# Homework Help: Confusion with units of work

1. Oct 2, 2012

### Mangoes

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. Oct 3, 2012

### voko

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
3. Oct 3, 2012

### ehild

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

4. Oct 3, 2012

### Mangoes

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.

5. Oct 3, 2012

### ehild

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
6. Oct 3, 2012

### voko

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.

7. Oct 3, 2012

### ehild

kilogram-force is more logical then "kilogram-weight" (kilogramsĂșly) as it was used in Hungary (sĂșly=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