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Homework Help: Power Problem

  1. Oct 19, 2011 #1
    1. The problem statement, all variables and given/known data

    Humans have about v = 60 cm3
    of blood per kilogram of body mass and blood makes a
    complete circuit in t = 20 s seconds. For a
    human body of thickness 0.2 m and height
    1.8 m, estimate the change in power output of
    the heart when the body is standing up on a
    horizontal surface compared to when the body
    is lying down on the same surface. Assume
    that the density of blood is ρ = 0.001 kg/cm3
    and the mass of the human body is 66 kg. Use
    g = 9.8 m/s

    2. Relevant equations
    P = W/t

    Conservation of energy

    3. The attempt at a solution

    Completely clueless really. I assume that I need to use conservation of energy to find the work.

    I set mgh as the PE of the body for both cases when h = 1.8 (standing horizontal) and h=0.3 (lying down)

    however, I have no idea how I could find the work that the blood does. Can anyone explain to me how I can go about doing that?
  2. jcsd
  3. Oct 19, 2011 #2


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    Homework Helper

    Do you know how to calculate the work needed to lift mass m to height h? You could use work done = potential energy gained, so W = mgh.
    It will be a bit tricky to see what mass of blood is involved and how much it is lifted in each case.

    Or is it lifted? You could argue that siphoning of the blood means there is no real lift involved at all. Oh, I shouldn't have mentioned that this thought - clearly you are asked to calculate lifting to a height so don't think about siphoning. Blood that initially falls down will have to be lifted back up to the heart while blood that is initially lifted will then fall back to the heart, so maybe a factor of 1/2 involved.
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