corrado33

A newton is a kg*m/s^2. A newton meter is kg*m^2/s^2.

A ft lb is... well... lb * ft. They aren't dimensionally equivalent. Or rather, you can't convert one to the other. It's missing a time component. How can both be used to measure torque?

Are we simply ignoring part of the lb*ft? Is it really lb*ft/s^2?

79pmooney

Here you get into the mess that is our English system of units. I am not good at describing what is going on, but I will do my best.

In the metric world we have weight (grams, kilograms) and force (newtons = kilogram * meter / seconds squared). In the English system we have two pounds, pounds mass (lbm) - not commonly used - and pounds force or just pounds (lbf = lbm * the acceleration of gravity or lbm * 32 feet / second squared)

So: lb*ft is really lbf*ft and the time component is there. Confused? Welcome to freshman engineering in America! The English system that the English no longer use! What does that say about us? (Thank you , Ronald Reagan, for sparing us from the metric system. )

Ben

fietsbob

Ask Napoleon Bonaparte , it was He who instituted the Metric System. because the other one Was English, I Expect.

https://en.wikipedia.org/wiki/Newton_metre Force in Newtons applied at the end of a lever measured in Meters .

[Though Sir Isaac Newton was English Born and had a lifetime Gig at Cambridge U]

HillRider

In simple and practical terms, N-m and foot-pounds (or inch-pounds) are convertible so if you have the torque spec in one unit system it's a simple calculation to convert it to another. The conversion is: 1 N-m = 0.74 ft-pounds or 8.85 in-pounds. In this case the pounds are pounds force, not mass.

BTW, despite the UK officially using the metric system, the older "Imperial" measures have by no means disappeared from daily usage.

fietsbob

The Royal Foot Used to Vary Depending on the Regent's foot's size during his Reign..

The Meter Ended that ... with the French Monarchy.

dsbrantjr

We are right up there with Myanmar and Liberia in clinging to Imperial measure; it's also in common use in the UK although they are officially metric.

DiabloScott

Torque is force times distance of the lever arm. Newton and pound are both units of force - they ARE dimensionally equivalent.

The bust is that English uses force units for mass also. And don't ever confuse pound with poundal or your spacecraft will miss its target.

dsbrantjr

Actually, it is correctly written lbf*ft or lbf-ft; lbf incorporates the s^2 term, as Ben points out.

prathmann

No, they are dimensionally equivalent. Both are the product of a force (i.e. Newtons or lbs.) times a distance (meters or feet).

1 N = 0.225 lb. and 1 m = 3.28 ft. leading to the conversion factor already given by HillRider

fietsbob

Italy bought British Machine tools, so.. you find the Italian BB thread is 36mm Diameter, with a 24 Thread per Inch Pitch.

Andy_K 

This is why freshman physics students in the U.S. don't understand the concept of mass. The real problem is that we think (incorrectly) that we can convert from pounds to kilograms.

dr_lha

We really should just be measuring torque in Joules.

corrado33

Ah! It's not "pound" it's "pound force." It's a different unit. People are just being lazy. Got it. Was confused earlier. (I'm a chemist, not an engineer.)

dr_lha

It's not really laziness, just historical. pound and "pound force" are interchangeable, as pound can both mean "force" and "mass". So "foot pound" is correct, but if you don't understand that it's talking about pound "the force" rather than pound "the mass" you might get confused.

The units are mainly there to tell you what it is you are measuring though, everyone knows foot pound is measuring torque. In the metric system, this is why we say "Newton Meter" for torque and not "Joule", even though they are dimensionally the same. Equally we don't just quote everything in unit combos of kg meter and seconds, even though it's perfectly possible to do that for most units (1 Nm = 1 J = 1 kg m^2 s^-2).

AnkleWork

In order to understand the vast difference between torque and work, you'll have to consider force as the vector quantity it actually is.

dr_lha

To be clear, I was not suggesting the Torque and Work are the same thing, only that they have the same units. My first comment in this thread was actually a joke.

RubeRad

When I was there in 2000, they did things like selling packages of meat by the 450g (~1lb), because the populace (or at least the previous generation) didn't really want to change. Much like us I guess.

dr_lha

I refuse to believe that the UK has gone metric until they start selling their beer in liters and measure their roads in kilometers.

RubeRad

Right, pints of beer are sacrosanct! You'd think they wouldn't mind switching to 500ml though, since it's a little more, but if you think that you don't realize that an English pint is 20oz, not 16oz like an american pint. 20oz~591ml, so they'd be losing one good gulp if they switched.

I seem to recall double mi/km on the road signs, but I might be wrong about that. Didn't have a car when I lived there. Petrol was sold in liters though, I'm pretty sure.

Earl Grey

Those really are the major holdouts I've noticed in everyday life there. That and, as RubeRad says, food etc is often sold in strangely familiar but non-round-numbers.

Wheever

There's an app for that: https://itunes.apple.com/us/app/conv...337224035?mt=8

habilis

It ain't work until it achieves some distance.

habilis

I have a torque wrench that measures both Newton-meters and inch-pounds. The numbers appear side by side on the handle, so you can see exactly how they equate.

HillRider

Petrol was sold in liters but priced in £/gallon when I was last there in the early 2000's. Imperial gallon, BTW.

AnkleWork

. . . or an increase in potential, or a few other things.