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How long does it take a ball to be back

  1. Mar 11, 2012 #1
    1. The problem statement, all variables and given/known data
    A ball is trown vertically straight up in the air. After t seconds the ball has reached the higth

    Question : after how many sconds is the ball back to the spot it was trown from ?



    2. Relevant equations
    I have figured out that i need to differentate.


    3. The attempt at a solution

    So i thought i had to solve it this was :

    60t – 5t2 --> 60-10t but it seems like this isnt the answer

    besides i have made a drawing of the information i got . The orange line shows how the ball is thrown straight up, and the blue shows that i have to solve how much that and the orange are when they are added when reffereing to time in seconds .
     

    Attached Files:

    Last edited: Mar 11, 2012
  2. jcsd
  3. Mar 11, 2012 #2

    PeterO

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    Homework Helper

    one formula for vertical motion is S = ut + 1/2 gt2 (acceleration under g)

    By comparison to s(t) = 60t – 5t2

    we can see the initial upward velocity is 60 m-1 and g is taken as 10 ms-2.

    In that case you can easily establish how long it takes to stop moving up, and it takes that much again to come back down.

    EDIT: you don't need differentiation - just find the values of t when S(t) = 0 - and one of those times is at time zero - when it all began.
     
  4. Mar 11, 2012 #3
    ok thank you
     
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