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i hope this post goes here... i was going to file it under [powerplant] but it's only somewhat related to that area.

for all the units and car jargon that i know, i never figured out what a pound-foot measured. i know it measures torque output, but ...? it's not as clearly intuitive as others like "miles per hour." can anyone help me out and explain lb-ft? i always want to explain it to people but i don't know how. thanks!
 

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A ft-lb is simply a measure of the amount rotational energy exerted to lift a one pound object one foot into the air against the acceleration due to the force of gravity.

This Rotational Energy can also be expressed in Joules (Newton-Meter).
 

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Kazbaeden said:
A ft-lb is simply a measure of the amount rotational energy exerted to lift a one pound object one foot into the air against the acceleration due to the force of gravity.

This Rotational Energy can also be expressed in Joules (Newton-Meter).
Ugh. Sorry, wrong answer :)

Unfortunately, the unit lb-ft (usually called ft-lb in my experience) is used for two completely different concepts. The one you described is a measure of energy. The one that's used in most car spec sheets is torque. Completely different thing.

First, a little background. Force is a measure of how hard something is pushing linearly. A 10 lb weight pushes down with a force of 10 lbs.

Torque is the rotational version of force. It measures how much effort a rotating shaft can exert.

Imagine you've got a shaft with a nut welded onto the end of it and you put a wrench on it and pull. If the wrench is a foot long and you pull on the wrench with 10 lbs of force, you're exerting 10 ft-lbs of torque.

Anybody who's ever tried to move a stuck nut knows you get more "leverage" (by which we really mean "torque") with a longer wrench. So, what happens if you apply the same 10 lbs of force to the end of a two foot long wrench? You get 20 ft-lbs of torque. You could also get 20 ft-lbs of torque by applying 20 lbs of force to the end of a 1 foot wrench. Or 40 lbs to a 6 inch wrench. Or 5 lbs to a 4 foot wrench. Just take the size of the size of the wrench (sometimes called the "lever arm") and multiply by the amount of force you're applying to the end, and you get the torque you're applying to the shaft.

Now, imagine the shaft's got nuts on both ends, and two guys each have a wrench on their respective end. One guy has a 2 foot long wrench and he's pulling with 10 lbs. The guy on the other end has a 6 inch wrench and he's trying to keep the shaft from turning. He'll have to apply 40 lbs of force to the end of his wrench to counter the 10 lbs of force the first guy is applying, because both of those combinations multiply out to 20 ft-lbs.

Anyway, this is basicly what's going on in a car. The piston is applying a force to the crankshaft, with maybe a 2-inch lever arm. This generates torque. This torque is transmitted through the transmission, differential, etc, until it gets to the tires, which apply a force to the road through, for example, an 8-inch lever arm (for a 16 inch tire).

Guess what happens if you apply 100 lbs of force to the end of a 5 foot wrench? You strip the nut, that's what happens, ha ha!
 

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Ugh. Sorry, wrong answer
No roy, actually it's not wrong. torque and energy have the same physics unit, and the above definition with earth potential energy is absolutely correct, even if it's more abstract than what you just describe.

Yes, the gravitation plays a role, because the leverage force expressed in the torque (by lb) is implicitly associated with earth gravitation (9.8 m/s^2). (On the moon you'll get 5 times smaller torque using the same object and with the same leverage length).

To be convinced here is another way to see why Energy and Torque are the same thing.

Power = Energy / Time (how fast the engine can provide energy)

Power = Torque x RPM (RPM SI unit is 1/second (1 RPM = 2 Pi / second) = 1/time)

That means: Energy / Time = Torque / Time -> Energy = Torque!!!

Yes, believe it or not, In Physics, Torque and Energy are the same thing, both can be expressed in N.m or Joules.
 

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Thanks Bruno, you saved me a good deal of typing. If I am wrong, I'd have a bone to pick with my newtonian mechanics professor. :p
 

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I agree with Roy... he was saying they were the same but that the question was in regards to TORQUE. I believe it should be explained in the way that it was meant. The question was what is lb-ft in regards to engines. And it was explained that it was the rotational energy measurement.
 

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Bruno just proved that Torque and Energy is the same thing. The specs in cars could easily say "Rotational Energy: 160 ft-lb" and it would be the same thing with the same meaning.

So, let's get one thing straight. The author expected a ft-lb to be something as clearly defined as a mile per hour. A ft-lb is much more abstract than that, because it's simply an arbitrary measurement of the amount of energy a system has. It's just a number, and the only way to understand it physically is the way I first described it, because that's how it was derrived. If you want to try to understand it in cars all you have to know is that a bigger number is better.

It's almost like a horsepower. What the hell is a horsepower anyway? It was measured by James Watt to be the amount of work a horse can do in one hour driving a water pump at a coal mine. It's a completely arbitrary number. Cars have nothing to do with horses and water pumps, but we still measure them with horsepower. We simply understand that the larger number means more it has more energy and thus more power, and we are content with that.
 

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Ya... and if I try and tow 160 horses with my car, I'm pretty sure the horses would win. Its just those god damn tires that wouldn't be able to stick to the ground! lol.

You know I'm joking now, right?
 

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Even though torque has a unit that is similar to potential energy, they are not the same thing. Torque is a force, force is not the same thing as energy.

Since we are in an automotive forum and we are not talking about potential energy, ft-lb refers to the force with which the engine crankshaft rotates. Given everything is the same, between two cars, the one with more torque will accelerate faster. But things are rarely the same between two cars of different make, so comparing crank torque is rather pointless. Just know that it exist and think about crank HP instead.
 

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explanation

I couldn't resist posting since I have a mechanical engineering background. This is the way I understand it:
Torque has units of WORK (work in Joules = force x distance)
Energy (or Power) is the work done per unit of time (or per revolution of the engine).
Torque figures tell you how much WORK the engine and drivetrain are capable of at a specific rpm. This is why truck owners want good "low-end" or low rpm torque, so that they can pull heavy loads at low engine speed.
Power or horsepower will tell you how quickly you can get to top speed and is directly related to acceleration.
Here's an example:
You're driving down the highway (in an automatic Mazda 3) and want to pass a slower vehicle. You wish to go from 70 to 100 mph in order to pass. You floor the gas and proceed to pass. Your car, which was cruising at a lower rpm drops a gear and the rpms jump (resulting in higher torque being rpoduced). Suddenly a huge gust of wind comes up at the front of your vehicle. If your car does not produce enough torque at that rpm to overcome the increased drag due to the wind you will not accelerate and will remain cruising at 70 mph.
Now say there's no wind that comes up. A car with more power (more work done per unit time) at that rpm will be able to accelerate faster to the new speed of 100 mph.

Hope this makes sense.
 

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Torque has units of WORK (work in Joules = force x distance)
Energy (or Power) is the work done per unit of time (or per revolution of the engine)
No, work and energy have the same unit. but power is defined as work done by unit of time, i.e., power = provided energy by unit of time.

P = dE / dt = F . l

Power (Watt, hp) is not energy/work (Joule, Calory, N.m, e.V)

To be totally correct, there is only one difference between torque and energy: torque is vectorial (vector pointed parallel to the rotation axis, don't ask me which direction, I never get it right), and energy is scalar (number.) Beside that, the concepts of torque and energy are interpreted differently only by humans, for god who creates the universe by physics laws, they are the same.
 

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(vector pointed parallel to the rotation axis, don't ask me which direction, I never get it right)
It doesn't really matter; you can define the direction. You just have to make sure you're consistant.
 

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Kazbaeden said:
(vector pointed parallel to the rotation axis, don't ask me which direction, I never get it right)
It doesn't really matter; you can define the direction. You just have to make sure you're consistant.
As long as you don't interact with anyone else, you're exactly right. However, the convention of the right hand rule (wrap right hand around rotational axis with fingers curling in direction of tangential force, then thumb points in direction of torque vector) eliminates the need to define your torque vector for each other person you deal with.

As for the definition of torque, it depends on who answers the question (for example, see http://golf.about.com/cs/golfterms/g/bldef_torque.htm). Everyone's basically talking about the same thing, but using terms that suit the individual's field of study. If one wants the definition of torque in the automotive sense, the (tangential force x moment arm) definition is better suited, IMHO.
 

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bluong1 said:
Torque has units of WORK (work in Joules = force x distance)
Energy (or Power) is the work done per unit of time (or per revolution of the engine)
No, work and energy have the same unit. but power is defined as work done by unit of time, i.e., power = provided energy by unit of time.

Thanks for the clarification on the term "energy". You are absolutely correct.
 
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