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Math Help - displacement vector

  1. #1
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    displacement vector

    A particle is tossed vertically upwards from a point O . At time t seconds later , the position vector of the particle from O is given by r=(12t-5t^2)j m . Find the time when the particle is at least 4m above O .

    shouldn't it be done liek this :

    12t-5t^2=4 , solving gives t=2 or t=2/5

    but the answer given is 1.6 s ??
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  2. #2
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    Hello hooke
    Quote Originally Posted by hooke View Post
    A particle is tossed vertically upwards from a point O . At time t seconds later , the position vector of the particle from O is given by r=(12t-5t^2)j m . Find the time when the particle is at least 4m above O .

    shouldn't it be done liek this :

    12t-5t^2=4 , solving gives t=2 or t=2/5

    but the answer given is 1.6 s ??
    Read the question: Find the time, not Find the times. 2-0.4 = 1.6.

    Grandad
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  3. #3
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    Quote Originally Posted by Grandad View Post
    Hello hookeRead the question: Find the time, not Find the times. 2-0.4 = 1.6.

    Grandad
    oh , so is it how long the particle would remain at least 4m away from O , to rephrase this ?
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  4. #4
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    Quote Originally Posted by hooke View Post
    oh , so is it how long the particle would remain at least 4m away from O , to rephrase this ?
    Yes.
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