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Homework Help: Free Fall of Rock off Tower

  1. Jan 29, 2012 #1
    1. The problem statement, all variables and given/known data

    A rock is thrown vertically upward with a speed of 17.0 from the roof of a building that is 60.0 above the ground. Assume free fall. In how many seconds after being thrown does the rock strike the ground?

    2. Relevant equations

    I used v^2 = vinitial^2 + 2a(y - yinitial) to find the speed when it hits the ground. I am trying to find the time and can't seem to locate a good equation to use.
     
  2. jcsd
  3. Jan 29, 2012 #2

    Delphi51

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    From your initial and final v's, you could get the time with Vf = Vi + a*t.
    Or start again and use good old y = Vi*t + ½at².
    Best to do it both ways as a check!
     
  4. Jan 29, 2012 #3
    I used Vf = Vi + a*t with Vf = 38.3, Vi = 17, a = 9.8 and came up with t = 2.17 but that is the incorrect answer for when the rock hits the ground. The rock went up at a starting point of 60m, then came down past 60m to hit the ground.
     
  5. Jan 29, 2012 #4

    Delphi51

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    I got a time of just over 5 and a half seconds by both methods. Of course I often make mistakes now - I'm over 60. Show your calc if you want a critique.
     
  6. Jan 29, 2012 #5
    t = (vf - vi)/a so (38.3-17)/9.8 unless i used the wrong formula
     
  7. Jan 29, 2012 #6

    Delphi51

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    Initial v is 17. Final v is -38.3. Difference is 55.3.
     
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