Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Oscillation of a Mass in Liquid (Densities and Cross-Sectional Area Known)

  1. Sep 6, 2008 #1
    1. The problem statement, all variables and given/known data

    A body of uniform cross-sectional area A = 1 cm^2 and of mass density p = 0.8 g/cm^3 floats in a liquid of density p0 = 1 g/cm^3 and at equilibrium displaces a volume V = 0.8 cm^3. Show that the period of small oscillations about the equilibrium position is given by:

    T = 2pi(V/gA)^(1/2)

    Where g is the gravitational field strength.

    2. Relevant equations

    T = 2pi(m/k)^(1/2)
    m = pV

    3. The attempt at a solution

    I think that, since the general solution for the period of an oscillation (in the relevant equations section) is very similar to the solution I'm looking for, that I'm probably trying to prove that what is under the square roots is equal to eachother, that is, m/k = V/gA.

    The mass of an object is m = pV. (I also found an equation stating that at equilibrium, the mass of an object floating in a liquid is m = p0V (density of the liquid times the volume of liquid displaced). I don't know which of these I want to use, or even how it's possible for them to both be true.)

    But if you use either of those equations (let's say m = pV because right now, that makes more sense to me :)), then you can plug that value for mass into the m/k = V/gA equation, and you end up with p = k/gA. So, I feel like if I can prove that, then I have my solution, but I don't know where to go from here.

    Some questions: What is the difference between m = pV and m = p0V, and which do I want to use? What is k in this problem? How is density related to gravity and cross-sectional area (or is it)? And, of course, am I on the right track at all? Thanks in advance! :)
  2. jcsd
  3. Sep 6, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    One's approach is correct. Think about the form of the equation for the oscillation of a mass m on a spring with constant, k. The restoring force is proportional to the displacement, x, from equilibrium. Similar, the bouyant force is related to the volume of the mass in the water, and there is some equilibrium depth. Push the mass down by some small displacement, and when the push is removed the bouyant force restores the mass to it's equilibrium position, or perhaps it bobs up and down if the push is removed instanteously.

    Note that the problem mentions small displacement, which implies an approximation of the restoring force, similar to kx.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook