Pounds/foot (and other nonsense) MEW 226

[Gordon WParticipant @gordonw]

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

[Robert DoddsParticipant @robertdodds43397]

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

[mickParticipant @mick65121]

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

[Capstan SpeakingParticipant @capstanspeaking95294]

Posted by Harold Hall 1 on 02/03/2015 15:28:40:

I am sorry Capstan but I was happy to be told I had got them the wrong way round but cannot accept that both torque and power have the same term for the unit of measure.

If I were to tell you that the result of experiment was 15 pound-feet can you tell me what that answer means?

Surely torque and power need differing units of measure, that is the way I was always taught, still it was a very long time ago.

Harold

Edited By Harold Hall 1 on 02/03/2015 15:31:08

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.

[Nicholas FarrParticipant @nicholasfarr14254]

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

[Harold Hall 1Participant @haroldhall1]

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

[Neil WyattModerator @neilwyatt]

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

[Harold Hall 1Participant @haroldhall1]

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

[JAParticipant @ja]

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

[Mark CParticipant @markc]

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

[Mark CParticipant @markc]

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

[Neil WyattModerator @neilwyatt]

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

[Neil WyattModerator @neilwyatt]

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!)

 

Tractive effort at 25% A.U.W	7.0	lbs force
	3.2	kgf
	31.2	newtons
velocity m/s	1.1	m/s
Power	33.5	watt
at 12 volts	2.8	amps
at 60% efficiency	4.7	amps

 

Neil

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

[Les Jones 1Participant @lesjones1]

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

Les.

[Mark CParticipant @markc]

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

Mark

[Jesse Hancock 1Participant @jessehancock1]

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

[jason udallParticipant @jasonudall57142]

I am sure that Neil has also taken into account. .peak efficiency of electric motors is when loaded at around 50% rpm.
…and torque at that rpm is half the stall torque….

Though different manufacturers mark the rating plate based on different compromises…
The calculations show the required power ..at what rpm is not considered. ..

[John OlsenParticipant @johnolsen79199]

The confusion arises because there is no easy way to distinguish that with torque the force is acting at right angles (or tangential if you prefer) to the distance measured, while with work the force is acting through the distance measured. So with torque, the multiplication should be a vector multiplication, not a scalar multiplication.

If that is too confusing, think of it like this…if you pull on your spanner lengthways, you will not undo the nut. You have to pull sideways.

On the other hand, if you pull sideways on a (miniature) train carriage, nothing will happen, but if you pull along the track it will move and you will have done work.

So whether we use pounds-foot or foot-pounds for torque, (or SI units) we really need a symbol in there that means that the force is acting at right angles to the moment arm. Otherwise it will always be ambiguous.

It gets even worse if you are looking at radio control servos, where they habitually leave out the distance and just talk about a servo being so many kg. (Yes, they use kg, not Newtons….)

On the motor thing, note that Jasons rule of thumb is fine for DC motors but does not apply to Induction motors.

John

[Mark CParticipant @markc]

I just read the bit that Capstan wrote "force in a radial direction aka torque" which leaves me starting to wonder if centrifugal force might be sneaking a look in as well perhaps?

Mark

[Mark CParticipant @markc]

Hey Les, I just read your post as well. I hold my hands up, you are right – it would have to be a square metre or even a randomly shaped one but it would defiantly need some area. It would however not have much pressure compared to most things in daily life. Now, if you could make a balloon big enough to trap lots of hot air you would have plenty of pressure if you got collecting in the right places….

Mark

[Mark CParticipant @markc]

John,

Perhaps we could call it Theta? 

Mark

PS. or even cos theta 

Edited By Mark C on 03/03/2015 00:34:32

[Enough!Participant @enough]

Posted by Harold Hall 1 on 02/03/2015 19:17:35:

that foot- pounds is used only for power and pounds-feet for force

Earlier in the thread, Harold, I was with you all the way but not this.

Power = work done per second

Work = force x distance moved through (in the direction of the force incidentally)

By convention (at least amongst Engineers), lb(f).ft is used when referring to a torque, ft.lb(f) when referring to work (or energy) and ft.lb(f)/sec when referring to power.

Torque and work have the same dimensions, yes, but they are not the same thing (not even close) so it is useful to express the units differently to avoid confusion …. I think we agree on this.

(with the "f" written as a subscript which I can't figure how to do in this editor).

[Enough!Participant @enough]

Posted by Gordon W on 02/03/2015 16:40:18:

And nobody has mentioned " slugs"

It's pretty much implied by the use of lb(f) as the force unit rather than poundals.

[Neil WyattModerator @neilwyatt]

> The calculations show the required power ..at what rpm is not considered. ..

Oh yes it was,! How do you think I calculated speed? I estimated the no-load speed at 3000 rpm and used 2000rpm and 60% efficiency for the calculations, knowing I needed a margin for going uphill.

Neil

I saw sin theta suggested, which as theta is 90 degrees equals 1…

So next time you calculate torque, don't forget to multiply by 1 to get the right answer

[jason udallParticipant @jasonudall57142]

Sorry Neil. .
Didn’t see that line in your post

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