English vs. SI units of mass and weight

  1. Hi,

    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. same place the Newton came from. F=ma===>W=mg, g=32.2ft/s^2

    where do any conversions come from?
  4. 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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