Pounds/foot (and other nonsense) MEW 226

I have now looked at Wikipedia which states

The foot-pound force (symbol: ft·lb_f or ft·lbf), or simply foot-pound (symbol: ft·lb) is a unit of work or energy

Also says Not to be confused with Pound-foot (torque) or Foot-poundal.

Then on another page

A pound-foot (lb·ft or lb_f·ft) is a unit of torque

I have to admit that it goes on to say

However, the torque unit is often still referred to as the foot-pound

To me that is a very risky thing to do and easily open to errors. Would be like using the term voltage for both pressure and power

Harold

Harold is correct.

I would always taught to remember "torque" as a "pound" at one "foot" as if using a force of one pound at the end of a lever one foot long. not a "foot at one pound" or ft-lb.

thus "pound-feet" is lb-ft is "Torque"

Len.

And nobody has mentioned " slugs" which had a brief life. Just as I was being taught, but not afterwards.No wonder I get confused.

Why not throw in a few ergs and dynes from the cgs system which pre-dates the mks system which both predate the SI system
As for the foot pound or pound foot its just a convention but its easier to roll of the tongue as foot pound whereas the metric system flows off the tongue more readily as Newton metre . Try repeating metrekilogram fast.

Bob D
Me thinks we have similar thoughts, Gordon

Edited By Robert Dodds on 02/03/2015 17:11:51

Did anyone notice if the reconditioned drill worked?????

You missed the "per minute," chap ;o)
Force (Torque) per time unit is indeed power.

"If I were to tell you that the result of experiment was 15 pound-feet can you tell me what that answer means?"
Without knowing the context I haven't the foggiest. These footpound terms are ridiculously vague.

This is why SI units (of which there are only seven principle ones) were introduced.

Hi, I have a good quality Torque wrench and on the illustration leaflet it is stated that the imprerial scale is foot pound.

However, quoting from my dictionary,

" Torque; 1. The moment of a force, a measure of its tendency to produce torsion and rotation about an axis, equal to the vector product of the radius vector from the axis of rotation to the point of application of the force by the force applied. 2. Broadly a turning or twisting force. [ Latin torquére, to twist ]"

To my understanding of reading this I have come to the conclusion that the term, ft.lbs. is the correct one, the moment equalling feet and the force equalling pounds, although I understand its meaning either way.

The OP's original concern is the use of the forward slash, which I understand as being divisonal in this context.

Regards Nick.

Edited By Nicholas Farr on 02/03/2015 19:11:40

I agree Capstan I did not fully take in the per time aspect of what you are saying. But now see we are basically in agreement. Foot pounds, on its own, is a quantity of force, but foot pounds needs quantifying by "per minute"if to be used for power. As you say the terms used like that are ridiculously vague.

That is why myself and others on this thread were taught, a very long time ago, that pounds-feet were used for power which eliminates the ambiguity. I am though surprised to learn that that approach has been shelved.

I think we can now agree to agree.

Harold

It doesn't matter if you work out length x force or force x length the answer is the same (they are commutative).

Any distinction between foot-pound and pound-feet is a semantic one, not a mathematical; or physical one.

I see this on wikipedia:

The SI unit for energy or work is the joule. It is dimensionally equivalent to a force of one newton acting over a distance of one metre, but it is not used for torque. Energy and torque are entirely different concepts, so the practice of using different unit names (i.e., reserving newton metres for torque and using only joules for energy) helps avoid mistakes and misunderstandings.^[7] The dimensional equivalence of these units, of course, is not simply a coincidence: A torque of 1 N⋅m applied through a full revolution will require an energy of exactly 2π joules.

Now I hate to admit it, but if the article had used newton-metres I would not have struggled!

Neil

I think I should opt out I see I have made a mistake I should have said

That is why myself and others on this thread were taught, a very long time ago, that foot- pounds is used only for power and pounds-feet for force which eliminates the ambiguity. I am though surprised to learn that that approach has been shelved. Too late to challenge the tutors though it was 66 years ago.

Harold

The National Physics Laboratory published a booklet many years ago titled Changing to the Metric System. It gave the Imperial units of moment of force (torque) as lbf ft. In my career this was translated to lbf.ft. I believe when assembling a dimension from basics the order of use is energy, mass, force, distance and then time but I have not seen this in print.

In addition lb is a pound mass while lbf is a pound force which is different due to Newton's laws. Things then get difficult: 1 lbf = 32.2 ft/s2 / 32.2 * 1 lb (I cannot type an superscript 2) where the first 32.2 is the acceleration due to gravity and the second 32.2 is a constant to get over the difficulty of expecting 1 lb mass to weigh 1lbf. This was always a problem with the Imperial system which the SI system overcame by introducing the Newton as the unit of force. So I now buy vegetables in kilograms!

JA

Uh oh Neil, You forgot the time in your last post (and I don't mean it was tea break time)..... a newton force acting over a metre - sounds closer to pressure to me!

Joules tie everything up nicely as they link kinetic energy to electrical energy (1 volt through 1 ohm for 1 second)

Mark

Out of interest (and I rarely take wiki at face value, but I will make an exception for this) I noticed this:

"The yottajoule (YJ) is equal to one septillion (10²⁴) joules. This is approximately the amount of energy required to heat the entire volume of water on Earth by 1 °C. The thermal output of the Sun is approximately 400 YJ per second."

Does that mean we would need 4 earth loads of water to turn the big yellow light out (and need to pump it all there in 1 second?

Mark

A newton-metre is a force of one newton on the end of a lever 1 metre long - torque.

The point of application of a force of 1 newton moves one metre, we call it a joule, so no confusion.

I see some mix-up between work and power:

• It can take a week for the newton to move a metre, it's still one joule.
• Move it in one second, that's a power of one watt.

Work is force time distance moved, power is the rate of doing work.

1 volt through 1 ohm for 1 second is 1 joule ate a power of 1 watt.

1 volt through 1 kilo-ohm for 1,000 seconds is 1 joule too, but only 1 milli-watt.

See I CAN do the metric stuff!

Neil

A separate post for clarity. Folks may be interested in the calculations I did to prove a 12V 80 watt motor would be sufficient for a small electric loco. The target speed was 3.6mph = ~1.6m/s (which is about four times scale speed!)

Neil

Edited By Neil Wyatt on 02/03/2015 20:45:52

Hi MarK,
A newton force acting over a metre is not pressure.
A newton force acting over a square metre is pressure.

Les.

I see, so now we have arrived at potential and kinetic energy as well... oooh this is going to run some!

Mark

Neil How did you arrive at 60% efficiency? Frictional norms found in similar Locos? Just interested.