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'm confused now too

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]

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]

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]

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]

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?

 

[Guest]

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]

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...

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.

 

[Guest]

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]

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]

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.

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]

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.