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Fluid pressure question in US units

  1. Sep 24, 2013 #1
    1. The problem statement, all variables and given/known data
    The ocean liner Titanic lies under 12500 feet of water at the bottom of the Atlantic Ocean. (The density of water is 62.4 lb/ft{}^3.)

    What is the water pressure at the Titanic? (Give your answer in both pounds per square foot and pounds per square inch.)


    2. Relevant equations
    pressure=ro*g*h


    3. The attempt at a solution
    at first, all I did was multiply 62.4*32(g in ft/s^2)*12500. The answer was wrong and then I think I found out why. I multiplied the units (lb/ft^3)*(ft/s^2)*ft and got lb/(ft*s^2) instead of lb/ft^2. Anyone get what I'm supposed to do for this problem? I never had this problem when working with metric units since kg/(m*s^2) converts nicely to N/m^2 Thanks!
     
  2. jcsd
  3. Sep 24, 2013 #2
    In the US system, the unit of mass that applies for use in Newton's second law and for getting the gravitational force on a body is the slug. The mass in slugs is equal to the mass in lbm divided by the average gravitational acceleration at the earth's surface 32.2. Alternately, some people express Newton's second law in a little different form:

    F = ma/gc

    where F is the force in lbf, m is the mass of the body in lbm, a is the acceleration in ft/sec2 and

    [tex]g_c=32.2 \frac{lb_m}{lb_f}\frac{ft}{sec^2}[/tex]

    The difficulty with the english system is the term pounds (lb) is used for both mass (lbm) and force (lbf); these are different entities.
     
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