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Kinematics: Throwing ball up the building with unknown initial velocity

  1. Oct 7, 2009 #1
    1. The problem statement, all variables and given/known data

    At the top of the building that's 65m high, a guy throws a ball upward.
    The ball comes back down and hits the ground in 4 seconds.

    A)What is the initial velocity when the guy threw the ball?

    B)What is the highest height of the ball?

    C)At what time does the ball comes back to him?


    I'm just stumped how the hell I can start off even.....

    There was a hw problem very similar to this but it gave initial velocity..this problem doesn't and I don't know how to start....

    Any advice would be fantastic..

    (This was my first physics quiz...is it even possible to solve this?!!?)

    Please help...


    2. Relevant equations

    all the kinematic equations..

    https://www.physicsforums.com/showpost.php?p=905663&postcount=2


    3. The attempt at a solution

    I can't...can't even start with A
     
  2. jcsd
  3. Oct 7, 2009 #2
    It should be possible. Think about splitting this problem up into a symmetric projectile motion part and an asymmetric projectile motion part (i.e. up and down are not equal). What do you know about the velocity of the ball as it passes the guy on the way down? (assuming no air resistance)
     
  4. Oct 7, 2009 #3
    I tried exactly that...but no avail...

    Well you can say when the ball reaches the top...and starts to fall you can do the following..

    Initial V= 0 m/s
    A= -10m/s/s
    but that's it..

    Still t and delta x are unknown...
     
  5. Oct 7, 2009 #4
    you can say that, but you can also say something when the ball reaches the side of the building going downward. remember that if it's allowed to travel the same distance up as it was down, the velocity going down will be directly related to the initial velocity going up.

    Perhaps you should set up your equations for a parabolic arc for a ball with an initial velocity v_i going up and coming down to the same height. What is final velocity before it hits the ground (at that height)?
     
  6. Oct 7, 2009 #5

    So how do I go about solving the initial v?

    Any step by step advice?
     
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