Very simple conversion problem

  • Thread starter coffeebird
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  • #1
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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
 

Answers and Replies

  • #2
TSny
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If there are 3.2808 ft in one meter, then how many meters are in one foot?
 
  • #3
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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?
 
  • #4
TSny
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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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  • #5
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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)
 
  • #6
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oh i see that's what you're saying, so what would a proper conversion look like step by step here?
 
  • #7
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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.

[tex]20 (ft)(lb)=20 (ft)(lb)\frac{3.448(N)}{1(lb)} \frac{1(m)}{3.2808(ft)}= 27.116(N)(m)[/tex]
 
  • #8
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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?
 
  • #9
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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.
 
  • #10
TSny
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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.)
 

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