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Very simple conversion problem

  1. Jan 5, 2013 #1
    1. The problem statement, all variables and given/known data
    convert 20lb*ft to Newton*meters


    2. Relevant equations



    3. 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
     
  2. jcsd
  3. Jan 5, 2013 #2

    TSny

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    If there are 3.2808 ft in one meter, then how many meters are in one foot?
     
  4. Jan 5, 2013 #3
    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?
     
  5. Jan 5, 2013 #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]
     
    Last edited: Jan 5, 2013
  6. Jan 5, 2013 #5
    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)
     
  7. Jan 5, 2013 #6
    oh i see that's what you're saying, so what would a proper conversion look like step by step here?
     
  8. Jan 5, 2013 #7
    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]
     
  9. Jan 5, 2013 #8
    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?
     
  10. Jan 5, 2013 #9
    Yes. That's right. I always do conversions this way, and it has never failed me in 50 years of experience.
     
  11. Jan 5, 2013 #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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