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Homework Help: Calculate force in english units

  1. Feb 20, 2013 #1
    1. The problem statement, all variables and given/known data
    Determine the force necessary to accelerate a mass of 20 lbm at a rate of 60 ft/s^2 vertically upward.

    2. Relevant equations
    How do you properly account for weight?
    What exactly is pound force?
    How do you go from pound mass (lbm) to pound force (lbf)?

    3. The attempt at a solution

    F - mg = ma
    F = m(g + a)

    Unfortunately This is how I would do it in SI units. However it is not correct in English units. I'm very confused on how it goes from pound mass to pound force
  2. jcsd
  3. Feb 20, 2013 #2


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    1 lbm is that amount of mass which, when accelerated at 32.174 ft/s^2, equals 1 lb force.

    F = ma applies to all measurement systems with consistent units.
  4. Feb 20, 2013 #3
    i understand that..but in the solutions. the equation was

    F - 20 = (20/32.2)(60)


    They took 20 on the left side to automatically be lbf even though it was originally lbm
  5. Feb 20, 2013 #4
    Sorry to be pedantic but it is incorrect to call it "english units" as we in the UK use SI/metric units for almost everything. Only our road network and cars use miles, same as in the USA I think. The correct term would be "imperial units" (as opposed to metric units).
  6. Feb 20, 2013 #5


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    A one pound mass weighs one pound on planet Earth. Since weight is mg, a one pound mass is really 1/32 of a slug. The slug is the proper mass unit in the imperial system and must be used when using f= ma.
  7. Feb 20, 2013 #6
    As PhantomJay indicated,

    m (slugs) x a (ft/s2) = F (lbf)

    1 lbm=(1/32.2) slug
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