# Very simple conversion problem

## Homework Statement

convert 20lb*ft to Newton*meters

## The Attempt at a Solution

this is driving me nuts....so if 1lb= 4.448N, then we have 4.448*20 N*ft, and if there are 3.2808ft in one meter, then we should have 4.448*20*3.2808 N*m! so why is the answer instead 27.116

TSny
Homework Helper
Gold Member
If there are 3.2808 ft in one meter, then how many meters are in one foot?

1/3.2808.... but what's weirding me out is something i just noticed....i mean if there is that much force per foot, then shouldn't there be more force per meter?

TSny
Homework Helper
Gold Member
It's not force per distance but force times distance. [Note added: 20 lb per ft would be written 20 lb/ft, but you have 20 lb*ft]

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that's actually how they wrote the problem in my online statics thing- exactly like i did (except with the multiplication dot in the middle which i don't know how to make here)

oh i see that's what you're saying, so what would a proper conversion look like step by step here?

Chestermiller
Mentor
A very simple foolproof trick for converting units is to write the conversion factor with units, and then to cancel out units as if they were algebraic variables.

$$20 (ft)(lb)=20 (ft)(lb)\frac{3.448(N)}{1(lb)} \frac{1(m)}{3.2808(ft)}= 27.116(N)(m)$$

okay, thank you : ) i guess i've just never really taken the time to write out proper conversions- i should probably start with that. so the unit you want goes on top, like meters/feet instead of feet/meters, right? and you mean 4.448 N per pound, i think?

Chestermiller
Mentor
okay, thank you : ) i guess i've just never really taken the time to write out proper conversions- i should probably start with that. so the unit you want goes on top, like meters/feet instead of feet/meters, right? and you mean 4.448 N per pound, i think?
Yes. That's right. I always do conversions this way, and it has never failed me in 50 years of experience.

TSny
Homework Helper
Gold Member
Coffeebird, you may not need it but there are lots of tutorials on the web for converting units. More on Chestermiller's method here. (Annoying music thrown in for free.)