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English vs. SI units of mass and weight

  1. Sep 18, 2005 #1

    We know that the SI unit for mass is the kg, and that for force is the newton, where 1 N = 1 Kg x the acceleration due to gravity.

    On the other hand, the English unit for mass is the slug, and that for force is the pound, and again we have a similar relationship based on Newton's second law.

    My question is, where does the american pound (lb) come from? and where does the conversion 1 kg = 2.2 lb come from?

  2. jcsd
  3. Sep 18, 2005 #2
    same place the Newton came from. F=ma===>W=mg, g=32.2ft/s^2

    where do any conversions come from?
  4. Sep 18, 2005 #3


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    that is actually not the definition of the Newton and it is, in fact, wrong. the Newton is the amount of force needed to accelerate 1 kg at a rate of 1 meter/sec^2. since the acceleration of gravity (on the Earth) is about 9.8 times that, then the amount of force that accelerates a kilogram at that rate is 9.8 N.

    1 pound of force will accelerate one slug at a rate of 1 ft/sec^2. in civil engineering on this side of the pond (U.S.), rather than talk of the slug, we define a pound mass (lbm) to be the amount of mass that weighs one pound (or exerts one pound force, lbf, onto the ground). so 1 lbf = 1 lbm * 32.174 ft/sec^2.

    oh geez. that goes back. i don't remember what the most fundamental conversion factor is. i think that 1 oz = 28.35 grams and 2.54 cm = 1 inch. i think those are the most fundamental numbers plus what the acceleration of gravity is.
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