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Direct proportion question

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    A ball falls vertically after being dropped. The ball falls a distance, d metres, in a time of t seconds.
    d is directly proportional to the square of t.



    I worked out the first question which was the formula:

    d=5t^2


    Another question was, calculate the time the ball takes to fall 605m

    So i did

    5t^2=d

    5t^2=605

    5t^2=121

    5t=11s

    That was the final answer, however i was wondering, do i not divide by 5 to get t on its own?

    If not for this one, do i ever? Or do you leave a number next to it always?


    Thank you!
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    did you not divide by 5 to get from 605 to 121
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    (Original post by TenOfThem)
    did you not divide by 5 to get from 605 to 121


    Aaah right yeh! So it would just be t^2=121

    then Sqrt that? :O?
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    yup
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    (Original post by DeathByBSOD)
    My Maths is kinda rusty, :s

    so i think its:

    d=5t^2

    5t^2=d

    5t^2=605

    5t=\sqrt 605

    5t=\sqrt 605
    Therefore,
    t=\sqrt 605/5 ~ 4.92s
    This is incorrect ... the OP now has the correct answer
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    (Original post by DeathByBSOD)
    My Maths is kinda rusty, :s but where did you get the 5 FROM?

    so i think its:

    d=t^2

    t^2=d

    t^2=605

    t=\sqrt 605


    Therefore,

    = 24.60s (2d.p)



    the 5 is from the first part of the question, when 20 is directly proportion to 4k so k=5
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    (Original post by IgorYakov)
    the 5 is from the first part of the question, when 20 is directly proportion to 4k so k=5

    Ah Ok thanks.

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Updated: April 19, 2012
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