Why do we say "pound feet of torque" when stating torque?

Question

This is kind of a grammar question, but when stating a measurement, like distance, force, time, or volume, we generally say 5 gallons, 50 lbs, or 10 miles etc. It is understood we are talking about the volume or the distance.

However, when we talk about torque, some times we say "Torque is 10 ft-lbs." Text books do and in engineering we do. But watching car videos or such, people also say "10 ft-lbs OF TORQUE".....isn't that redundant? We know the 10 ft-lbs corresponds to torque, and it the measure/amount of torque.

I know English isn't perfect. Is it just habit and clarification, as to not confuse it with moments, energy, work, as they share common units?

Answer 1

It's convention and a judgement about whether the meaning will be clear if we state a quantity without also stating its meaning.

We also might say the engine has "2 liters displacement" (rather than just "2 liters"), or "200 braking horsepower" (rather than just "200 hp").

On the other hand if we say a milk jug has "2 liters" we can be pretty confident the listener will understand we're talking about the capacity of the jug to hold milk rather than some other characteristic of the milk jug.

Answer 2

Yeah, it's because of the other useful quantities which have those units.

Answer 3

It is because pound-feet correspond to torque, but not only torque. Energy has the same physical dimension of $\mathrm{kg \cdot m^2 \cdot s^{-2}}$ or $\mathrm{lbs \cdot ft^2\cdot s^{-2}}$, yet they are not directly comparable.

In case of torque this is the result of a cross product of two vectors $\vec{r} \times \vec{F}$ (length of the lever arm multiplied by the force); while with energy this is a dot product $\vec{r} \cdot \vec{F}$ (force multiplied by the distance over which it acted, while also considering the cosine of the angle between them).

Another bad habit that can be seen here is the implicit assumption of standard gravity $g$. This used to be common with SI units and "kilogram-force" $\mathrm{kg_F}$ or kiloponds $\mathrm{kp}$, but is now seen very rarely. However, with Imperial and US customary units this usage is still fairly common, "pounds" are often used both as a unit of mass, and as a unit of force, written as $\mathrm{lbf}$ or $\mathrm{lb_f}$.

So a "pound foot" could, in different context, also mean a pound-foot of energy – that is, the energy expended in moving one pound (of force) over a distance of one foot, while implicitly assuming standard gravity $g_0$. However, historically this was solved in English by changing the order and calling this unit a foot-pound instead. The unit can still be encountered in various texts, for instance when describing muzzle energies of small arms.

Answer 4

But watching car videos or such, people also say "10 ft-lbs OF TORQUE".....isn't that redundant?

It is necessary to specify torque since ft-lbs is also a unit for work which is not the same thing as torque.

Hope this helps.