Why can't I understand Torque?

[a_unique_person]

I can understand power. The ability of an engine to push a car. I find the distinction with torque a little hard to understand. I did physics at High School, and could plug in the values to the equations.

I just don't understand it instinctively.

Why is twisting power in an engine so different to the horsepower?

[The Fool]

As far as I know "horsepower" is calculated on how much work can be done with a certain amount of "torque" ie horspower is a function of torque and time (for a spinning car engine).

[QuarkChild]

I've never heard of torque being described as twisting "power." Usually it's compared to force.

One can apply a force linearly, say by pushing against a wall. One can apply a force that rotates an object, like when you push a door open. In this case it's a little more complicated, because how hard you have to push depends on how far away you are from the hinges of the door. Near the hinges, you have to push pretty hard (try it); it's much easier if you push on the far side (where door handles are.) Mathematically this is expressed as Torque = r x F, where r is the distance from the hinges [or pivot point.] Note that a higher r allows a smaller force [F] to produce the same amount of torque.

Um, is that what you were talking about?

[QuarkChild]

I started my post before I saw The Fool's comment. Now I get the horsepower thing.

:)

I don't know much about cars.

[LucyR]

Originally posted by a_unique_person
I can understand power. The ability of an engine to push a car. I find the distinction with torque a little hard to understand. I did physics at High School, and could plug in the values to the equations.

I just don't understand it instinctively.

Why is twisting power in an engine so different to the horsepower?

A_U_P,

Linear momentum, p,: product of mass with velocity.
Force, F,: derivative w.r.t. time of p.
Work, w,: dot product of F with displacement.
Power, P,: derivative w.r.t. time of w.

(I think you might be confusing force, work and power.)

Angular momentum, L, is the angular analogue of p: cross product of r with p, with r being the vector from the axis of rotation to the particle in question.

Torque, tau, is the angular analogue of F: derivative w.r.t. time of L, i.e. r x F. tau is orthogonal to F.

As far as a car is concerned: in low gear (a small gear turning a big gear) it has a large torque, but a low top speed, and vice versa.

Or I suppose you can think of tightening a nut: a short spanner does it quickly, but with a long spanner you can make it tighter, even though your strength is constant.

[Thumbo]

Originally posted by a_unique_person
I can understand power. The ability of an engine to push a car. I find the distinction with torque a little hard to understand. I did physics at High School, and could plug in the values to the equations.

I just don't understand it instinctively.


Think of the gear chain in a old style mechanical clock.

They all run off the spring. With perfect gears, conservation of energy says you could extract the same power from any of them.

Now try and stop the gears with your finger. It's realy easy with the fast moving ones. The slower ones are harder. The slowest of all will just keep moving inexorably. The difference is the torque.

Or, to rephrase LucyR's post above, torque is to rotation what force is to linear movement.

[Crossbow]

Originally posted by a_unique_person
I can understand power. The ability of an engine to push a car. I find the distinction with torque a little hard to understand. I did physics at High School, and could plug in the values to the equations.

I just don't understand it instinctively.

Why is twisting power in an engine so different to the horsepower?

To: a_unique_person

I can understand why you may be confused, people (even those who know better) tend to throw around the terms 'force', 'power', and 'energy' almost interchangably, however as these terms are applied in the scientific sense, they have very different, but related meanings.

To explain, in many practical applications (i.e. car repair) Torque is typically measured in units of 'foot-pounds' (look at a Torque wrench sometime).

However, Power is in units of 'horsepower' (i.e. the power rating of the car's engine), and one horsepower is equal to 550 foot-pounds/second. In other words, a 1 hp engine should be able to (assuming there is no friction or other losses at play) lift 550 weight off the ground by a distance of one foot every second; or a 225 lb weight two feet off the ground in one second; or a 5500 pound weight one foot off the ground in ten seconds; and so on.

Now then, in the case of Torque, it is established by the application of a force applied over a distance via a lever (note, there is no time stipulation). LucyR brought up an excellent point when she described the amount of Torque is proportional to the length of the wrench one is working with. One cannot readily increase ones body strength, however one can readily change the size of the wrench and thus vary the amount of applied Torque.

I hope this helps!

[scotth]

Perhaps an example taken to the extreme will help.

Horsepower is a measure of an ability to do work.

You can have torque without any rotation. If you are on a steep hill with a bicycle, and you put preasure on the pedal but fail to move the bike (and you) any distance up the hill, you have created/applied a torque but didn't perform any work.

You can have rotation without any (useful) torque. If you have a device that spins freely at 1000 rpm, but quits spinning as soon as on load is put on it, it has rotation but cannot do any work.

Horsepower requires both torque and rotation.

Did this help at all?

[scotth]

Originally posted by The Fool
As far as I know "horsepower" is calculated on how much work can be done with a certain amount of "torque" ie horspower is a function of torque and time (for a spinning car engine).

Horsepower is a function of torque and rotational speed. It is not time dependant.

A stalled electric motor (I use this instead of an engine because electric motor can readily produce torque at 0 rpm) can provide torque all day. If it is not allowed to spin, no work is accomplished and no horsepower is made.

[garys_2k]

Put a wrench on a tight bolt and try to get it loose. You're applying torque (rotational force), equal to the force you're applying to the wrench handle (force) times the distance your hand is from the bolt's axis (distance). Torque = force * distance.

You know about rotational speed, revolutions per minute (RPM). That's analogous to straight ahead velocity.

Horsepower = Torque * RPM / 5252, where torque is measured in lb-ft (one pound of force applied a foot away from the axis).

So, horespower can increase by either increasing the torque an engine can provide at a given speed, or increasing the speed that it can provide a given torque at. Small Formula One engines have VERY high horsepower ratings because they turn at insane RPMs and still provide moderate torque. As far as the car's concerned, with the right gearing it will all be the same: more acceleration.

So, torque is force and horsepower is torque times rotational speed. Simple?

All excellent explanations. But to understand it instinctively, simply think of it as force applied in a circle.

[Plutarck]

Could it be said, loosely, that "an object in motion tends to stay in motion" can be rephrased to say that "an object that is spinning tends to stay spinning"? When an object is moving linearly, IE from point A straight to point B, then the...uh...well, the 'something' with which it does so is called "force". When an object is spinning, the "something" with which it does so is called "torque".

So when you try to stop a falling bowling ball, you are fighting against Force; when you try to stop a spinning gear, you are fighting against Torque.

Is this correct?

I too previously have very little understanding of what torque actually means. However, if I am correct, then I now have a solid intuitive knowledge of what it is that is being talked about. I hope that is the case, 'cause it makes sense to me now ;)

[scotth]

Originally posted by Plutarck
Could it be said, loosely, that "an object in motion tends to stay in motion" can be rephrased to say that "an object that is spinning tends to stay spinning"? When an object is moving linearly, IE from point A straight to point B, then the...uh...well, the 'something' with which it does so is called "force". When an object is spinning, the "something" with which it does so is called "torque".

So when you try to stop a falling bowling ball, you are fighting against Force; when you try to stop a spinning gear, you are fighting against Torque.

Is this correct?

I too previously have very little understanding of what torque actually means. However, if I am correct, then I now have a solid intuitive knowledge of what it is that is being talked about. I hope that is the case, 'cause it makes sense to me now ;)

You may have it, but your statements are not a convincing arguement for it.

A spinning object would stay spinning unless a force acted on it. Spinning is a type of motion, so spinning is a more specific statement of the idea that an "object in motion tends to stay in motion".

A torque would be that force that could do either, start the spinning or stop the spinning. Any net change in an objects rate of spin (as long as its mass and shape are assumed to be constant) would be caused by a torque.

However, a torque can exist independant of any actual rotation. If you try to loosen a rusted bolt and fail, you have just applied a measurable torque to the bolt, but no spinning has occurred because you did not overcome the friction that has it stuck in place.

Looking at this statement in particular, "So when you try to stop a falling bowling ball, you are fighting against Force; when you try to stop a spinning gear, you are fighting against Torque."

In either case, you are fighting a force called inertia. A torque is a force applied in a way to cause (a change in) a rotation. To say "fighting a torque" in this instance is pretty sloppy use of the term and casts a bit of doubt our your mastery of the concept.

A force is the most generic term used to describe what is necessary to cause some type of movement. A torque is a subset of or specific type of force that causes or tries to cause rotational movement.

[garys_2k]

Go up to the nearest wall and lean against it. You're applying plain-vanilla force.

Go to the nearest door and turn the doorknob. You're applying torque to it.

If you apply enough force to the wall to put a hole in it or knock it down, your force did work (expended energy). If you applied enough torque to the doorknob to actually make it turn, or you twisted it off, you did work to that, too (expended more energy).

If you are able to knock down walls and twist off doorknobs on a routine basis, whoa!

[garys_2k]

I'll try to explain, but scotth did a good job...
Originally posted by Plutarck
Could it be said, loosely, that "an object in motion tends to stay in motion" can be rephrased to say that "an object that is spinning tends to stay spinning"?

Yes, both are equally true.

When an object is moving linearly, IE from point A straight to point B, then the...uh...well, the 'something' with which it does so is called "force".

Not quite, the something that it has is "momentum," a property that can be found my multiplying the object's mass times its velocity.

When an object is spinning, the "something" with which it does so is called "torque".

Nope, that's momentum, too. In this case, "rotational momentum." You can find that by multiplying the spinning object's rotational speed (eg RPM) by it's "moment of inertia" (a property analogous to mass, but that includes where the mass is located on the body with respect to its axis of rotation -- big flywheels have heavy rims to increase this). Don't worry about this, it's not important here.

So when you try to stop a falling bowling ball, you are fighting against Force; when you try to stop a spinning gear, you are fighting against Torque.

Almost. You APPLY force to the falling bowling ball to slow it down, and you apply torque to the spinning gear to change its speed.

Edit to fix a fubar. All set now.

Originally posted by garys_2k

Almost. You APPLY force to the falling bowling ball to slow it down, and you apply torque to the spinning gear to change its speed.

I think he's confused between momentum and force. Momentum is what keeps a moving object moving (or spinning); force is what moves it in the first place, or slows it down again.

Garys_2k said it very well with his doorknob example. Torque IS force, applied in a circle. That's all there is to it, it's not a complicated concept at all.

Think about a torque wrench. It allows you to tighten a bolt with an exact amount of force. But the force is torque because you are applying it in a circular motion.

[garys_2k]

Originally posted by sundog


I think he's confused between momentum and force. Momentum is what keeps a moving object moving (or spinning); force is what moves it in the first place, or slows it down again.


And I think I may have confused momentum and inertia!

[Plutarck]

Originally posted by sundog


I think he's confused between momentum and force. Momentum is what keeps a moving object moving (or spinning); force is what moves it in the first place, or slows it down again.

Garys_2k said it very well with his doorknob example. Torque IS force, applied in a circle. That's all there is to it, it's not a complicated concept at all.

Think about a torque wrench. It allows you to tighten a bolt with an exact amount of force. But the force is torque because you are applying it in a circular motion.


Ooooo! I see now, and I think that NOW I got it. I don't quite get momentum precisely as it relates to other...'stuff', but I think I finally have figured out what the hell torque is (is momentum a kind of force in the same sort of roundabout way that torque is a force? I'm guessing "no", but I of course am almost entirely guessing on this one).

So, torque is, basically - if not precisely - force applied in a circle. If you push a rock, that's force; if you open a plastic Coke bottle top, that's torque. Torque is 'a' force - precisely, a specific kind of force.

Right?

[garys_2k]

I'll try these in reverse order.
Originally posted by Plutarck

So, torque is, basically - if not precisely - force applied in a circle. If you push a rock, that's force; if you open a plastic Coke bottle top, that's torque. Torque is 'a' force - precisely, a specific kind of force.

Right?


Yes, you've got it. That is entirely correct -- torque is "twisting force."

... is momentum a kind of force in the same sort of roundabout way that torque is a force? I'm guessing "no", but I of course am almost entirely guessing on this one).

Momentum is a bit harder to grasp, because it's more of a mathematical property than something we can "make" ourselves. For "linear momentum," or the momentum of an object moving in a straight line, it is the object's mass multiplied by its speed.

A car and a baseball both going sixty miles per hour have the same speed but vastly different values of momentum. But, if you could get that baseball moving VERY fast, you could get both the sixty mile per hour car and the super fast baseball to have the same momentum if the products of their mass times speeds were equal.

Rotational momentum is just about the same, but it involves the rotational speed multiplied by the object's moment of inertia. Have you ever watched a figure skater do a spin, and seen the spin accelerate when she pulls her arms in close to her body? She is using the fact that her rotational momentum is being conserved and that by reducing her moment of inertia (mass distribution around her spin axis) her rotational speed must increase to keep the same total momentum.

Is this clear? I'll go back and edit my earlier post when my brain took my hands hostage and forced me to write "inertia" instead of momentum. Really, it wasn't my fault!

[scotth]

Originally posted by Plutarck
So, torque is, basically - if not precisely - force applied in a circle. If you push a rock, that's force; if you open a plastic Coke bottle top, that's torque. Torque is 'a' force - precisely, a specific kind of force.

Right?

Sounds like you have it now.

Originally posted by Plutarck



Right?

Exactly right! :)

[Walter Wayne]

A slighty more formal definition from www.dictionary.com

torque - the measure of a force's tendency to produce torsion and rotation about an axis

A note: Whenever a torque is applied, there are two forces involved. When you twist a door knob, you apply opposite forces on top and bottom. When you turn a lever you apply a force on one side, and an opposing force is applied at the fulcrum.

Walt

Originally posted by Walter Wayne

A note: Whenever a torque is applied, there are two forces involved. When you twist a door knob, you apply opposite forces on top and bottom. When you turn a lever you apply a force on one side, and an opposing force is applied at the fulcrum.

Walt

No, I don't think so. A torque can be applied as a single vector to a point on a rotating disc. I remember having to work out problems about it. Think of a train engine wheel being pushed around by its whatchamacallit.

Also, when you turn a doorknob, you apply force in the same direction on the top and bottom, not the opposite direction (relative to the doorknob, which is what counts). If this weren't so, you'd be fighting yourself and the doorknob wouldn't move. In this scanario, every place your hand touches the doorknob, a torque is being applied, adding up to the total torque applied to the doorknob.

[LucyR]

It seems my previous post requires a few more statements.

Horsepower is merely a unit of power. It is not 'a function of torque and rotational speed'. It is a constant numerical quantity.

The SI unit of power is the Watt. 1 Horsepower ~ 750 Watts. By definition: one Watt is one Joule of work performed in one second. One Joule is the amount of energy associated with the application of a force of one Newton over a distance of one meter. One Newton is the force required to impart an acceleration of one meter/second^2 to an object with a mass of one kilogram.

As I stated earlier power is a measure of how quickly a system can do work. Detonating a piece of dynamite may release as much energy as digesting a jam doughnut, in which case the capacity of both systems to do work is the same. Their respective powers are, however, very different.

Both work and power are scalar quantities.

Since angular momentum is the cross product of the position vector and the linear momentum, it is perpendicular to the plane containing r and p. Think of an artillery shell or rifle bullet, these may be spun about an axis of symmetry that is parallel to the gun barrel, thus imparting an angular momentum that points in the same direction. The shell will tend to 'resist' change in its direction due to conservation of angular momentum, and is hence more accurate. This also explains why bicycles are stable when moving, but are not when stationary.

Torque is not a 'kind of force'. There is only one definition of force, viz. the time rate of change of linear momentum. In the case of constant mass this is written as F = ma, where a is the acceleration. The unit of torque is the Newton.meter. You may think of torque as the angular analogue of force.

Both torque and force are vectors. However, since torque is the time rate of change of angular momentum it is perpendicular to plane containing r and F. The magnitude of the torque reduces to rF only when F is perpendicular to r.

Momentum is not 'what keeps a moving object moving'. An object has momentum by virtue of its mass and its velocity. Nothing is needed to keep an object moving in the absence of external forces.

Plutarck,

Remember what I said earlier about the definition of force in terms of linear momentum. Perhaps this helps you to get a handle on the concept of momentum.

[garys_2k]

A couple of comments...
Originally posted by LucyR
Since angular momentum is the cross product of the position vector and the linear momentum, it is perpendicular to the plane containing r and p.

Do you really think that makes it clearer for a person that is new to the concept?

Torque is not a 'kind of force'.

Which was quickly followed by:

You may think of torque as the angular analogue of force.

And, where "a kind of" = "the angular analogue"

we have by substitution:

You may think of torque as a kind of force.

That is to say, without the pedantry.

Momentum is not 'what keeps a moving object moving'. An object has momentum by virtue of its mass and its velocity. Nothing is needed to keep an object moving in the absence of external forces.

Sorry, but inertia is what is needed to keep an object moving. No inertia, no coasting. It is the definition of inertia.

[scotth]

Originally posted by LucyR
It seems my previous post requires a few more statements.

Horsepower is merely a unit of power. It is not 'a function of torque and rotational speed'. It is a constant numerical quantity.


While this is strictly true, this thread was in reference to an engine. In this instance, horsepower is a function of torque and rotational speed.

Originally posted by LucyR

Momentum is not 'what keeps a moving object moving'. An object has momentum by virtue of its mass and its velocity. Nothing is needed to keep an object moving in the absence of external forces.


From dictionary.com:

Momentum: Impetus of a physical object in motion.

So the best word is really inertia:

Inertia: the tendency of a body at rest to remain at rest or of a body in straight line motion to stay in motion in a straight line unless acted on by an outside force.


"What keeps a moving object moving" is a perfectly valid way of stating this.

[Tez]

not sure this'll help the specific question at hand, but a great demonstration none-the-less.

Take a broom - one with a wooden handle. Find the point where you can balance it on your finger, and mark that point with a pen.

Cut the broom at the marked point.

Feel the weight of both pieces. Are they the same?

Once you understand that you understand the difference between force and torque...

[Thumbo]

Originally posted by Tez
not sure this'll help the specific question at hand, but a great demonstration none-the-less.

Take a broom - one with a wooden handle. Find the point where you can balance it on your finger, and mark that point with a pen.

Cut the broom at the marked point.

Feel the weight of both pieces. Are they the same?

Once you understand that you understand the difference between force and torque...
...and you'll need a new broom.

[garys_2k]

Originally posted by Tez
not sure this'll help the specific question at hand, but a great demonstration none-the-less.

Take a broom - one with a wooden handle. Find the point where you can balance it on your finger, and mark that point with a pen.

Cut the broom at the marked point.

Feel the weight of both pieces. Are they the same?

Once you understand that you understand the difference between force and torque...
What would this have to do with torque? You'll have a longish piece of wood and a shorter piece of wood with the broom head on it. They will weigh the same. I'm not sure what this proves.

[Tez]

try it gary

[garys_2k]

Originally posted by Tez
try it gary
No thanks, I don't have any spare brooms I want to trash. What is it you're supposed to see or feel?

[Tez]

because the broom is asymmetric, the two pieces wont weight the same.

The balance point is when the torque around that point is 0, not when the weight of the two pieces is the same.

Say the (point particle!) head of the broom weighs 5 Kg, the broom is 2m long, and the wood has a (uniform) mass per unit length of 1 kg/metre. (Dont let anyone attack you with such a broom).

Let "x" be the distance from the head of the broom to the balance point.

Then the balance occurs when

5*x+M*x*(x/2)=(L-x)/2*M*(L-x)

putting M=1,L=2 gives some value of x which I cant be bothered working out (I've ignored "g" - acceleration due to gravity becoz it cancels from both sides).

Whatever that value of x is, use it to calculate the mass of the two pieces - they will not be the same (see if you can guess which will be greater before the calculation!)...

[LucyR]

Gary,

I was not meaning to be unduly pedantic, and I was not attempting to belittle you, or the other posters.

The point is that there are perfectly good extant and distinct definitions of both force and torque, so why not use them?

I do not know if A_U_P understands the cross product or not, but it's nevertheless important for understanding the distinction between the two concepts.

I do not think that 'a kind of' and 'an analogue of' are synonymous. I'd say that a Labrador is 'a kind of' dog but not 'an analogue of' a dog. An analogue would presumably be something that fulfills a similar role in the same, or a different context, but is nevertheless intrinsically different. The way I think about it force and torque are analogues in the sense that they are both vectors, they can both give rise to motion, and can both be expressed as the time-derivative of momentum or a quantity that involves momentum. On the other hand, force produces linear motion, torque produces angular motion. I must say that I don't believe its unusual to describe the following quantities as analogues of one another:

linear displacement, velocity, acceleration, momentum /angular displacement, etc.
force/torque.

When I said that momentum is not what keeps an object moving, I was replying to Sundog who stated that it is. I said that an object has a momentum by virtue of its mass and its velocity. I said that nothing is needed to keep an object moving in the absence of external forces. I assume in this discussion we are limiting ourselves to Newtonian particles having non-zero rest masses. Mass is a quantitative measure of inertia, i.e. no mass - no inertia - no object. Inertia is intrinsic. Let me know if you're not happy with this.

In any case, I appreciate being able to have these discussions, and do not want to end up merely pissing people off.


Sundog,

Dictionary definitions are not always appropriate. The common meaning of terms such as force, momentum, work, and power, are often only distantly related to their scientific definitions (and this is after all the science forum). Once again, momentum is defined as the product of an object's mass and its velocity, no more no less. An object is accelerated by a force, it possesses a momentum as a result of that force. Its changing momentum is indicative of the continued application of a net force, etc., etc., etc.

In the absence of external forces, the object either remains at rest or continues moving in the same direction. This is indeed the principle of inertia. However, since we regard mass as a measure of inertia, I think it sounds rather superfluous to state that an object needs inertia to continue in a str line etc. No inertia implies no mass, which in turn implies no object. Is this reasonable?

[a_unique_person]

Originally posted by Tez
not sure this'll help the specific question at hand, but a great demonstration none-the-less.

Take a broom - one with a wooden handle. Find the point where you can balance it on your finger, and mark that point with a pen.

Cut the broom at the marked point.

Feel the weight of both pieces. Are they the same?

Once you understand that you understand the difference between force and torque...

thanks for the input all of you. This has generated more discussion than i expected.

Thanks for also putting this in the correct words I should have used, the difference between force and torque is what I am after.

I will simulate the experiment with something cheaper than a broom, I don't have any spare old ones lying around at the moment.

[garys_2k]

Lucy, I wasn't trying to be aggravating to you either. I merely wanted to point out that people that have a difficult time grasping what torque is, can't picture it, usually aren't helped with strictly scientifically and technically precise explanations. I didn't feel that adding confusing concepts like vectors (or even moments) was appropriate. Discussing cross products with someone that hasn't grasped the basics may make them feel that it's all too complicated and not worth the bother. Unless the person specifically asks about the vectors and such I tend to leave them out at first.

Describing torques as a "rotary force" is, imho, completely appropriate. Both cause acceleration, although one does so for linear motion and the other for rotary motion. Yes, I really believe that describing torque as a "type of force" is better than "an analogue of" because they do almost the same thing.

The distinction between force's linear effects and torque's rotational one was being emphasized, so I guess I don't see any harm in calling it "rotational force." We may agree to disagree here.

Likewise, your technically correct DEFINITION of horsepower was not as good (again, imho) at illustrating the point of how torque relates to a vehicle's engine output as the simple equation. By showing the relationship between the AMOUNT of horsepower it generates to its torque and speed it's easier to see how things work. Yes, of course, power is energy divided by time, but what that more general, esoteric concept means when thinking of an engine that generates 275 lb-ft. of torque at 3800 RPM and 320 Hp at 5600 RPM isn't particularly helpful.

I do like to start slowly and, given enough time, would likely start with more fundamental concepts. But when a post to a website forum asks for a simple understanding of the concepts of torque and power, I think it's easier and more appropriate to use analogies and reasonable simplifications that are easily grasped.

Regarding momentum and its relationship to "what keeps bodies in motion," you are correct. Momentum is entirely a mathematical construct (unlike torque, which is a physical phenomena we can create with a twist of the wrist). The correct term for what makes things tend to stay at one velocity is inertia. Similar to mass, yes, but when jumping back and forth between linear motion and rotational motion concepts in this case I guess the most technically correct term fits better. Most people have heard of inertia but many don't know what it really represents. Of course, it basically IS mass when considering linear motion but it's a bit more complicated when discussing rotational motion.

[garys_2k]

Originally posted by Tez
because the broom is asymmetric, the two pieces wont weight the same.
<snip>

OK, yes, of course. Thanks! That is an interesting illustration of torque, where the two pieces balance because of equal torques around the pivot but each has different weights.

Sundog, after you do this experiment, note that the longer piece, being farther from the pivot, will need less weight to balance the shorter, heavier piece. This is because the balance requires that the TORQUES of each balance, and that happens when the torque of the left piece is the same as the torque of the right.

Torque(L) = Torque(R)

Weight(L) * Length (L) = Weight(R) * Weight(R)

If the left side has more weight than the right it will have a proportionately shorter length, so that both products are the same and the equation balances.

Thanks, Tez, good one!

[LucyR]

Gary,

Thanks for getting back to me. I think we're pretty much in agreement.

Anyway, judging by his latest post, it looks as though A_U_P seems quite happy with what we've collectively written.

Sundog,

Dictionary definitions are not always appropriate. The common meaning of terms such as force, momentum, work, and power, are often only distantly related to their scientific definitions (and this is after all the science forum).

The equations are of course essential to understanding the concepts, but keep in mind the context: we are trying to make this clear to a layman. The dictionary definition given is, in fact, entirely compatible with the definition you gave, which is of course correct. I realized that I had used "momentum" where I meant "inertia." The dictionary definition is what mane me realize my error.



I think it sounds rather superfluous to state that an object needs inertia to continue in a str line etc.

Yes, to someone used to thinking about these things. (I don't think I said or implied that an object "needs" inertia to keep moving.) This is not a scientific debate, though; it's an attempt to convey intuitive understanding of these concepts to someone unfamiliar with them. I think in this context, my wording of the idea was perfectly appropriate, as was my description of torque as "force applied in a circle". Your very correct notation that it is actually the angular analogue doesn't help the layman much. I think.

But all of us together got the job done! :)

[diddidit]

I wrapped my brain around it this way:

Torque is something you make with a rotational engine, like an electric motor.

That motor operates at some rotational speed, so speed is also something you make.

Power, being the product of torque and speed, is something you get.

SO: A crane requires a certain amount of torque from its motor, based on the mechanical advantage of the gears and rigging and the lifted load. That amount of torque is constant whether you are lifting that load at 5 feet per minute or 10 fpm. But, if the 10 fpm version uses an 1800 rpm motor, the 10 fpm one needs a 3600 rpm motor. The 3600 rpm motor will have the same torque as the 1800 but with twice the speed it will have twice the power.

(I was in the crane+hoist bidness until a few years ago).

did

Originally posted by diddidit
I

SO: A crane requires a certain amount of torque from its motor, based on the mechanical advantage of the gears and rigging and the lifted load. That amount of torque is constant whether you are lifting that load at 5 feet per minute or 10 fpm. But, if the *5* fpm version uses an 1800 rpm motor, the 10 fpm one needs a 3600 rpm motor. The 3600 rpm motor will have the same torque as the 1800 but with twice the speed it will have twice the power.


Sounds right to me, with the noted correction.

[clusterm2]

Originally posted by garys_2k
Put a wrench on a tight bolt and try to get it loose. You're applying torque (rotational force), equal to the force you're applying to the wrench handle (force) times the distance your hand is from the bolt's axis (distance). Torque = force * distance.

You know about rotational speed, revolutions per minute (RPM). That's analogous to straight ahead velocity.

Horsepower = Torque * RPM / 5252, where torque is measured in lb-ft (one pound of force applied a foot away from the axis).

So, horespower can increase by either increasing the torque an engine can provide at a given speed, or increasing the speed that it can provide a given torque at. Small Formula One engines have VERY high horsepower ratings because they turn at insane RPMs and still provide moderate torque. As far as the car's concerned, with the right gearing it will all be the same: more acceleration.

So, torque is force and horsepower is torque times rotational speed. Simple?



If that was the case then the torque and horsepower curves, when graphically illustrated, would have the same shape...They don't

[garys_2k]

Originally posted by clusterm2




If that was the case then the torque and horsepower curves, when graphically illustrated, would have the same shape...They don't
The equation is correct but engines may not behave the way you may think they do.

Engines produce only torque and speed, horsepower is a calculated value showing how much energy they are producing per unit of time. Engine torque is multiplied with gears to create a lateral force on the contact patch of the tire where it meets the road. More torque means more force and more acceleration.

Horsepower is a measure of "how quickly" the engine can produce that torque (note to LucyR, please give me some latitude here). Producing a very high torque at a low speed, like a truck engine, is fine for applications where ultimate acceleration isn't needed because the total POWER, torque multiplied by RPM, doesn't have to be too high. But if you want plenty of acceleration for a sustained period of time, like a dragster, go for horsepower and the proper gearing.

The horsepower curve usually peaks a bit after the torque curve because it benefits from both torque and speed. Once the engine's friction (which increases as a quadratic function of engine speed, with constant, linear and squared terms) becomes too great, and induction and cylinder filling losses increase at higher speeds, the torque drops with increasing speed faster than the speed itself increases. This causes the horsepower to drop with increasing RPM after the peak: torque is going down too fast.

So, engines ONLY produce torque and speed. Horsepower is proportional to their product. Does this help?

[Iconoclast]

Originally posted by garys_2k
The horsepower curve usually peaks a bit after the torque curve because it benefits from both torque and speed.

Usually? It's mathematically impossible for the power peak to occur before the torque peak. Furthermore, only with a (theoretical) torque profile that drops immediately to zero after it's peak will the power and torque peaks occur at identical engine speeds. So, for real world engines, the power peak must ALWAYS occur after the torque peak.

[clusterm2]

Originally posted by garys_2k

The equation is correct but engines may not behave the way you may think they do.

Engines produce only torque and speed, horsepower is a calculated value showing how much energy they are producing per unit of time. Engine torque is multiplied with gears to create a lateral force on the contact patch of the tire where it meets the road. More torque means more force and more acceleration.

Horsepower is a measure of "how quickly" the engine can produce that torque (note to LucyR, please give me some latitude here). Producing a very high torque at a low speed, like a truck engine, is fine for applications where ultimate acceleration isn't needed because the total POWER, torque multiplied by RPM, doesn't have to be too high. But if you want plenty of acceleration for a sustained period of time, like a dragster, go for horsepower and the proper gearing.

The horsepower curve usually peaks a bit after the torque curve because it benefits from both torque and speed. Once the engine's friction (which increases as a quadratic function of engine speed, with constant, linear and squared terms) becomes too great, and induction and cylinder filling losses increase at higher speeds, the torque drops with increasing speed faster than the speed itself increases. This causes the horsepower to drop with increasing RPM after the peak: torque is going down too fast.

So, engines ONLY produce torque and speed. Horsepower is proportional to their product. Does this help?



Still doesn't explain a very flat torque curve along with a steep horsepower curve like say on a Buell. Here there appears to be no relationship between the two.

[scotth]

Originally posted by clusterm2




Still doesn't explain a very flat torque curve along with a steep horsepower curve like say on a Buell. Here there appears to be no relationship between the two.

I wide flat torque curve will always cause a steadily rising HP curve.

If torque stays constant across and RPM range, the product of torque * RPM (HP) will have to go up across that range.

[Iconoclast]

Originally posted by clusterm2
Still doesn't explain a very flat torque curve along with a steep horsepower curve like say on a Buell. Here there appears to be no relationship between the two.
Well sorry mate, but rotational power is defined as the product of torque and angular velocity. Either the curves you have are wrong or you haven't done some test calculations against them to check whether or not the power at each point on the curve is equal to the torque at the same point multiplied by the angular velocity at that point.

[garys_2k]

Originally posted by Iconoclast

Usually? It's mathematically impossible for the power peak to occur before the torque peak.
True.

Furthermore, only with a (theoretical) torque profile that drops immediately to zero after it's peak will the power and torque peaks occur at identical engine speeds.

Not at all. All you need is for the torque curve to drop faster per RPM increase than the RPM increases. For example, if the torque drops off at a 3X downslope with a 2X increase in speed, the horsepower will drop after the torque peak. Some engines, usually real stump pullers, have torque and horspower curves with nearly identical peak points. There's no real reason they couldn't be at identical points.

[Iconoclast]

Originally posted by garys_2k
There's no real reason they couldn't be at identical points.
Yes there is. It's called calculus.

[garys_2k]

Originally posted by Iconoclast

Yes there is. It's called calculus.
Really?

What physical impossibility is expressed in this hypothetical torque output?

RPM - - - - Tq - - - - Hp
500 - - - - 75 - - - - 7.14
1000 - - - 85 - - - - 16.2
1500 - - - 100 - - - 28.6
2000 - - - 65 - - - - 24.8
2500 - - - 50 - - - - 23.8
3000 - - - 25 - - - - 14.3

Using advanced engine control you can pretty much make a torque and horsepower curve anything you want, within the physical limits of the hardware, cam, induction. I know, see my home state for a hint at how I know.

[scotth]

Originally posted by garys_2k

Really?

What physical impossibility is expressed in this hypothetical torque output?

RPM - - - - Tq - - - - Hp
500 - - - - 75 - - - - 7.14
1000 - - - 85 - - - - 16.2
1500 - - - 100 - - - 28.6
2000 - - - 65 - - - - 24.8
2500 - - - 50 - - - - 23.8
3000 - - - 25 - - - - 14.3

Using advanced engine control you can pretty much make a torque and horsepower curve anything you want, within the physical limits of the hardware, cam, induction. I know, see my home state for a hint at how I know.

Try doing this with a little finer RPM resolution and you will almost certainly find that it fails.

In real life, torque curves do not drop rapidly enough after peak for peak HP and peak torque to coincide.

[garys_2k]

Originally posted by scotth


Try doing this with a little finer RPM resolution and you will almost certainly find that it fails.

In real life, torque curves do not drop rapidly enough after peak for peak HP and peak torque to coincide.
Oh, but we CAN now make torque curves drop as fast as we want. "Traction control" schemes can allow us to manipulate an engine's torque curve dynamically, setting the peak or slopes to almost arbitrary values.

I was only pointing out that there is no inherent, mathematically-based reason that a torque and power peak couldn't happen at the same engine speed. As long as the torque falls off past that speed faster than the speed itself increases, the power will also decrease.

[scotth]

Originally posted by garys_2k

Oh, but we CAN now make torque curves drop as fast as we want. "Traction control" schemes can allow us to manipulate an engine's torque curve dynamically, setting the peak or slopes to almost arbitrary values.

I was only pointing out that there is no inherent, mathematically-based reason that a torque and power peak couldn't happen at the same engine speed. As long as the torque falls off past that speed faster than the speed itself increases, the power will also decrease.

I would agree that if a torque curve has a very sharp break downward starting at its peak, HP and torque would be at a peak at the same RPM.

Generally, without some out of the ordinary contrivance this doesn't happen.