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Real quick help needed with KINEMATIC EQUATION

  1. Dec 10, 2012 #1
    Number7-3.png

    So I'm given...

    In the y-direction
    vo = 0 m/s
    vf = 7.5 m/s (is the speed of the ball thrown straight up)
    g = -9.81 m/s2 (when it falls back down)

    yo = 0 m
    yf = 1.5 m (at this height above the ground, the bat connects with the ball)

    t = ?

    I tried doing this...

    yf = yo + vyot + (1/2)gt2
    1.5 = 0 + (0)t + (1/2)(-9.81)t
    1.5 = -4.905t
    0.30581 = t

    :(
    Help?
     
  2. jcsd
  3. Dec 10, 2012 #2

    TSny

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    Projectile motion starts just after the ball leaves the bat. So, the initial velocity is not zero. The final velocity would be the velocity the ball strikes the ground (not given).

    Similarly, think about the initial and final values of y.
     
  4. Dec 10, 2012 #3
    Ok... So

    vo = 7.5 m/s (initial speed of the ball when thrown straight up)
    vf = ____ m/s (the final speed when the ball strikes the ground)
    g = -9.81 m/s2 (when it falls back down)

    yo = 1.5 m
    yf = 0 m (at this height above the ground, the bat connects with the ball)

    t = ?

    Like that?
     
  5. Dec 10, 2012 #4

    TSny

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    Yes. Good.
     
  6. Dec 10, 2012 #5
    yf = yo + vyot + (1/2)gt2

    0 = 1.5 + 7.5t + (1/2)(-9.81)t2
    0 = 1.5 + 7.5t - 4.095t2
    0 = - 4.095t2 + 7.5t + 1.5

    Quadratic formula:

    [ -b ± √(b2 - 4ac) ] / 2a
    = [ -7.5 ± √(7.52 - 4(-4.095)(1.5)) ] / 2(-4.095)
    = [ -7.5 ± √(56.25 +24.57) ] / -8.19
    = [ -7.5 ± √(80.82) ] / -8.19
    = ( -7.5 ± 8.989 ) / -8.19
    = ( -7.5 - 8.989 ) / -8.19
    = -16.48999444 / -8.19
    = -16.48999444 / -8.19
    t = 2.013 s


    What did I do wrong??
     
    Last edited: Dec 10, 2012
  7. Dec 10, 2012 #6

    TSny

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    Looks like just a slip. Half of 9.81 is not 4.095
     
  8. Dec 10, 2012 #7
    Oh yeah...i just realized that..the answer is supposed to be A :)

    Can you help me out with one more?

    Number9-1.png

    I tried doing....
    a2 + b2 = c2 after making my triangle...

    3.22 + b2 = 122
    b2 = 144-10.24
    b = √(133.76)
    b = 11.56

    And then I tried doing

    12*sin (18.9) = 3.89 m
     
  9. Dec 10, 2012 #8

    haruspex

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    Don't you think gravity might come into it somewhere?
     
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