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Discussion Starter · #1 ·
As the title says.
 

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Whats the difference in ft lbs or lb ft? For years engine torque was defined as ft lbs. Seems now its in vogue to express the value in lb ft.
 

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Discussion Starter · #3 · (Edited)
Whats the difference in ft lbs or lb ft? For years engine torque was defined as ft lbs. Seems now its in vogue to express the value in lb ft.

In a physics textbook, the definition of work (energy) is force times distance. The units should be lb-ft, although it is usually expressed as ft-lbs.

The definition of torque is lever arm (distance) times force. The units should be ft-lbs, although it is usually expressed as lb-ft. :confused:
According to the commutative law of multiplication,which applies to units as well as numbers, either would work. It is just a matter of convention.


I am really just responding to the last thread which was closed before I could give my input.
 

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Elk Whisperer (Super Moderator)
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As explained, it is pound feet. The amount of "force" (pounds) applied to a beam of a known length (in feet) then reduced to pound feet.

An engine's torque is measured in pound feet (or the metric equivalent) A torque wrench, as an example, is calibrated in pound feet (or pound inches) the amount of weight in pounds on the end of a one foot (or one inch) long beam needed to torque a bolt to the proper "tightness". Hence, 12 pound inches equals one (1) pound foot.

Horse power (HP) is a unit of work as described (or calculated) as weight over time and distance. One HP is the ability to raise 550 pounds, one foot in one second (or ~ 750 watts in the metric system)

Torque and HP are two separate units of work measured in different ways. Horse powered "tumbling rods" (or horse powers) were the first devices to convert linear work (HP) into rotational force (torque) Along comes the steam traction engine and farmers wanted to know "How many HP is that" so a "scale" was made (by James Watt) to convert torque to Brake Horse Power (BHP). One can sometimes see the scale used at a "threshing bee" or steam show where restored stationary steam engines and steam traction engines are "put to work". A fascinating way to spend a day.

F2G1D, I closed that thread because it was traveling way beyond the scope of the original question, not out of spite.

RJ
 

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Discussion Starter · #5 ·
I've been known to flog a dead horse. :)

What I am saying is, both torque and work have units of distance times force in some order. In the case of torque, the force is perpendicular to the 'distance'. In the case of work, the force is parallel to the 'distance.' The use of lb-ft versus ft-lb is an attempt to distinguish between the two.
 

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Elk Whisperer (Super Moderator)
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I've been known to flog a dead horse. :)
Me too :eek:

The use of lb-ft versus ft-lb is an attempt to distinguish between the two.
I understand.

In "olden times" wasn't "energy" measured in foot pounds, as in the use of a pendulum of a known weight being struck by an object of a known weight and the distance the pendulum moved (measured in feet)? If memory serves, the first chronographs were like this, requiring lengthy calculations to reduce the "energy" to feet per second?

RJ
 

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The Hog Whisperer (Administrator)
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ft-sec (initial speed) of the pendulum is not a direct measurement of kinetic energy. How high the pendulum ultimately goes (vertical distance above where it started, not arc length) can be used to calculate KE, along with the mass of the pendulum.

Unfortunately it gets confusing when both torqe (true 'force') and KE (energy) are expressed in similar units. But they aren't the same thing at all.
 

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Discussion Starter · #8 ·
Me too :eek:



I understand.

In "olden times" wasn't "energy" measured in foot pounds, as in the use of a pendulum of a known weight being struck by an object of a known weight and the distance the pendulum moved (measured in feet)? If memory serves, the first chronographs were like this, requiring lengthy calculations to reduce the "energy" to feet per second?

RJ

Yes.




V = [M + m)/m]Sqrt(2gh)
 

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The Hog Whisperer (Administrator)
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I mis-spoke, you can get KE from the initial speed of the pendulum.... but if you can't directly measure the speed (and if you could measure the speed of the pendulum, you could probably measure the speed of the bullet).... but if you measure the vertical climb of the pendulum, then you can calculate the speed.

And then calculate KE. I think that's how it goes. But not really measuring 'force' as far as I can tell.
 

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The Hog Whisperer (Administrator)
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Or another way to try and clear things up.... you can have force with no movement (torque). But you can't have KE without movement. An object at rest has no KE (relative to the rest of its environment).

Or maybe that doesn't help at all :p
 

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Discussion Starter · #11 ·
I mis-spoke, you can get KE from the initial speed of the pendulum.... but if you can't directly measure the speed (and if you could measure the speed of the pendulum, you could probably measure the speed of the bullet).... but if you measure the vertical climb of the pendulum, then you can calculate the speed.

And then calculate KE. I think that's how it goes. But not really measuring 'force' as far as I can tell.

The beauty of the ballistic pendulum is that momentum is conserved during the collision, though energy isn't.

Energy is conserved after the collision, though momnentum isn't.
 

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" the definition of work (energy) is force times distance. The units should be lb-ft, although it is usually expressed as ft-lbs.

The definition of torque is lever arm (distance) times force. The units should be ft-lbs, although it is usually expressed as lb-ft. "

Didn't the Cheshire Cat say that in Alice in Wonderland?
 
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Discussion Starter · #13 ·
" the definition of work (energy) is force times distance. The units should be lb-ft, although it is usually expressed as ft-lbs.

The definition of torque is lever arm (distance) times force. The units should be ft-lbs, although it is usually expressed as lb-ft. "

Didn't the Cheshire Cat say that in Alice in Wonderland?
It came to me in a dream.
 
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Torque and HP are two separate units of work measured in different ways.
They’re really not. Power (whether horse or otherwise) is the amount of work accomplished in a given amount of time. Torque has no time component, and in fact nothing has to be accomplished in order to have torque. IOW, power requires both that something be accomplished (movement) AND that it happen in some amount of time. Torque means neither.
 

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Right. You can apply torque and have nothing happen, as when you apply a wrench to a rusted bolt head that doesn't break loose. You still applied the torque. It just wasn't enough to overcome the rust adhesion. No energy, other than heat from stretching the metals, was put into the wrench or bolt. That sounds funny because the person applying the torque had to put biological energy into creating the force, but that's all internal to the human. He didn't succeed in getting it into the bolt.

Work is applying force over a distance. The longer the distance or the greater the force, the more work is done. When, at the end, the thing you did the work on is free to coast forward, it carries with it the energy that was put into it by the work and can go on to apply that energy to doing work on something else, as when a bullet strikes a target.
 

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I used to know how to convert, but I'm pretty sure in Physics we always used Newton-meters (N-m), and never remember it called meter-Newtons. lol
 

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Discussion Starter · #17 ·
I used to know how to convert, but I'm pretty sure in Physics we always used Newton-meters (N-m), and never remember it called meter-Newtons. lol

...asnd Joules. Don't even get me started on the Poiseuille!
 

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Discussion Starter · #18 ·
Torque means neither.

Not only can work be related to force and distance, in a rotating environment, work

can be related to torque and angular displacement. An example would be the energy

loss due to rifling torque on a bullet.
 

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It's been some time that I have posted. Glad to be back. So here are my two cents. Foot pounds as in a bullet or car engine is foot-pound force (dot product). Weight is the pound mass. The torque of a torque wrench is Foot-pound mass (cross product).
 

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