Torque.. what the @#$ is Torque?

[mooeypoo]

Right, so I am taking a physics class this semester. Most of it is just reminding myself of my Highschool physics, but I encountered a conceptual problem I still remember struggling with in High School too.

 

Disclaimer: I have no problems understanding the *formulas*. That's not my problem; I can either memorize them, or add them up to my formula page. The "what is torque Mathematically" is not the question here.

 

What is Torque? Physically. Can anyone please explain to me what is this force?

 

I understand it is created by rotation, but I can't manage to imagine it, or understand how I can 'feel' it. If I

rotate something around my hand - like a wheel - where do I feel this force? What effect does it have?

 

I guess what I'm looking for is the physical "realistic" manifestation of Torque, and since I can't manage to actually imagine/detect it myself I am having troubles understanding what it is...

 

Help?

 

 

Thanks in advance!

 

~moo

[ydoaPs]

What is Torque? Physically. Can anyone please explain to me what is this force?

 

I understand it is created by rotation, but I can't manage to imagine it, or understand how I can 'feel' it. If I

rotate something around my hand - like a wheel - where do I feel this force? What effect does it have?

Torque can be thought of as how hard something is twisted. Like how tightly you tighten a nut on a bolt. It's basically a force that causes a rotation. I'm not sure if I'm explaining it very well, but you get a feel for torque by changing a tire on your car.

[DrDNA]

Rotational force.

The force that is at a right angle to the axis of rotation.

Pedal a bike and look down at the pedals and the sprockets. Your feet are applying torque ......the force that is making the sprocket turn and the wheel turn.

If you have a wrench you can play with some nuts or bolts. The torque (the force you apply with the wrench at a right angle to the nut or bolt) will loosen or tighten the bolts or nuts. If you have a "torque wrench" handy you can easily measure how much torque you are applying to the nuts or bolts.....if you are out shopping in Sears, the tool section should have some torque wrenches for you to play with and see it for yourself. Hope that helps.

[MrMongoose]

It's just a complex pattern of shear stresses, which means sliding layers within a crsytal over each other and in doing so, seperating atoms slightly, which means electromagnetic force.

 

All the T=Fr stuff just comes from summing all the seperate contributions.

 

If you're looking for something more fundamental then you wont find it because torque really isn't that fundamental.

[ydoaPs]

Rotational force.

The force that is at a right angle to the axis of rotation.

Pedal a bike and look down at the pedals and the sprockets. Your feet are applying torque ......the force that is making the sprocket turn and the wheel turn.

If you have a wrench you can play with some nuts or bolts. The torque (the force you apply with the wrench at a right angle to the nut or bolt) will loosen or tighten the bolts or nuts. If you have a "torque wrench" handy you can easily measure how much torque you are applying to the nuts or bolts.....if you are out shopping in Sears, the tool section should have some torque wrenches for you to play with and see it for yourself.

 

Rotational force.

The force that is at a right angle to the axis of rotation.

Pedal a bike and look down at the pedals and the sprockets. Your feet are applying torque ......the force that is making the sprocket turn and the wheel turn.

If you have a wrench you can play with some nuts or bolts. The torque (the force you apply with the wrench at a right angle to the nut or bolt) will loosen or tighten the bolts or nuts. If you have a "torque wrench" handy you can easily measure how much torque you are applying to the nuts or bolts.....if you are out shopping in Sears, the tool section should have some torque wrenches for you to play with and see it for yourself. Hope that helps.

 

oops

 

 

oops

 

All the T=Fr stuff just comes from summing all the seperate contributions.

I thought it was a cross product.

[math]\vec{\tau}={\vec{F}}X{\vec{r}}[/math]

 

It's been a while since I've done the math, but that doesn't mean I can't use a torque wrench.

[DrDNA]

You have to have a Y chromosome to understand it. Sorry but its the law.

[MrMongoose]

Well yeah, vectorially. But as I missed off the vector arrows, I was clearly talking about the scalar magnitudes, sheesh

 

I was just trying to appeal to the high school nostalgia in him. I'm sure it's not as patronising as telling him to use a wrench and "feel" the torque!

[ydoaPs]

I was just trying to appeal to the high school nostalgia in him. I'm sure it's not as patronising as telling him to use a wrench and "feel" the torque!

 

She said She wanted to feel the force.

[MrMongoose]

I have this awful affliction where I can't look at someone's avatar without assuming it's them... You means she's not Dexter?

[mooeypoo]

Rotational force.

The force that is at a right angle to the axis of rotation.

Pedal a bike and look down at the pedals and the sprockets. Your feet are applying torque ......the force that is making the sprocket turn and the wheel turn.

Wait wait.. sorry to be an idiot here but.. when I pedal, I rotate the wheel.. it's even in the same "direction" as the rotation.. where is the Torque?

 

Fro that matter.. if I would "push" on the wheel from "the side" (IE 90 degrees from the rotation -- where the "torque" is supposed to be..)... would it start rotating?

 

 

If you have a wrench you can play with some nuts or bolts. The torque (the force you apply with the wrench at a right angle to the nut or bolt) will loosen or tighten the bolts or nuts. If you have a "torque wrench" handy you can easily measure how much torque you are applying to the nuts or bolts.....if you are out shopping in Sears, the tool section should have some torque wrenches for you to play with and see it for yourself. Hope that helps.

I'll look those up... when I'm using a wrench (regular one, never heard of 'torque wrench' before) i'm just ROTATING the screw. Of course it takes force, but the force is directional to the rotation, and not 90 degrees to it..

 

 

Egh this is so confusing.

[MrMongoose]

Another way to think about it is just to think of a simple bending beam. If you rotate one end by applying a force, it bends, and thuscompared to a neutral unstretched axis, one side is stretched and the other is compressed, so they will provide moments by virtue of Hookes Law onto whatever is attatched to the other end (or equally back to the source). You can kind of follow a pseudo-inductive path through the system as a whole.

 

Yet another way of looking at it is to grab a ruler in one hand so that the tip of your finger is on top of it at the one end and your thumb holds it up about 2 inches along the underside. Now apply a force to the other end with your other hand, and two discrete forces will act on your finger and thumb. Torque is just the sum of all those forces internally between atoms along the ruler.

[brainfart94]

Here's a simple two word definition: angular momentum.

 

It's simply the force appied to an object that would cause it to move in an angular path. There's nothing there that needs to be "fluffed" up and complicated.

[iNow]

I'll look those up... when I'm using a wrench (regular one, never heard of 'torque wrench' before) i'm just ROTATING the screw.

When tightening a bolt on a nut, you sometimes don't want to tighten it too hard. There are many applications where, if tightened too little or too much, the system won't operate properly. To resolve this, they invented the "torque wrench" which is almost entirely the same as a regular wrench, but which "stops" tightening when a certain limit (which you set according to what you're doing) is reached. So, you tighten... keep tightening.. keep tightening... but then, you reach the amount of torque the wrench is set to and it won't let you tighten the bolt any more (whereas, if you were using a regular "non-torque wrench," you could keep tightening the nut with sheer brute strength).

 

Many of these wrenches, when you reach the torque limit you've set, will just start clicking so your twisting motion is not longer transfered to the nut.

 

 

Physically, I don't really understand the physics of torque, or the explanations, but I always think of my car or motorcycle engines. There's this "feeling" I get when accelerating... that sense the engine is really making the tires spin with extremely little difficulty... the knowledge that pushing the gas pedal just a bit too hard is going to break the friction point of the tires because so much of the engine power is reaching the axle and making the vehicle go... like the tire floats above the road because the engine is spinning it so easily and so quickly... That's how I think of torque.

 

So, on a little vespa scooter... not a lot of torque. On a huge Harley Davidson V-rod... "crap your pants" amount of torque. In a little eco-friendly tiny "smart car"... not a lot of torque. In a 1970s American muscle car, or better yet, a modern day drag racer... large amounts of torque.

 

There's the "I understand it enough to know what it is when I see it, but I can't really teach it" description.

[ydoaPs]

Wait wait.. sorry to be an idiot here but.. when I pedal, I rotate the wheel.. it's even in the same "direction" as the rotation.. where is the Torque?

 

The torque is actually perpendicular to the plane of the force and the rotation.

[MrMongoose]

The torque is actually perpendicular to the plane of the force and the rotation.

 

The rotation is actually perpendicular to the plane of the force and the rotation.

[iNow]

The rotation is actually perpendicular to the plane of the force and the rotation.

 

WTF? How could rotation be perpendicular to itself?

[MrMongoose]

Well The torque is generally colinear with the rotation. Yourdadonapogostick said the rest.

[mooeypoo]

The torque is actually perpendicular to the plane of the force and the rotation.

 

Right.. so where do I "feel" it?

[Mr Skeptic]

Torque is

[math]\vec{\tau}={\vec{F}}X{\vec{r}}[/math]

This vector is perpendicular to both the force and the radial vector; you would say that it is in the direction of the axis of rotation. The sign of the vector shows whether the torque is clockwise or counterclockwise.

[mooeypoo]

Yah, I know that.

 

Where do I feel it though? What's its physical manifestation? I don't get *that* part.. it's so 'vague' to me, it's confusing.

[ydoaPs]

The direction of torque is really just to say if rotation is clockwise or counterclockwise. You can get a feel for torque by turning a tight bolt to make it tighter. Torque is how much force is making the rotation.

 

A torque wrench is just a socket set with a gauge that tells you how much torque you are applying.

[DrDNA]

Think about a simple steam engine. Like an old train.

The piston comes out of the cylinder from the steam presssure and turns the crank.

The piston is perp to the axle (the axis).

Choo Choo....chuga....chuga....choooooooo...........

 

That's torque baby!

[mooeypoo]

Think about a simple steam engine. Like an old train.

The piston comes out of the cylinder from the steam presssure and turns the crank.

The piston is perp to the axle (the axis).

Choo Choo....chuga....chuga....choooooooo...........

 

That's torque baby!

 

I was certain that choochoo trains use steam to rotate the axil similarly to a 'windmill' rotating by wind... but that's not quite perpendicular to the motion.

 

If I ask for a drawing (or.. resource with a drawing?) will that sound REALLY rude?..

 

I just.. I get what you're saying .. kida.. I just.. don't SEE it.. that's what is bothering me :\ I don't know *why* this is such a problem, btw, for me.. it just is.. I'm having REALLY big troubles with it.

[Severian]

I think you may be thinking about a torque as only originating from one force, which is incorrect and can lead to confusion.

 

If I apply a force to an object and it doesn't move that must mean that there is a force pushing back (Newton's third law). Now that force is pushing exactly opposite to the force I am applying (ie. along the same line). But what would happen if they weren't along the same line?

 

Imagine a table in your office. If I push it I can overcome the fristion and move it about. Now imagine a 'friend' pushing it in the opposite direction - if he pushes from exactly opposite you the table will stay still. But if you are pushing in slightly different directions, the table will turn. The two forces acting in slightly different directions are applying a torque to the table making it turn.

 

You actually don't need the friend to see this - friction will do the job for you. Imagine you push the table from off centre (so you are not pushing towards the centre-of-mass). The frictional force opposing the move does act towards the centre of mass, so your forces are not aligned, you are applying a torque and the table will turn.

 

Alternatively, for a pedal on a bike, you are applying a force to move the pedal and the bit of metal attaching it to the bike. But the bike is holding the pivot - it applies a force to keep the pedal from falling off, but the force is not in line with the force you are applying (since it is at the pivot) and you have an unbalanced torque.

 

Just like an unbalanced force produces a change in linear momentum, an unbalanced torque causes a change in angular momentum and the thing turns.

[swansont]

If I apply a force to an object and it doesn't move that must mean that there is a force pushing back (Newton's third law). Now that force is pushing exactly opposite to the force I am applying (ie. along the same line).

 

But that's not a proper application of Newton's third law. If I apply a force, there will always be a force pushing back on me, regardless of the motion of the object. Pushing on a motionless object and having it not move is the result of some second force on the object. Motion of objects are the result of forces on them; the reaction force is exerted on me.