Okay, I’ve noticed that on every torque wrench I’ve ever seen the readout is in Newtons (which is a unit of force and torque is a force). However, they also read out in foot pounds which is a unit of ENERGY and that is wrong. POUNDS are a unit of force so why do they say foot-pounds? To me this makes as much sense as having a measuring cup that measures cubic centimeters and square inches.
You got it all wrong. Torque is not a force but force multipled by the length of the arm. Foot*pounds is correct. Newtons is not.
There seems to be widespread confusion between the foot-pound (the unit of work required to raise a mass of one pound a distance of one foot) and the proper SAE unit of torque, which is the pound-foot. A pound-foot is the torque exerted by a pound of mass at a distance of a foot from the axis of torsion.
This confusion apparently extends to many Web-based conversion tables, and the makers of torque wrenches. I have two, from different manufacturers, both of which incorrectly read “foot-pounds.” (Until just a few days ago, when I read an item about the pound-foot in the current issue of Road & Track, the confusion also extended to me.)
Here’s a site that gets it right.
Why is this distinction important? It’s the same unit; same exact dimensions and value.
You’d better check those torque wrenches again. The metric measurement should say N-m, for Newton-meters. That’s what it says on my wrench.
No, not really. When you exert a force of 1 lb for a distance of 1 ft you have done 1 lb*ft of work. It is a unit of work or energy.
OTOH, torque is not work or energy. It is the force multiplied by the length of the arm. To obtain the work done you cannot multiply the force by the length of the arm but you have to multiply the force by the distance traveled which is the arc of circumference traveled.
If I am turning a wheel with a torque of 1 ftlb, for every complete turn I have done 2pi() lb*ft of work.
Different concepts even if the dimensions are the same.
At my job everything is measured in inch/pounds. In fact last week I got me a brand new 15 in/lb cordless screwgun in my tool kit. No more dragging air hoses around the airplane!!!
OK, I agree they are different concepts, and I realize that SI uses different units for the two (Joule vs. Newton-meter). But I’m still not sure if it’s important to distinguish between “pound-foot” and “foot-pound” - I can’t think of any other compound units where the order matters.
It should be N-m, not just N.
The difference between ft-lbf versus lbf-ft seems IMO to be one of parlance. Equation-wise, they have the same dimensions. And in some professions, like mine, parlance wars are the equivalent of “Enterprise versus Star Destroyer”.
Conventions exist for a reason. It is only a matter of being clear about whether you are talking about torque or work. As long as it is clear to everybody you can say it whichever way you want but following conventions makes it easier for everybody to understand and avoid mistakes.
But the N-m is the proper metric unit of both work and torque. Different concept, same dimensions…but same unit as well.
And yes, the OP should notice that his torque wrench, unless very poorly designed, has N-m on it. Force times Length
The difference between work and torque (even using the same dimenstions) becomes clear when working in vector math. For the unfamiliar, vectors are quantities having both magnitude and direction.
Torque is the cross product of force and the length of the moment arm. The result is the torque vector, which is perpendicular to both.
T = F x r
Work is the dot product of force and the distance over which it is applied. The result is a scalar. That is, it has magnitude, but no direction.
W = F • d
Nitpick: don’t you need to specify g? Torque and Energy should be force x distance not mass x distance iirc.
Anyway, after some googling, it appears that:
- They do in fact have the same units.
- But the convention is to use J or foot-pounds for energy and Nm or pound-feet for torque, to try and avoid confusion.
- While it’s normal to gloss over this, torque is defined by physicists as a vector, (while energy is definately a scalar) so they can’t be compared.
I don’t know why I opened this post, but I am really glad I did; I wish my physics instructor had explained this as neatly as Shade.
So this is why my pruning sheers and bolt cutters have such long handles and why I hold my hammer at the far end rather than at the forte, right?
If I finally understand torque, it’s why levers work, right?
Thanks to all for the quick replies.
Actually, no. Though most people tend to think of the pound as the American system’s unit of mass, it is actually the unit of force, analogous to S.I.'s newton. Mass in the American system is measured in slugs.
Almost no one needs to know this, since almost any application where the distinction is important would use S.I. units.
OK, fair enough. But I think “a mass of one pound” is not the clearest way to express “a mass weighing one pound” as opposed to “a pound-mass.”
j66: Thanks. Yes, that’s why levers work
Torque is actually a pseudovector, but so what? Velocity is a vector and the speed of light is a scalar, but they can still be compared. And they still use the same units.
For a magnetic dipole mu in a magnetic field, the torque is given by:
[ul]tau = mu × B[/ul]and the interaction energy is given by:
[ul]U = -mu · B[/ul]How could they possibly not have the same units?
T = r x F
The resultant vector ends up in the opposite direction if you do it backwards.
You are correct. Thanks for the friendly reminder. I was spouting from memory, and wasn’t thinking about the non-commutativity of vector multiplication.
Then again, it would have been more conventional to write t (“tau” if it doesn’t format right on everyone’s machines) instead of T, as well.
But I think my point was still clear.