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Homework Help: Calculations involving freefall

  1. Mar 3, 2009 #1
    1. The problem statement, all variables and given/known data
    A baseball pitcher throws a ball vertically upward and catches it at the same level 4.2s later

    a) with what velocity did the pitcher throw the ball?

    2. Relevant equations

    3. The attempt at a solution

    I put this into givens

    t = 4.2 s
    displacement = 0 m if the ball goes up and then comes back at the same level I figured it has a displacement of 0

    acceleration = -9.81 m/s

    Vf = 0 m/s since it goes up then reaches a climax and it was already thrown with an initial velocity that you are trying to find

    so I decided to use the vf = vi + a(t)

    so.. then I rearranged it so that it was

    vf - ( a ) ( t ) = vi

    which gave me

    -(-9.81)(4.2) = vi

    and then I got 41.2 but thats not the right answer :S so im not sure if its just that im using a wrong equation or that I have the wrong givens I don't know exactly what im doing wrong please point me in the right direction.
  2. jcsd
  3. Mar 3, 2009 #2


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    With y representing the vertical direction you might want to take a look at

    [tex] y = y_o + v_o t + \frac{1}{2} a t^2 [/tex]
  4. Mar 3, 2009 #3
    Remember that your using vf as 0 which is at its climax. The time you used is for the total time (up and down) not just the time to get to the climax.
    Last edited: Mar 3, 2009
  5. Mar 3, 2009 #4
    Alright I got the answer thank you very much. Also I have a test tommorow on this whole kinematics unit and so far I know ab out half what would be my best bet? Should I just keep doing questions until I know them all and asking for help if I can't figure it out myself? I really want to do well as I did so poorly on the quiz :S.
  6. Mar 3, 2009 #5


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    50 years of experience has taught me that the best bet is NOT to put off studying until the night before the exam!
  7. Mar 3, 2009 #6
    Yeah, it's not the best thing to do :s but I have no idea where my priorities are well..that and my time management skills I have put off doing physics homework until today :S. Hopefully all my questions can be answered on the forums by tomorrow. Its just I feel that I don't have the trained mindset for physics just yet, and actually not learning kinematics scares me for whats to come especially since I want to be an engineer. Btw thank you very much for your help as well as Imperitor.
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