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Homework Help: Physics/mechanics, forces accelaration

  1. Apr 2, 2010 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown horizontal out of a window 18m above the ground. it is thrown horizontally at 5.0ms. show that it takes about 2 seconds to drop.

    2. Relevant equations

    S=ut+1/2at^2

    3. The attempt at a solution

    u=0
    a=9.8
    s=18
    18=0t+1/2*9.8t^2
    36=9.8t^2
    sqrt3.6734=t
    t=1.92

    is this near enough too 2 to be the right answer or have i made a mistake some where i was not sure if the horizontal force would have any effect on the time taken to drop. Any help would be much appreciated.
     
  2. jcsd
  3. Apr 2, 2010 #2
    That's perfect. 1.92 is certainly "about 2 seconds" in anyone's book!
    The horizontal component of the ball's motion is independent of the vertical, gravitational, component; so your method is fine.
    (That horizontal velocity of 5m/s doesn't come into this calculation, but is probably needed for further calculations, such as how far away from the window does the ball land?)

    Oh, and welcome to Physics Forums!
     
  4. Apr 2, 2010 #3
    Thanks i just needed conformation because i got to the answer partially using trial and error to find the right method and I'm impressed you managed to guess the exact next question on the paper.
     
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