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Kinematics problem

  1. Jul 30, 2016 #1
    1. The problem statement, all variables and given/known data
    Concept Simulation 2.3 provides some background for this problem. A ball is thrown vertically upward, which is the positive direction. A little later it returns to its point of release. The ball is in the air for a total time of 8.21 s. What is its initial velocity? Neglect air resistance.

    2. Relevant equations

    3. The attempt at a solution
    known values
    xf = 0 because it comes back to where it was released from
    xo = 0 because initial displacement is going to be 0
    a = -9.8m/s2
    t = 8.21 s
    so couldn't you solve by using the kinematic formula xf=xo+vo+1/2at2
    and then solving for vo
    leaving you with -vo=xo+1/2at2-xf
    and because both the initial displacement and final displacement are 0
    -vo = 1/2 at2 ????
    so -vo=-9.8⋅8.212
    which gives me -330.62 which would be just 330.62m/s but it is incorrect. pls helpp

  2. jcsd
  3. Jul 30, 2016 #2


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    Can you spot the mistake in this equation?
  4. Jul 30, 2016 #3
    ahhhh i havent put in vo t
    because the equation is really xf=xo+vot+1/2at2 right?
  5. Jul 30, 2016 #4


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    Yes, good.
  6. Jul 30, 2016 #5
    so by just dividing my answer by t (8.21) ill get the vo right?
  7. Jul 30, 2016 #6


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    Yes. But be careful with the signs. In your first post you wrote
    But you have a negative sign on both sides, so this will give a positive value for v0.
  8. Jul 30, 2016 #7
    yeah okay, thank you!
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