 = 4m/s

    e) find the acceleration of the car

    f''(x) which is -2

    Please could someone help with question a) and check the rest? Many thanks in advance.
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  2. #2
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    b - d are fine.

    For a), it's asking you "when does the car get back to A?", in other words, "when is the distance from A again 0?".

    So it's asking you to find t when s = 0.
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  3. #3
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    Quote Originally Posted by Prove It View Post
    b - d are fine.

    For a), it's asking you "when does the car get back to A?", in other words, "when is the distance from A again 0?".

    So it's asking you to find t when s = 0.

    When the acceleration is negative does that mean the car is slowing down?
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  4. #4
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    so lets see

    10t - tsquared = 0
    - tsquared + 10 t = 0

    What do you do from here?
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  5. #5
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    No, the negative tells the direction it is going in.

    For the other part

    -t^2 + 10t = 0

    t(-t + 10) = 0

    t = 0 or -t + 10 = 0.

    Go from here.
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  6. #6
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    or t = 10

    Thanks!
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