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Homework Help: Basic Buoyant Force HW Question

  1. Feb 28, 2008 #1
    [SOLVED] Basic Buoyant Force HW Question

    First time doing this, so bare with me:)

    1. The problem statement, all variables and given/known data

    A 53kg girl dives off a raft 4m square floating in a freshwater lake. By how much does the raft rise?

    Density of water = 1000kg/m^3

    Use the equation for buoyant force. Then solve the equation for V.

    Of course, volume = L x W x H.

    g = 9.8 m/s^2

    Fb = weight of girl in Newtons

    3. The attempt at a solution

    My teacher gives us the equation for buoyant force as Fb = (density)(volume)(acc.due to gravity).

    The answer is to be given in cms (centimeters).

    Fb = (1000)(???)(9.8)

    I am trying to get the volume, knowing that there is 4m squared. I am just missing the height.

    Thanks for any ideas...
  2. jcsd
  3. Feb 28, 2008 #2
    Okay, just looking over my notes one more time. My teacher said to use the weight of the girl (53kg) to equal Fb.

    How do I convert 53kg to Newtons? I can use online calculators, but that is not helping me learn. It looks like it is 9.8 x 53 = 519.4 Newtons

    So I should be getting Fb = (1000)(V)(9.8) or 519.4 = (1000)(V)(9.8).

    I just have to solve for V it looks like.
    Last edited: Feb 28, 2008
  4. Feb 28, 2008 #3
    I am trying to solve for volume. I have 4m ^2 given in the problem.

    Okay, the formula to work from is:

    1. Fb = (Density in kg/m^3)(Volume)(Acc. Due to Gravity)
    2. 519.4 = (1000 kg/m^3)(V)(9.8 m/s^2)
    3. 519.4 = 9800V
    4. Volume = 18.87 (units?)

    Is this correct so far?

    How do I convert this to centimeters.?
  5. Mar 2, 2008 #4

    Shooting Star

    User Avatar
    Homework Helper

    How do you get 4 from 3?

    Are you able to understand the concept behind what you are doing? Refer to this similar problem in PF and see.

  6. Mar 17, 2008 #5
    Sorry for the late response. I figured out the problem.

    The professor made a typo in our question, and she corrected it. I solved it very quickly after that.

    Thanks anyways!
  7. Oct 16, 2011 #6
    Re: [SOLVED] Basic Buoyant Force HW Question

    if Buoyant force equals to weight of girl in Newtons
    Fb = W
    (Density in kg/m^3)*(Volume)*(Acc. Due to Gravity) = (mass)*(Acc. Due to Gravity)
    (Density in kg/m^3)*(Volume) = (mass)

    Volume = A.h

    (Density in kg/m^3)*(A.h) = (mass)

    h = (mass) / (Density in kg/m^3)*(A) = 0.01325 m = 1,325 cm

    Last edited by a moderator: Apr 26, 2017
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