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Conversion between lbf and Newtons

  1. Aug 3, 2005 #1
    I know there are 4.48 N in 1 pound force. The only way I can derive this is dividing 9.8 m/s^2 by 2.2 lbm/kg. The result shows that there 4.48 N in 1 lbm, but shouldn't it be that there are 4.48 N in 1 lbf. Can anyone clear this up or show a better derivation? Thanks
  2. jcsd
  3. Aug 3, 2005 #2


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    On the surface of the earth, which is indicated by your using 9.81, one pound force IS one pound "mass". (I put "mass" in quotes because, strictly speaking, the pound IS a unit of force, not mass. When you talk about "one lbm" you mean the mass that, at the surface of the earth, would weigh one pound.)
  4. Aug 3, 2005 #3
    so then it would be valid to say that 1 lbf = 1 lbm*ft/s^2 when g has a magnitude of 32.2 ft/s^2?
  5. Aug 3, 2005 #4


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    You are correct. Using Newton's 2nd Law in the fps system, it is expressed as:

    [tex]F = m \frac{a}{g_c}[/tex]

    In this formulation, [tex]a[/tex] is the local acceleration due to gravity and [tex]g_c[/tex] = 32.17405 lb·ft/(lbf·s²)

    You will run into different opinions on the use. I am of the opinion that one should always designate what you are talking about, i.e. [tex]lb_m[/tex] or [tex]lb_f[/tex]
    Last edited: Aug 3, 2005
  6. Aug 3, 2005 #5
    This is why the metric system rocks.
  7. Aug 3, 2005 #6
    You couldn't possibly be anymore correct about that statment. stupid slugs, lbm, and lbf!!!!
  8. Aug 4, 2005 #7


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    Confession: I admit that sometimes, when I am just not thinking correctly, I'll convert a problem to SI units to do the calculations. Shhhh....don't say that too loudly.
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