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Homework Help: Different units in Bernoulli's equation's terms?

  1. Nov 23, 2006 #1
    When I check units in the terms in Bernoulli's equation, they do not match:
    p1 + 1/2 (row) v1^2 = p2 + 1/2 (row) v2^2

    On the left hand side of the equation, the first term, pressure must have units of force/unit area; say lb/ft^2. The units in the second term are the product of

    row, the density in, say, lb/ft^3
    v1^2, the square of speed, say, ft^2/sec^2,

    or [lb/ft^3][ft^2/sec^2] = [ lb/(ft sec^2) ] and this is different from the [lb/ft^2] of the first term. Why?

  2. jcsd
  3. Nov 23, 2006 #2
    A lb isn't a unit of force, it's a units of mass.
  4. Nov 23, 2006 #3
    I thought so!

    I haven't done physics in a log time and I am rusty on the basics. I thoght I had the problem that you mention but was not sure. I'll try again using proper force units and I'll come back.
  5. Nov 23, 2006 #4
    I am back!

    After reviewing my units, I found that in the SI system, the unit of force is the pound (lb), the unit of mass is the slug and that of acceleration is ft/s^2. I remember that 1lb = 1 slug ft/sec^2. Armed with this knowledge, I atacked the problem as follows:

    pressure (p) has units of lb/ft^2 = (slug ft)/(sec^2 ft^2) =
    slug/(ft sec^2)

    velocity has units of ft/sec

    row, the density has units of slug/ft^3

    Then, the second term in the equation must have units of:

    (row)v^2 = [slug/ft^3][ft^2/sec^2] = slug/(ft sec^2)] which matches the pressure units of the first term.

    Thanks for the reply. It motivated me to arrive at the right answer.
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