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Equation of uniformly accelerated motion

  1. Apr 20, 2010 #1

    fcb

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    1. The problem statement, all variables and given/known data

    A ball is thrown vertically upwards at 16ms^-1. What is the velocity after 2 seconds
    (acceleration is 9.8 m/s/s)

    2. Relevant equations
    v = u+at
    V=velocity
    u= Initial speed
    a=acceleration down to earth
    t=Time
    3. The attempt at a solution

    I dont quite understand it.
     
  2. jcsd
  3. Apr 20, 2010 #2
    v = u + at

    v = final velocity, u = initial velocity, a = acceleration (i.e. gravity of 9.8 m/s^-1 downwards), t = time

    v = 16 + (-9.8)(2)
    = -3.6 m/s^-1, i.e. the ball is now traveling downwards.
     
  4. Apr 20, 2010 #3

    fcb

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    Thanks so much

    You're a life saver.
     
  5. Apr 20, 2010 #4

    fcb

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    Ohh and is the displacement 51.6m?
     
  6. Apr 20, 2010 #5
    I could be wrong on this, but wouldn't the displacement be 0? There is no motion in the horizontal plane, there is only motion in the vertical plane and the ball returns to its inital starting position, hence displacement is 0.
     
  7. Apr 20, 2010 #6

    fcb

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    Well the answer book said 12.4m

    Using the equation: r=ut + 1/2at^2
    r = displacement
    u = Initial
    t = time
    a = acceleration
     
  8. Apr 20, 2010 #7

    tiny-tim

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    Hi fcb! :smile:

    You've got a sign wrong … this u and a have opposite signs. :wink:
     
  9. Apr 20, 2010 #8

    fcb

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    I thought to myself that can't be right because gravity is 9.8m/s/s relative. So maximum number in 2 seconds can only be 19.6m/s

    thanks heaps lad.
     
  10. Apr 20, 2010 #9
    @tiny-tim : Well his equation is kind of correct, as in fact it's when you substitute acceleration that the - sign comes up. Though you can easily forget to substitute the sign too.

    @dink87522: You kinda confused acceleration units there, it's ms-2 instead of ms-1. I know it's a mistake though, don't worry.
     
  11. Apr 20, 2010 #10

    fcb

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    Is m/s/s the same as ms^-2?
     
  12. Apr 20, 2010 #11

    tiny-tim

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    (try using the X2 tag just above the Reply box :wink:)

    Yes. :smile:
     
  13. Apr 20, 2010 #12

    fcb

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    I am on my phone so I can't use it. Well not I currently know of
     
  14. Apr 20, 2010 #13

    tiny-tim

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    ah! you can type [noparse]"" and "" instead.[/noparse] :wink:
     
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