- #1
dE_logics
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Torque is when a couple is formed...and for a moment, a couple needs not be there right?
Difference between torque and moment
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- Difference Moment Torque
In summary, torque is a measure of the moment of a force about a point, and can also refer to a pair of equal-and-opposite forces with a purely rotational effect. It is often used interchangeably with the term "moment", although in engineering, torque is specifically used to refer to the moment of a couple. Torque and moment are both important concepts in physics and engineering, and can be used to explain rotational motion and equilibrium. While torque and moment are often used synonymously, they have slightly different meanings and applications in different contexts.
- #2
tiny-tim
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Hi dE_logics! 
"torque" sometimes means a couple, and sometimes not.
From the PF Library on https://www.physicsforums.com/library.php?do=view_item&itemid=175" …
Torque is the moment of a force about a point.
…
"A torque" is also the name of a pair of equal-and-opposite forces which are not in-line, and so have a purely rotational effect.
…
"Torque" vs. "moment":
The words "torque" and "moment" (of force) mean the same.
However, "torque" tends to be used when there is an axle or pivot to be turned around, while "moment" tends to be used in essentially non-rotational situations, such as analysis of forces on a beam.
"torque" sometimes means a couple, and sometimes not.
From the PF Library on https://www.physicsforums.com/library.php?do=view_item&itemid=175" …
Torque is the moment of a force about a point.
…
"A torque" is also the name of a pair of equal-and-opposite forces which are not in-line, and so have a purely rotational effect.
…
"Torque" vs. "moment":
The words "torque" and "moment" (of force) mean the same.
However, "torque" tends to be used when there is an axle or pivot to be turned around, while "moment" tends to be used in essentially non-rotational situations, such as analysis of forces on a beam.
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- #3
sganesh88
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You topple if you get down from a rapidly moving bus because of the torque exerted by friction alone..
Moment and Torque are not always the same. Moment is a blanket term. Moment of a force is torque. Moment of momentum is angular momentum. Moment of a vector X, is R x X.
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- #4
TVP45
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Although torque and moment of a force are the same, engineers tend to use moment (I believe an older term) as a way of suggesting motion (a cognitive sticky, if you will).
- #5
dE_logics
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tiny-tim said:
You mean in case the moment by each force is not balanced, then it will make a torque.
So moment is only in statics and it turns to torque in kinetics.
The 3 answers sound pretty confusing.
- #6
tiny-tim
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dE_logics said:
You seem to be expecting English to have the same rigour as maths.
In English, "torque" is a bit vague, and in particular, there is a difference between torque of a force and "a torque".
Don't worry!
… the context usually makes it clear. - #7
TVP45
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The two terms really are the same. See this article
http://en.wikipedia.org/wiki/Distinction_without_a_difference
http://en.wikipedia.org/wiki/Distinction_without_a_difference
- #8
dE_logics
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:uhh: Let's just talk science here...
No 100% English vocab.
BTW how do you even relate it to the English torque?...or other meanings?
No 100% English vocab.
BTW how do you even relate it to the English torque?...or other meanings?
- #9
sganesh88
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Balanced or unbalanced, moment of a force about a point is called torque. All you need is a force and a reference point. A force doesn't cease to exist just because it is neutralized by some other force.dE_logics said:
- #10
dE_logics
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sganesh88 said:
Yeah, that's why we have rotation.
So torque and moment are the same thing.
Or is it that, when there is no equilibrium, then its torque, else moment.
- #11
sganesh88
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A torque is a torque is a torque..
- #12
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Moment of force is a synonym for torque. They mean exactly the same thing. Using the term "moment", sans the "of force" qualifier, is physicists being a tad lazy. There are lots of other "moments". In addition to those already mentioned, moment of inertia.
- #13
dE_logics
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:uhh:
- #14
realXenuis
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Moment is the tendency for a force(s) to create rotation about a point.
In Physics: Moment is Torque is Moment. Done.
In Engineering: Moment is Moment. Torque is the Moment of a Couple.
A Couple is a Moment with ZERO NET FORCE.
A Moment of a Couple can be moved anywhere on the body and cause the same rotation. It is thus sometimes called a "Free Vector". Though this may seem non-intuitive, it is a real characteristic of a Couple.
"Equilibrium" is not an intrinsic necessity to qualify a Moment or a Couple. If there is rotational motion, there is definitely no Moment equilibrium (sum of Moments = 0). However, if it's a Statics context, there would definitely be translational equilibrium. If there is a resultant Force or Moment, there is spin, and thus no equilibrium. If you are BALANCING a system, then you will set forces and moments to zero and then FIND equilibrium. I hope that clears it up.
In Physics: Moment is Torque is Moment. Done.
In Engineering: Moment is Moment. Torque is the Moment of a Couple.
A Couple is a Moment with ZERO NET FORCE.
A Moment of a Couple can be moved anywhere on the body and cause the same rotation. It is thus sometimes called a "Free Vector". Though this may seem non-intuitive, it is a real characteristic of a Couple.
"Equilibrium" is not an intrinsic necessity to qualify a Moment or a Couple. If there is rotational motion, there is definitely no Moment equilibrium (sum of Moments = 0). However, if it's a Statics context, there would definitely be translational equilibrium. If there is a resultant Force or Moment, there is spin, and thus no equilibrium. If you are BALANCING a system, then you will set forces and moments to zero and then FIND equilibrium. I hope that clears it up.
- #15
Studiot
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This sems to be a very old thread to resurrect, although the discussion is perennial.
- #16
realXenuis
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True, Studiot. I'm in a Statics class and came across this thread just yesterday (and several other similar). There seems to be no real closure to some of them (uncontested misinformation, missing information, questions unanswered) so I thought I'd at least clear this one up.
- #17
Studiot
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There are two real differences between a moment (or couple) and torque.
Firstly Torque is not limited to a single revolution.
Secondly moments (and couples) are planar beasts - they exist in a plane. Torque, on the other hand, is three dimensional and has the ability to transfer moments from one plane to another.
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- #18
realXenuis
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Studiot said:
Moments/couples certainly exist in 3-D. Their resultant vector projects into the third dimension. This is no different than a Torque.
Couples transfer moments the same as Torques, being free vectors.
Is a Moment limited to a single revolution? Please, show me where or how.
- #19
Studiot
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Perhaps you can display a non planar couple?
If the two forces constituting the couple are not in the same plane how can there be a zero force resultant?
A single force (line) and a single fulcrum (point in space)can only be planar.
- #20
realXenuis
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Studiot said:
Yes. But we're not talking about a Force and a point, we're talking about a Force and a Position vector, with origin at a point. If we're not talking about two vectors, then we can't be talking about a Moment or a Couple.
- #21
Studiot
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Every force has a moment about every single point in space.
You can have a position vector if you like, but it does not add anything except complication.
To create a moment all you require is a point and one single force.
Elementary geometry prescribes that these be coplaner, since you can always establish a plane containing a given line and a given point.
If you introuduce a second non concurrent force to the system you can create a couple.
If you apply this couple to a material body you can cause rotational motion or bending.
Bending is not torque.
If you apply a second couple to the same body you can cause torsion or power transfer. This is the three dimensional effect I was referring to. This is how screwdrivers, socket sets and engine shafts work.
You can have a position vector if you like, but it does not add anything except complication.
To create a moment all you require is a point and one single force.
Elementary geometry prescribes that these be coplaner, since you can always establish a plane containing a given line and a given point.
If you introuduce a second non concurrent force to the system you can create a couple.
If you apply this couple to a material body you can cause rotational motion or bending.
Bending is not torque.
If you apply a second couple to the same body you can cause torsion or power transfer. This is the three dimensional effect I was referring to. This is how screwdrivers, socket sets and engine shafts work.
- #22
realXenuis
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Every force has a moment about every single point in space.
No, a force CAN create a moment about any convenient point on a rigid body when the distance between this point and any point of application along the line of force is represented as a position vector. This representation is not just pedantic, it represents the physical connection between an applied force and an axis. If you just have a point in space and a force in space, then you just have a point in space and a force in space. They don't interact.
This is all described mathematically (and physically) and conforms to vector space rules, including the vector (cross) product. This can be found in any Statics or Physics book. If you can point me to any evidence to the contrary, I'd love to see or be directed to it.
You can have a position vector if you like, but it does not add anything except complication.
No, it's not that I can, but I MUST have a position vector. Conversely, YOU can have a point and a vector, but you won't have a torque, moment, or couple. Holding onto the idea that a point and a vector (force) is all that's needed for a moment is what is adding the complication.
To create a moment all you require is a point and one single force.
Just stating this again does not make it any more valid. You need two vectors to create a Moment.
Elementary geometry prescribes that these be coplaner, since you can always establish a plane containing a given line and a given point.
Elementary geometry prescribes that two parallel forces that don't reside along the same line of action are indeed coplaner, the key term here being Forces, not Moments. A Moment requires a third dimension, for the resultant moment vector. Again, constricting a Moment to 2-D is really just another way of saying the MAGNITUDE of a moment.
If you introuduce a second non concurrent force to the system you can create a couple.
No, you MUST introduce a second force to create a couple, but that's not enough, the force also has to have equal magnitude, opposite direction, and not reside along the line of action of the first force. Just having another force be concurrent is not enough, actually.
If you apply this couple to a material body you can cause rotational motion or bending.
Absolutely it CAN cause rotation (providing there is not equilibrium). I'm not sure why you're introducing bending moments. This is a discussion about Moments, Torques, and Couples.
Bending is not torque.
And Bending doesn't qualify a Moment.
If you apply a second couple to the same body you can cause torsion or power transfer. This is the three dimensional effect I was referring to. This is how screwdrivers, socket sets and engine shafts work.
I don't disagree here.
I can point you to countless references corroborating my understanding of a Moment, including the maths involved. They will all explicitly necessitate a position vector, and not just a point. This vector is most certainly not a "complication", it is a strict necessity. Here is one:
http://www.engin.brown.edu/courses/en3/notes/Statics/moments/moments.htm
I'll provide more if you ask.
Again, if you can point me to any definition of a moment that does not require a position vector, I would love to see it. If you can describe, physically, a Moment involving a point in space and a force, and nothing between them, I'd love to hear it. If you can describe mathematically how to get a perpendicular resultant moment vector, mathematically, from a plane containing a point (scalar) and a vector, then please, I'd love to hear it.
No, a force CAN create a moment about any convenient point on a rigid body when the distance between this point and any point of application along the line of force is represented as a position vector. This representation is not just pedantic, it represents the physical connection between an applied force and an axis. If you just have a point in space and a force in space, then you just have a point in space and a force in space. They don't interact.
This is all described mathematically (and physically) and conforms to vector space rules, including the vector (cross) product. This can be found in any Statics or Physics book. If you can point me to any evidence to the contrary, I'd love to see or be directed to it.
You can have a position vector if you like, but it does not add anything except complication.
No, it's not that I can, but I MUST have a position vector. Conversely, YOU can have a point and a vector, but you won't have a torque, moment, or couple. Holding onto the idea that a point and a vector (force) is all that's needed for a moment is what is adding the complication.
To create a moment all you require is a point and one single force.
Just stating this again does not make it any more valid. You need two vectors to create a Moment.
Elementary geometry prescribes that these be coplaner, since you can always establish a plane containing a given line and a given point.
Elementary geometry prescribes that two parallel forces that don't reside along the same line of action are indeed coplaner, the key term here being Forces, not Moments. A Moment requires a third dimension, for the resultant moment vector. Again, constricting a Moment to 2-D is really just another way of saying the MAGNITUDE of a moment.
If you introuduce a second non concurrent force to the system you can create a couple.
No, you MUST introduce a second force to create a couple, but that's not enough, the force also has to have equal magnitude, opposite direction, and not reside along the line of action of the first force. Just having another force be concurrent is not enough, actually.
If you apply this couple to a material body you can cause rotational motion or bending.
Absolutely it CAN cause rotation (providing there is not equilibrium). I'm not sure why you're introducing bending moments. This is a discussion about Moments, Torques, and Couples.
Bending is not torque.
And Bending doesn't qualify a Moment.
If you apply a second couple to the same body you can cause torsion or power transfer. This is the three dimensional effect I was referring to. This is how screwdrivers, socket sets and engine shafts work.
I don't disagree here.
I can point you to countless references corroborating my understanding of a Moment, including the maths involved. They will all explicitly necessitate a position vector, and not just a point. This vector is most certainly not a "complication", it is a strict necessity. Here is one:
http://www.engin.brown.edu/courses/en3/notes/Statics/moments/moments.htm
I'll provide more if you ask.
Again, if you can point me to any definition of a moment that does not require a position vector, I would love to see it. If you can describe, physically, a Moment involving a point in space and a force, and nothing between them, I'd love to hear it. If you can describe mathematically how to get a perpendicular resultant moment vector, mathematically, from a plane containing a point (scalar) and a vector, then please, I'd love to hear it.
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- #23
Studiot
- 5,440
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That was a big post, with many points to discuss.
Remembering the thread title I will take the last one first.
If we have a shaft and apply a single couple to it we simply have a rotating shaft. No torque is involved.
If we apply a second equal counter acting couple coaxially then the shaft will not rotate it will twist.
This is torque.
I think you agree with this?
Alternatively we can apply a lesser couple (brake?) and extract work.
I am sorry to disillusion you but a bending moment is a true moment.
If you take the case of the root an encastre cantilever, there are two support reactions.
One of these is a moment, not a force, nor yet a couple. In other cases a bending moment may be the result of a couple.
With regard to vectors, vectors are not necessary to consider moments. And moments certainly do not need three dimensions. Take the calculation for the centroid of an L shaped lamina, so beloved of mechanics exam questions. The point in question is in free space, and not part of the body at all. Yet we take moments about it.
Nor is the third dimension required for a rotational vector, which incidentally is not a true vector at all, since it does not obey the commutative law of addition.
In my experience and if one has learned one's lessons well at a particular level, one of the most difficult things to do is to unlearn these lessons when proceeding to the next level. I found this particularly so in the case of vectors.
go well
Remembering the thread title I will take the last one first.
If we have a shaft and apply a single couple to it we simply have a rotating shaft. No torque is involved.
If we apply a second equal counter acting couple coaxially then the shaft will not rotate it will twist.
This is torque.
I think you agree with this?
Alternatively we can apply a lesser couple (brake?) and extract work.
I am sorry to disillusion you but a bending moment is a true moment.
If you take the case of the root an encastre cantilever, there are two support reactions.
One of these is a moment, not a force, nor yet a couple. In other cases a bending moment may be the result of a couple.
With regard to vectors, vectors are not necessary to consider moments. And moments certainly do not need three dimensions. Take the calculation for the centroid of an L shaped lamina, so beloved of mechanics exam questions. The point in question is in free space, and not part of the body at all. Yet we take moments about it.
Nor is the third dimension required for a rotational vector, which incidentally is not a true vector at all, since it does not obey the commutative law of addition.
In my experience and if one has learned one's lessons well at a particular level, one of the most difficult things to do is to unlearn these lessons when proceeding to the next level. I found this particularly so in the case of vectors.
go well
- #24
realXenuis
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That was a big post, with many points to discuss.
It was big because you are kitchen-sinking.
Remembering the thread title I will take the last one first.
If we have a shaft and apply a single couple to it we simply have a rotating shaft. No torque is involved.
No, we have a shaft and a couple. Rotation is not necessary, we still have a torque and a moment. We have a body, acted upon by forces in the configuration of a Couple. Because it happens to be rotating changes nothing. There is mass, and it's being acted upon by rotational (as opposed to translational) forces. There IS torque/moment. And it DOES include a position vector. These are the kind of posts that add to the confusion. This is why I posted in the first place.
I think you're probably trying to use specifically an example of a FRICTIONLESS SHAFT. Ignoring that there is no such thing in reality, still, you are rotating a body when you apply a couple. When analyzing the system, the shaft would be included in any free body diagram. If that body has mass, then you have Torque (couple). There is no getting around that. If there is no body (mass) then you have nothing to apply force to. This is a simple concept, and building up complication does not make your argument work any better. In fact, you made my argument for me: a couple applied to a shaft made the shaft rotate. It had to have torque/moment. You would agree now, right?
If we apply a second equal counter acting couple coaxially then the shaft will not rotate it will twist.
Sure, it CAN twist. And you don't even need a second couple.
This is torque.
Well, yes. There was torque/moment in the first place. If we apply another torque, sure there is still torque.
I think you agree with this?
Yes, i agree if you have torque and you apply a torque, there can still be torque. Yes, we agree!
Alternatively we can apply a lesser couple (brake?) and extract work.
Sure.
I am sorry to disillusion you but a bending moment is a true moment.
Don't be sorry, I'm quite illusioned! You introduced the qualifier "Bending". I don't disagree that this is a moment, but it's a distinction, a certain way to look at a moment. My point was, there is no need to introduce Bending Moment into our discussion, as Moment did just fine, and it has not changed your argument. A torque is still a moment.
If you take the case of the root an encastre cantilever, there are two support reactions.
One of these is a moment, not a force, nor yet a couple. In other cases a bending moment may be the result of a couple.
Show me an example, online.
With regard to vectors, vectors are not necessary to consider moments.
Yes, they are. I've given you an example, as evidence. Please, show (not tell) me yours, won't you?
And moments certainly do not need three dimensions.
Yes, they do. Moments NEED 3 dimensions. It is in the example I've give you. Show me yours.
Take the calculation for the centroid of an L shaped lamina, so beloved of mechanics exam questions. The point in question is in free space, and not part of the body at all. Yet we take moments about it.
Not exactly. The point is the center mass (of a plane), and the moment can be taken using this (or any arbitrary) point. Still, a Position Vector is taken from this point. The real physical interaction is actually happening along the L-shaped body. The force is being transferred along it. You still HAVE to have a vector. You still HAVE to have something for the force to act upon. A dot and a force in space are just a dot and a force. Please point me to an example of a centroid with a force acting upon it resulting in a Moment but NOT containing a POSITION VECTOR.
Nor is the third dimension required for a rotational vector, which incidentally is not a true vector at all, since it does not obey the commutative law of addition.
A third dimension is necessary for a Moment. It has to. A moment vector, not so incidentally, IS in fact a vector. I covered this in "pseudo vector" above. The math still works. We just use conventions for the direction of it's projection. Please, point me to an example of a Moment which DOES NOT result in a Moment vector pointing in a dimension not occupied by the plane of the forces in which the Moment was created. Please. I beg you.
In my experience and if one has learned one's lessons well at a particular level, one of the most difficult things to do is to unlearn these lessons when proceeding to the next level. I found this particularly so in the case of vectors.
I agree with you here. That is why I posted here originally. It seems particularly important to post accurate and specific, as well as factual, information about math/physics/engineering on of all things a Physics forum. I have, and still do, source them, and know well the frustration of misinformation.
I've pointed you to a website supporting my claims. Please, show me/us some evidence supporting your claims. I don't think that's too much to ask. If you're right, there should be a multitude of reputable sources out there you could quickly link to.
It was big because you are kitchen-sinking.
Remembering the thread title I will take the last one first.
If we have a shaft and apply a single couple to it we simply have a rotating shaft. No torque is involved.
No, we have a shaft and a couple. Rotation is not necessary, we still have a torque and a moment. We have a body, acted upon by forces in the configuration of a Couple. Because it happens to be rotating changes nothing. There is mass, and it's being acted upon by rotational (as opposed to translational) forces. There IS torque/moment. And it DOES include a position vector. These are the kind of posts that add to the confusion. This is why I posted in the first place.
I think you're probably trying to use specifically an example of a FRICTIONLESS SHAFT. Ignoring that there is no such thing in reality, still, you are rotating a body when you apply a couple. When analyzing the system, the shaft would be included in any free body diagram. If that body has mass, then you have Torque (couple). There is no getting around that. If there is no body (mass) then you have nothing to apply force to. This is a simple concept, and building up complication does not make your argument work any better. In fact, you made my argument for me: a couple applied to a shaft made the shaft rotate. It had to have torque/moment. You would agree now, right?
If we apply a second equal counter acting couple coaxially then the shaft will not rotate it will twist.
Sure, it CAN twist. And you don't even need a second couple.
This is torque.
Well, yes. There was torque/moment in the first place. If we apply another torque, sure there is still torque.
I think you agree with this?
Yes, i agree if you have torque and you apply a torque, there can still be torque. Yes, we agree!
Alternatively we can apply a lesser couple (brake?) and extract work.
Sure.
I am sorry to disillusion you but a bending moment is a true moment.
Don't be sorry, I'm quite illusioned! You introduced the qualifier "Bending". I don't disagree that this is a moment, but it's a distinction, a certain way to look at a moment. My point was, there is no need to introduce Bending Moment into our discussion, as Moment did just fine, and it has not changed your argument. A torque is still a moment.
If you take the case of the root an encastre cantilever, there are two support reactions.
One of these is a moment, not a force, nor yet a couple. In other cases a bending moment may be the result of a couple.
Show me an example, online.
With regard to vectors, vectors are not necessary to consider moments.
Yes, they are. I've given you an example, as evidence. Please, show (not tell) me yours, won't you?
And moments certainly do not need three dimensions.
Yes, they do. Moments NEED 3 dimensions. It is in the example I've give you. Show me yours.
Take the calculation for the centroid of an L shaped lamina, so beloved of mechanics exam questions. The point in question is in free space, and not part of the body at all. Yet we take moments about it.
Not exactly. The point is the center mass (of a plane), and the moment can be taken using this (or any arbitrary) point. Still, a Position Vector is taken from this point. The real physical interaction is actually happening along the L-shaped body. The force is being transferred along it. You still HAVE to have a vector. You still HAVE to have something for the force to act upon. A dot and a force in space are just a dot and a force. Please point me to an example of a centroid with a force acting upon it resulting in a Moment but NOT containing a POSITION VECTOR.
Nor is the third dimension required for a rotational vector, which incidentally is not a true vector at all, since it does not obey the commutative law of addition.
A third dimension is necessary for a Moment. It has to. A moment vector, not so incidentally, IS in fact a vector. I covered this in "pseudo vector" above. The math still works. We just use conventions for the direction of it's projection. Please, point me to an example of a Moment which DOES NOT result in a Moment vector pointing in a dimension not occupied by the plane of the forces in which the Moment was created. Please. I beg you.
In my experience and if one has learned one's lessons well at a particular level, one of the most difficult things to do is to unlearn these lessons when proceeding to the next level. I found this particularly so in the case of vectors.
I agree with you here. That is why I posted here originally. It seems particularly important to post accurate and specific, as well as factual, information about math/physics/engineering on of all things a Physics forum. I have, and still do, source them, and know well the frustration of misinformation.
I've pointed you to a website supporting my claims. Please, show me/us some evidence supporting your claims. I don't think that's too much to ask. If you're right, there should be a multitude of reputable sources out there you could quickly link to.
- #25
Studiot
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This isn't a p__ing contest.
It is supposed to be an opportunity in learning and understanding.
So if you really don't understand how an encastre cantilever works, simply ask and I will explain.
Do you have any idea what a concurrent force is or what the implications of such a force might be, because the above response suggests otherwise?
For all you know the shaft might be floating freely in space. I did not specify any other forces/agents/moments acting quite deliberately.
Because I wanted to make the point that if the only such agent you apply to said shaft is a single couple, it must perforce rotate indefinitely. Your jumble of a response has completely turned that round.
So, no I don't agree, I stand by what I originally said.
- #26
realXenuis
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This isn't a p__ing contest.
You've managed to confuse "p___ing contest" with requesting evidence. So, where's that evidence?
It is supposed to be an opportunity in learning and understanding.
Absolutely, as well as discussion. I will go further and say that it's destructive to learning to provide misinformation. So, you've disagreed with my representations of these things. We've articulated specific points of contention. I've provided evidence. You've changed your tactics, and have provided no evidence. Show some evidence, specific to the points we disagree on. This is simple.
So if you really don't understand how an encastre cantilever works, simply ask and I will explain.
You've misunderstood. I simply asked for an example. I don't believe I mentioned my knowledge of the subject. If it were true that I did NOT know how an encastre cantilever works, would this change the arguments you've given? It doesn't have to veer down the path of "I know more than you". THAT would be a p___ing contest. Now, if you link to an example, we can go from there, no?
Do you have any idea what a concurrent force is or what the implications of such a force might be, because the above response suggests otherwise?
Sure. But a concurrent force does not imply equal magnitude. That is why i went further in describing what's needed for a Couple. It's not good enough to just have a second nonconcurrent force. So yes, I would say that I do know what it means. Would you agree?
For all you know the shaft might be floating freely in space. I did not specify any other forces/agents/moments acting quite deliberately.
I understand your approach. That is what i addressed with "..Frictionless...". I'm not sure you're understanding or acknowledging mine. Here, let me be very explicit:
With..OR without, any rotation, if you have a couple applied to any body, be it shaft or whatever, you indeed do have to have a torque/moment. To ignore this or say it's not true is to ignore engineering, physics, and math. A couple REQUIRES two forces. a body to apply them to, a position vector, and a third dimension. I have given evidence of this, and will give more if you just ask. If you disagree with this somewhat specific representation, please, give evidence that does not consist of you just restating your position.
Because I wanted to make the point that if the only such agent you apply to said shaft is a single couple, it must perforce rotate indefinitely. Your jumble of a response has completely turned that round.
Rotation, be it indefinite or definite, is not at issue here. You're confusing concepts. You CAN include rotation in your example, but it absolutely does not change the fact that a couple includes a moment. How about we go all out and say a single couple causes rotation of a shaft which spins indefinitely until the end of human kind. The couple used to spin it still created a moment.
I'm sorry if my response was a jumble to you. I could answer with 2 line restatements of my opinion, but that would not really get us anywhere.
I think it's 3 responses now since i asked for something, anything, that could be considered evidence. If you are right, then show some evidence. What can it hurt you? To the contrary, propagating misinformation serves no constructive purpose to the thread.
If through this convo we can clear up some ideas, then we've done something. I've given evidence that supports my points, so that anyone reading this post can verify my representations. Otherwise, I'm just a person with an opinion. You've challenged my representations, and that's fine. But you've provided no backup. You've had every opportunity to back it up. The longer you choose deliberately to avoid presenting any evidence, the weaker your argument is becoming, and with good reason.
It' doesn't have to be a long reply. Just reply with a link. Could anything be easier?
So, no I don't agree, I stand by what I originally said.[/QUOTE]
Right. Then back it up.
You've managed to confuse "p___ing contest" with requesting evidence. So, where's that evidence?
It is supposed to be an opportunity in learning and understanding.
Absolutely, as well as discussion. I will go further and say that it's destructive to learning to provide misinformation. So, you've disagreed with my representations of these things. We've articulated specific points of contention. I've provided evidence. You've changed your tactics, and have provided no evidence. Show some evidence, specific to the points we disagree on. This is simple.
So if you really don't understand how an encastre cantilever works, simply ask and I will explain.
You've misunderstood. I simply asked for an example. I don't believe I mentioned my knowledge of the subject. If it were true that I did NOT know how an encastre cantilever works, would this change the arguments you've given? It doesn't have to veer down the path of "I know more than you". THAT would be a p___ing contest. Now, if you link to an example, we can go from there, no?
Do you have any idea what a concurrent force is or what the implications of such a force might be, because the above response suggests otherwise?
Sure. But a concurrent force does not imply equal magnitude. That is why i went further in describing what's needed for a Couple. It's not good enough to just have a second nonconcurrent force. So yes, I would say that I do know what it means. Would you agree?
For all you know the shaft might be floating freely in space. I did not specify any other forces/agents/moments acting quite deliberately.
I understand your approach. That is what i addressed with "..Frictionless...". I'm not sure you're understanding or acknowledging mine. Here, let me be very explicit:
With..OR without, any rotation, if you have a couple applied to any body, be it shaft or whatever, you indeed do have to have a torque/moment. To ignore this or say it's not true is to ignore engineering, physics, and math. A couple REQUIRES two forces. a body to apply them to, a position vector, and a third dimension. I have given evidence of this, and will give more if you just ask. If you disagree with this somewhat specific representation, please, give evidence that does not consist of you just restating your position.
Because I wanted to make the point that if the only such agent you apply to said shaft is a single couple, it must perforce rotate indefinitely. Your jumble of a response has completely turned that round.
Rotation, be it indefinite or definite, is not at issue here. You're confusing concepts. You CAN include rotation in your example, but it absolutely does not change the fact that a couple includes a moment. How about we go all out and say a single couple causes rotation of a shaft which spins indefinitely until the end of human kind. The couple used to spin it still created a moment.
I'm sorry if my response was a jumble to you. I could answer with 2 line restatements of my opinion, but that would not really get us anywhere.
I think it's 3 responses now since i asked for something, anything, that could be considered evidence. If you are right, then show some evidence. What can it hurt you? To the contrary, propagating misinformation serves no constructive purpose to the thread.
If through this convo we can clear up some ideas, then we've done something. I've given evidence that supports my points, so that anyone reading this post can verify my representations. Otherwise, I'm just a person with an opinion. You've challenged my representations, and that's fine. But you've provided no backup. You've had every opportunity to back it up. The longer you choose deliberately to avoid presenting any evidence, the weaker your argument is becoming, and with good reason.
It' doesn't have to be a long reply. Just reply with a link. Could anything be easier?
So, no I don't agree, I stand by what I originally said.[/QUOTE]
Right. Then back it up.
- #27
Studiot
- 5,440
- 9
OK so let's examine your apparently oh so reasonable statements shall we?
I stated
"If we have a shaft and apply a single couple to it we simply have a rotating shaft. No torque is involved."
Which you flatly contradicted
"Rotation is not necessary"
Well you are wrong as I subsequently stated.
But you are still trying to obscure this very important point.
well either
You do understand that a moment is necessary as a support reaction at one end of a cantilever, to prevent the other end from collapsing, in which case you are wasting my time demanding an example which is downright rude and inconsiderate,
or,
You do not understand this and would rather argue than simply say 'what do you mean please explain' Once again if you need an explanation then have the good grace to accept it in the form offered rather than trying to prescribe the form as though you know what you are talking about. It is not a question of example, but proper explanation.
In this passage you clearly state what you think are conditions for two concurrent forces to form a couple.
It is a physical impossibility for two or any number of concurrent forces to form a couple. This is basic stuff.
Just remember that you are the one who revived this thread with the stated intention of clearing up some alleged misstatements by others.
I stated
"If we have a shaft and apply a single couple to it we simply have a rotating shaft. No torque is involved."
Which you flatly contradicted
"Rotation is not necessary"
Well you are wrong as I subsequently stated.
But you are still trying to obscure this very important point.
well either
You do understand that a moment is necessary as a support reaction at one end of a cantilever, to prevent the other end from collapsing, in which case you are wasting my time demanding an example which is downright rude and inconsiderate,
or,
You do not understand this and would rather argue than simply say 'what do you mean please explain' Once again if you need an explanation then have the good grace to accept it in the form offered rather than trying to prescribe the form as though you know what you are talking about. It is not a question of example, but proper explanation.
In this passage you clearly state what you think are conditions for two concurrent forces to form a couple.
It is a physical impossibility for two or any number of concurrent forces to form a couple. This is basic stuff.
Just remember that you are the one who revived this thread with the stated intention of clearing up some alleged misstatements by others.
- #28
realXenuis
- 12
- 0
OK so let's examine your apparently oh so reasonable statements shall we?
I stated
"If we have a shaft and apply a single couple to it we simply have a rotating shaft. No torque is involved."
Which you flatly contradicted
"Rotation is not necessary"
Well you are wrong as I subsequently stated.
If I am wrong, show me some evidence. I told you why you were wrong, and gave evidence. You're just repeating yourself. Typing in a summary of how we disagreed and how you stated i was wrong is just keeping the minutes. Again, show me the evidence that I'm wrong.
But you are still trying to obscure this very important point.
Tell me exactly what I've attempted to obscure, and when. You seem to be kitchen-sinking again. This is getting pretty weak.
well either
You do understand that a moment is necessary as a support reaction at one end of a cantilever, to prevent the other end from collapsing, in which case you are wasting my time demanding an example which is downright rude and inconsiderate,
or,
You do not understand this and would rather argue than simply say 'what do you mean please explain' Once again if you need an explanation then have the good grace to accept it in the form offered rather than trying to prescribe the form as though you know what you are talking about. It is not a question of example, but proper explanation.
or,
I do understand it, having covered it in at least 3 classes now, including the one I'm in, and see no point in allowing you to travel along another line of reasoning to add complication.
or,
I didn't understand your verbal description of the example and simply desired another one rather than go through this with you.
You can choose any of the above. It doesn't matter. But, I certainly was not trying to be rude. If you reread what I said and how I said it, there's nothing rude about it. I simply wanted an example, as opposed to your verbal description.
Having said that, we can of course use that example. I'll go down that road with you, but I can assure you, neither you nor I will stumble across any couples without a position vector. I have proof, do you?
In this passage you clearly state what you think are conditions for two concurrent forces to form a couple.
Oh I see now. No, sorry that was a typo, i left "non" off of the front. However if you re-read it w/"non" on the front, my reply will then make sense. So, after you've re-read that, feel free to let me know if you disagree with what I was saying and why.
It is a physical impossibility for two or any number of concurrent forces to form a couple. This is basic stuff.
Yes, it is. Again, sorry for the typo. I'll add, knowing a Moment has to have a position vector is basic stuff (statics) as well. I admit my typo. Do you admit you're wrong?
Just remember that you are the one who revived this thread with the stated intention of clearing up some alleged misstatements by others.[/QUOTE]
It's hard not to remember it when I posted it in my very last reply.
Again, you refuse to give any proof. I'm sure you must understand how that might make someone look, right? If I had to guess, I'd assume you just don't have any proof. I mean, you'd have presented it by now, instead if quibbling about whether I understand (of all things) cantilevers, right?
This should be simple, and to the point, and contrast any difference we have in our understanding of moments and couples. I'll ask a question, and I'll answer it and then you can answer it:
Does a Couple ALWAYS require BOTH a force vector AND a position vector?
I answer yes. Your answer: ____________
Does a Couple or Moment ALWAYS REQUIRE 3 dimensions to exist in?
I answer yes. Your answer:_____________
Does a Couple applied to a shaft, causing it to rotate (or not), ALWAYS REQUIRE a Moment?
I answer yes. Your answer:_____________
Does realXenuis's understanding of the concept of a cantilever in any way change any of the arguments he or Studiot has made about any of their previously articulated, and on the part of realXenius, proven, arguments?
I answer no. Your answer:______________
I stated
"If we have a shaft and apply a single couple to it we simply have a rotating shaft. No torque is involved."
Which you flatly contradicted
"Rotation is not necessary"
Well you are wrong as I subsequently stated.
If I am wrong, show me some evidence. I told you why you were wrong, and gave evidence. You're just repeating yourself. Typing in a summary of how we disagreed and how you stated i was wrong is just keeping the minutes. Again, show me the evidence that I'm wrong.
But you are still trying to obscure this very important point.
Tell me exactly what I've attempted to obscure, and when. You seem to be kitchen-sinking again. This is getting pretty weak.
well either
You do understand that a moment is necessary as a support reaction at one end of a cantilever, to prevent the other end from collapsing, in which case you are wasting my time demanding an example which is downright rude and inconsiderate,
or,
You do not understand this and would rather argue than simply say 'what do you mean please explain' Once again if you need an explanation then have the good grace to accept it in the form offered rather than trying to prescribe the form as though you know what you are talking about. It is not a question of example, but proper explanation.
or,
I do understand it, having covered it in at least 3 classes now, including the one I'm in, and see no point in allowing you to travel along another line of reasoning to add complication.
or,
I didn't understand your verbal description of the example and simply desired another one rather than go through this with you.
You can choose any of the above. It doesn't matter. But, I certainly was not trying to be rude. If you reread what I said and how I said it, there's nothing rude about it. I simply wanted an example, as opposed to your verbal description.
Having said that, we can of course use that example. I'll go down that road with you, but I can assure you, neither you nor I will stumble across any couples without a position vector. I have proof, do you?
In this passage you clearly state what you think are conditions for two concurrent forces to form a couple.
Oh I see now. No, sorry that was a typo, i left "non" off of the front. However if you re-read it w/"non" on the front, my reply will then make sense. So, after you've re-read that, feel free to let me know if you disagree with what I was saying and why.
It is a physical impossibility for two or any number of concurrent forces to form a couple. This is basic stuff.
Yes, it is. Again, sorry for the typo. I'll add, knowing a Moment has to have a position vector is basic stuff (statics) as well. I admit my typo. Do you admit you're wrong?
Just remember that you are the one who revived this thread with the stated intention of clearing up some alleged misstatements by others.[/QUOTE]
It's hard not to remember it when I posted it in my very last reply.
Again, you refuse to give any proof. I'm sure you must understand how that might make someone look, right? If I had to guess, I'd assume you just don't have any proof. I mean, you'd have presented it by now, instead if quibbling about whether I understand (of all things) cantilevers, right?
This should be simple, and to the point, and contrast any difference we have in our understanding of moments and couples. I'll ask a question, and I'll answer it and then you can answer it:
Does a Couple ALWAYS require BOTH a force vector AND a position vector?
I answer yes. Your answer: ____________
Does a Couple or Moment ALWAYS REQUIRE 3 dimensions to exist in?
I answer yes. Your answer:_____________
Does a Couple applied to a shaft, causing it to rotate (or not), ALWAYS REQUIRE a Moment?
I answer yes. Your answer:_____________
Does realXenuis's understanding of the concept of a cantilever in any way change any of the arguments he or Studiot has made about any of their previously articulated, and on the part of realXenius, proven, arguments?
I answer no. Your answer:______________
- #29
Studiot
- 5,440
- 9
Here is a simple plane frame, with three identical cantilevers, loaded with weights, W.
I have chosen to place the origin at the bottom left hand corner, so all position vectors are measured from here, by definition.
I have shown the support reactions for each cantilever (of course they are all equal).
However since the cantilevers are located in different places the postion vectors to their respective roots, A, B and C are different.
I repeat, despite different position vectors their support reaction moments are identical.
Why do I need three dimensions to analyse this frame? All the necessary analysis is independant of the third, z, axis.
What benefit do I derive from employing position vectors in the analysis?
It is true that for instance the method of tension coefficients (which does use position vectors) might be employed.
Your other claim is that torque and moment and couples are identical.
Please describe the two forces required to create a couple acting at A, B and C
If a moment or couple were applied to a cantilever in a plane parallel to the yz plane this would produce a torque on the cantilever about the x axis. This effect of this torque would be felt all along the cantilever, not just in the plane where it is applied.
Would the support reaction moments have any such effect anywhere else on the z axis, other than in the plane of the frame? Obviously not.
I think that covers all your beefs.
I have chosen to place the origin at the bottom left hand corner, so all position vectors are measured from here, by definition.
I have shown the support reactions for each cantilever (of course they are all equal).
However since the cantilevers are located in different places the postion vectors to their respective roots, A, B and C are different.
I repeat, despite different position vectors their support reaction moments are identical.
Why do I need three dimensions to analyse this frame? All the necessary analysis is independant of the third, z, axis.
What benefit do I derive from employing position vectors in the analysis?
It is true that for instance the method of tension coefficients (which does use position vectors) might be employed.
Your other claim is that torque and moment and couples are identical.
Please describe the two forces required to create a couple acting at A, B and C
If a moment or couple were applied to a cantilever in a plane parallel to the yz plane this would produce a torque on the cantilever about the x axis. This effect of this torque would be felt all along the cantilever, not just in the plane where it is applied.
Would the support reaction moments have any such effect anywhere else on the z axis, other than in the plane of the frame? Obviously not.
I think that covers all your beefs.
Attachments
- #30
realXenuis
- 12
- 0
Studiot said:
No, it in no way does. You could simply answer yes/no to the questions in my last reply. That's all this thread needs at this point.
- #31
Studiot
- 5,440
- 9
Boosting your post with a copy of mine in no way enhances the paucity of your reply.
If you read my post properly you would have seen that I displayed one example that negates all your theory.
Let me ask you this one question.
Your 3 dimensional theory of moments has to work in all cases and situations, to be universally valid.
That means it has to work in the case where we are restricted to 2 dimensions.
If it is unable to accomplish this then it fails.
If you read my post properly you would have seen that I displayed one example that negates all your theory.
Let me ask you this one question.
Your 3 dimensional theory of moments has to work in all cases and situations, to be universally valid.
That means it has to work in the case where we are restricted to 2 dimensions.
If it is unable to accomplish this then it fails.
- #32
realXenuis
- 12
- 0
Studiot said:
This makes no sense. You're just making up sentences now. Are the rest of your 1600 posts on the physicsforum of this high quality?
You really shouldn't reply. Why would you want to intentionally mislead people about Moments on a Physicsforum thread?
- #33
Studiot
- 5,440
- 9
Perhaps a little history might help?
Ca 250 BC
The mechanics of turning effects was known to the ancient world for example the principle of levers attributed to Archimedes.
1725
The term moment was introduced and formally defined by Varignon in his book
'Nouvelle Mechanique.'
“The moment of a force, P, about a point O is defined as the product of that force into the perpendicular OM drawn to its line of action from O, this perpendicular being reckoned positive or negative according as it lies to the left or right of the of the direction of P."
1750 – 1804
St Vennant investigated the torsion of prismatic bars and posed St Vennant’s Problem.
He did not however introduce new concepts in turning.
1804 -1806
Poinsot published his book 'Elements de Statique' and the theorem that bears his name.
This introduced two things. He defined and introduced the term ‘couple’ and the theorem which states that in 3 dimensions any system of forces may be reduced to a single force plus a couple, in a plane perpendicular to the line of action of the force.
He clearly defined his couple to exist in a plane.
1912
Lamb, one of the most prominent applied mathematicians of his time, proposed that the term ‘torque’ be introduced to replace ‘couple’
Lamb 'Statics' p52.
“Since a couple in a given plane is for the purposes of pure statics sufficiently defined by its moment, it has been proposed to introduce a name torque or twisting effect which shall be free from the irrelevant suggestion of two particular forces.”
This suggestion was not, however generally adopted.
Indeed the three most influential texts ( in this subject) of that era and since carried on as before.
1926 Love ‘A Treatise on the Mathematical Theory of Elasticity’
1936 Southwell ‘Theory of Elasticity’
These both refer to ‘Torsional Couples’ for the 3D effects described in St Vennant’s Problem.
1934
Timoshenko published the third standard text, ‘Theory of Elasticity’ and clearly establish torque in this 3D role.
In fact most authors in the second part of the 20th century have followed the notation set by Timoshenko in elasticity.
What I have said is in total accord with the established Timoshenko convention. It has suited most purposes well and I see no compelling reason to change it.
Ca 250 BC
The mechanics of turning effects was known to the ancient world for example the principle of levers attributed to Archimedes.
1725
The term moment was introduced and formally defined by Varignon in his book
'Nouvelle Mechanique.'
“The moment of a force, P, about a point O is defined as the product of that force into the perpendicular OM drawn to its line of action from O, this perpendicular being reckoned positive or negative according as it lies to the left or right of the of the direction of P."
1750 – 1804
St Vennant investigated the torsion of prismatic bars and posed St Vennant’s Problem.
He did not however introduce new concepts in turning.
1804 -1806
Poinsot published his book 'Elements de Statique' and the theorem that bears his name.
This introduced two things. He defined and introduced the term ‘couple’ and the theorem which states that in 3 dimensions any system of forces may be reduced to a single force plus a couple, in a plane perpendicular to the line of action of the force.
He clearly defined his couple to exist in a plane.
1912
Lamb, one of the most prominent applied mathematicians of his time, proposed that the term ‘torque’ be introduced to replace ‘couple’
Lamb 'Statics' p52.
“Since a couple in a given plane is for the purposes of pure statics sufficiently defined by its moment, it has been proposed to introduce a name torque or twisting effect which shall be free from the irrelevant suggestion of two particular forces.”
This suggestion was not, however generally adopted.
Indeed the three most influential texts ( in this subject) of that era and since carried on as before.
1926 Love ‘A Treatise on the Mathematical Theory of Elasticity’
1936 Southwell ‘Theory of Elasticity’
These both refer to ‘Torsional Couples’ for the 3D effects described in St Vennant’s Problem.
1934
Timoshenko published the third standard text, ‘Theory of Elasticity’ and clearly establish torque in this 3D role.
In fact most authors in the second part of the 20th century have followed the notation set by Timoshenko in elasticity.
What I have said is in total accord with the established Timoshenko convention. It has suited most purposes well and I see no compelling reason to change it.
- #34
realXenuis
- 12
- 0
Studiot said:
- #35
- 15,464
- 690
This thread has degenerated into a sophomoric debate over terminology. Moreover, the terms being debated have slightly different means in physics and engineering, as well as from text to text.
Thread locked pending moderation.
Thread locked pending moderation.
Last edited:
1. What is the difference between torque and moment?
Torque and moment are two terms used to describe the rotational force applied to an object. Torque is the measure of the force that causes an object to rotate around an axis, while moment is the measure of the tendency of an object to rotate around a fixed point.
2. How are torque and moment related?
Torque and moment are closely related as they both involve rotational forces. Torque is the actual force applied to an object, while moment is the measure of the effect of that force on the object's rotation.
3. Can torque and moment be measured in different units?
Yes, torque is typically measured in units of force multiplied by distance, such as newton-meters or pound-feet. Moment, on the other hand, is measured in units of force multiplied by distance squared, such as newton-meters squared or pound-feet squared.
4. What are the practical applications of torque and moment?
Torque and moment are important concepts in engineering and physics, and have many practical applications. They are used in the design and analysis of machines and structures, such as engines, motors, and bridges. They are also important in understanding the forces involved in sports, such as golf swings and baseball pitches.
5. How can torque and moment be calculated?
To calculate torque, you must know the force applied and the distance from the point of rotation. The formula for torque is torque = force x distance. Moment can be calculated by multiplying the torque by the distance from the point of rotation to the point where the force is applied.
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