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Homework Help: Simple harmonic motion and buoyant force

  1. Apr 18, 2009 #1
    1. The problem statement, all variables and given/known data

    A cylindrical wooden log is loaded with lead at one end so that it floats upright in water. The length of the submerged portion is L = 2.56m. The log is set into vertical oscillation.
    (a) Show that the oscillation is simple harmonic
    (b) Find the period of oscillation

    Neglect dampening effect by water

    2. Relevant equations

    3. The attempt at a solution

    So I drew three pictures, one of the log in equilibrium, one with the log slightly raised, and one with the log slightly more submerged.

    Case 2: Slightly Raised

    [tex]F_b < W[/tex]

    [tex]F_net = F_b - W = -m*a[/tex]

    [tex]\rho_w * (A*L - A*\Delta y) * g - m*g = -m*\frac{dy^2}{dt^s}[/tex]

    and, if this is correct, i'm not given the cross sectional area A or the density of the log, so not sure where to go.

    Case 3: Slightly Pushed Down

    [tex]F_b > W[/tex]

    [tex]F_net = F_b - W = m*a[/tex]

    [tex]\rho_w * (A*L + A*\Delta y) * g - m*g = m*\frac{dy^2}{dt^s}[/tex]

    Same problem as case 2.

    Some insight on where to go next would be nice :D

    I'm thinking maybe trying to find the pressure differences at each delta y, but I'm sure there is a simpler way than that?

  2. jcsd
  3. Apr 18, 2009 #2
    Any clues?
  4. Apr 18, 2009 #3


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    Staff Emeritus
    Science Advisor

    One knows W = mg, where m is the mass of the log.

    At the equilibrium point, Fb = W. What is the expression for Fb in terms of ρ, where ρ = density of water, and L.

    Let [itex]\xi[/itex] be the displacement from equilibrium, so one must be concerned about [tex]\ddot{\xi}[/tex].

    Think about the restoring force per unit length.
  5. Apr 19, 2009 #4
    At equilibrium, we have Fb = W = Weight of displaced water = ρw * g * V = ρw * g * L * A

    Where L * A is the length times the cross sectional area - but I am not given A, but that is a volume in terms of L?
    Last edited: Apr 19, 2009
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