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

 

[HallsofIvy]

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 $$lb_f$$

 

[zwtipp05]

This is why the metric system rocks.

 

[abercrombiems02]

You couldn't possibly be anymore correct about that statement. 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.