Conversion between lbf and Newtons

abercrombiems02

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

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HallsofIvy

Homework Helper
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.)

abercrombiems02

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?

FredGarvin

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

$$F = m \frac{a}{g_c}$$

In this formulation, $$a$$ is the local acceleration due to gravity and $$g_c$$ = 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. $$lb_m$$ or $$lb_f$$

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zwtipp05

This is why the metric system rocks.

abercrombiems02

You couldn't possibly be anymore correct about that statment. stupid slugs, lbm, and lbf!!!!

FredGarvin

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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