Very simple conversion problem

[coffeebird]

Homework Statement

convert 20lb*ft to Newton*meters

Homework Equations

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]

If there are 3.2808 ft in one meter, then how many meters are in one foot?

 

[coffeebird]

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]

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]

 

[coffeebird]

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)

 

[coffeebird]

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

 

[Chestermiller]

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.

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

 

[coffeebird]

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]

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]

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.)