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Time taken to reach the water's surface

  1. Jun 9, 2009 #1
    1. The problem statement, all variables and given/known data

    A thin spherical shell with a mass of 3.30 kg and a diameter of 0.225 m is filled with helium (density = 0.180 kg/m3). It is then released from rest on the bottom of a pool of water that is 3.77 m deep. Neglecting frictional effects, determine the value of that acceleration.
    Correct, computer gets: 7.91E+00 m/s^2

    How long does it take for the top of the shell to reach the water's surface?
    2. Relevant equations

    3. The attempt at a solution

    so i know that to reach the top of the container it would travel 3.77 m and that vinitial = 0m/s and we know that acceleration is 7.91m/s^2 so i used the equation

    [tex]\Delta[/tex]d= Vi*t+0.5at2
    and when i solve this i get a negative under the root which i know isnt right..pls help
  2. jcsd
  3. Jun 9, 2009 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    In doing this problem, you've implicitly defined the "up" direction as being positive. So why is your delta d negative?
  4. Jun 9, 2009 #3
    even if i put my delta d as positive i still get the wrong answer
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