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Confusion with units of work

  1. Oct 2, 2012 #1
    Hey there,

    I'm in a math class and we're covering some applications to physics and although I completely understand the math behind it and am getting the correct answers, I'm a little iffy on how some of the units work.

    Would anyone be kind enough to tell me whether or not my thought process here is correct? Since my book covers the theory behind the math, it doesn't really cover how the units work.

    Work = Force*Distance
    Force = Mass*Acceleration

    Let's say that we measured mass in kg and acceleration in m/s^2.

    F = (kg)(m/s^2) = Newton

    Plugging this into work and taking distance to be measured in meters:

    W = [(kg)(m/s^2)]*(m) = (kg)(m^2/s^2) = Joules

    Is the above correct?

    Also, my book uses notation I'm unfamiliar with (ft - lb) and calls it foot-pounds.

    Would it be dimensionally equal to Joules but using feet and pounds instead?
    That is, would it be equal to (lb)(ft^2/s^2)?

    Thanks for any help.
    I'm just concerned I'm not understanding this correctly and on a test I'll write units that don't mean what I actually mean to say.
     
  2. jcsd
  3. Oct 3, 2012 #2
    The SI units are correct. For foot-pounds: http://en.wikipedia.org/wiki/Foot-pound_(energy [Broken])
     
    Last edited by a moderator: May 6, 2017
  4. Oct 3, 2012 #3

    ehild

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    It is correct. Work is force *distance, the unit is newton-meter=kg*m^2/s^2 and it has the name "joule" (J)

    As far as I know, "Pound" is unit of weight and that is force. So you can measure work in ft-lb units (foot*pound) .http://en.wikipedia.org/wiki/Foot-pound_(energy)

    ehild
     
  5. Oct 3, 2012 #4
    Oh wow, I'm being an idiot with the foot pounds thing.

    Guess I'm too used to using pounds as a measure of mass in everyday life.

    Thanks guys.
     
  6. Oct 3, 2012 #5

    ehild

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    It is really very confusing. In the old times when I was at school, we used "kg " as unit of weight and the unit of work was m-kg. Later it was said the kg(weight) is the force the Earth attracts an 1 kg mass. The same happened to the pound, I guess.

    See http://hyperphysics.phy-astr.gsu.edu/hbase/mass.html

    ehild
     
    Last edited: Oct 3, 2012
  7. Oct 3, 2012 #6
    Technically, "kg" as a unit of force was named "kilogram-force" and denoted as "kgf". Of course everybody abbreviated that to just kilogram and kg, and confusion frequently ensued.
     
  8. Oct 3, 2012 #7

    ehild

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    kilogram-force is more logical then "kilogram-weight" (kilogramsúly) as it was used in Hungary (súly=weight). It was very confusing, as it is with the different meanings of "pound". I read it is force in the US? http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/slug.html#c1

    ehild
     
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