Some interesting discussion :
http://www.physicsforums.com/showthread.php?t=279526
Torque
Moderators: slparry, Gromit, Paul
It's a very weird place!tanneman wrote:Infinite, it is used only for calculations. You would not be able to see it with the naked eye, that is why the theory regarding physics, chemistry and maths works.
Dimensionless, except for length (unless it is an infinitely thin disc! )
Going back to my question "is there any place on a spinning disc or shaft where the angular velocity is zero?". If the centre is at infinity then every part of the disc will experience angular velocity! And yet books tell you that the centre is motionless. Go figure!
Is infinity truly the right expression for the centre? We never reach infinity and yet the centre or axis "point" must be there?
What I'm trying to tell you is that it is just a point in the exact centre of a disc spinning in a perfect plane. It is theoretical because you cannot physically pin the centre and secondly it is very difficult to get a disc to spin in a perfect plane. So for all calculations and arguments and reasoning we use the theory. If you are building something based on theory then the practical implications needs to be taken in consideration.
Eg. making a mirror for a space telescope. The surface has to absolutely distortion free. In theory it is but in practice it can be achieved to within an acceptable percentage of perfect. That is why it takes like almost a year to polish depending on the size in a dust free clinical environment.
So for a spinning disc the centre is in the exact centre, we cannot see it with the naked eye but we know it is there.
Eg. making a mirror for a space telescope. The surface has to absolutely distortion free. In theory it is but in practice it can be achieved to within an acceptable percentage of perfect. That is why it takes like almost a year to polish depending on the size in a dust free clinical environment.
So for a spinning disc the centre is in the exact centre, we cannot see it with the naked eye but we know it is there.
'Let me check my concernometer.'
Thanks for reply.tanneman wrote:What I'm trying to tell you is that it is just a point in the exact centre of a disc spinning in a perfect plane. It is theoretical because you cannot physically pin the centre and secondly it is very difficult to get a disc to spin in a perfect plane. So for all calculations and arguments and reasoning we use the theory. If you are building something based on theory then the practical implications needs to be taken in consideration.
Eg. making a mirror for a space telescope. The surface has to absolutely distortion free. In theory it is but in practice it can be achieved to within an acceptable percentage of perfect. That is why it takes like almost a year to polish depending on the size in a dust free clinical environment.
So for a spinning disc the centre is in the exact centre, we cannot see it with the naked eye but we know it is there.
In "understanding physics" by Isaac Asimov he freely uses the term "torque" when explaining the principle of a fulcrum. He (not surprisingly) uses the seesaw as a real world example too.Merecat wrote:I wasn't going to.............but I can't help myself.
I don't see how you can have torque using a fulcrum. A fulcrum is a pivot point.
In its simplest form a seesaw.
Put small child on one end seesaw goes down.
Put large child on the other end, large child goes down, small child goes up.
Ends of seesaw describe arcs in the air but the forces, applied and resultant are in straight lines, so they are plain and simple forces and not torque.
In the seized bolt scenario you mention you put spanner on bolt and stand on spanner, your mass exerts a vertical force on the spanner, which in turn tries to twist the nut off the bolt, that is the torque
The length of the spanner to the centre of the bolt (ft) and your mass (lbs)
If the bolt is well seized, or been cross threaded by some ham fisted oaf it will remain stuck fast despite the applied torque
We may need a longer spanner or save ourself several pages by using a gas axe!!
The long and short of it is by using a fulcrum the resultant motion will be linear.
Put a spanner on a bolt and the resultant motion will be a rotation
To answer my own question, the torque still applies even if the loads are in balance, so there is no movement, as you've all said.
But it definitely is ok to class the force at a distance as "torque" even in a fulcrum application. What's interesting is that there is no twisting in that scenario.
Picking up on manfred's point about movement at molecular level, if he means "strain" then, yes, I see that as movement but not continuous movement. It will settle to a position. Or break!
What I need to find out next is whether "force at a distance" or "moment" is a "torque" in all applications or whether there has to be rotation, or centre of potential rotation involved. Eg, are the books on my bookshelf causing a torque on the shelf brackets? Is the bottom edge of the bracket acting as a fulcrum point?
In physics and maths there is more than one way to come to the same conclusion. It is how you apply a formula that will influence the answer. When building a structure you will most likely work with moments to calculate the beam sizes and some torque calculations to determine the twist on the structure from an unbalanced load. In the automotive application we refer to the strength of the engine, the potential to rotate a shaft. From that we can calculate kW, gear ratios and the strength of an alloy needed for reliable operation of the drive train.
I would simplify to say that torque is a movement through an arc. Moments are loads on a beam and tend to bend, looking head on to the beam the twist from an unbalanced load can be classified at torque.
I would simplify to say that torque is a movement through an arc. Moments are loads on a beam and tend to bend, looking head on to the beam the twist from an unbalanced load can be classified at torque.
'Let me check my concernometer.'
I see what you mean. When I said that the seesaw fulcrum version of torque doesn't involve twist I was assuming the opposing loads to be in line. I was thinking of a typical real world application of a seesaw which tends to use trunnions as the pivots. I meant that the trunnion "shafts" won't twist. But it was wrong of me to just assume perfectly in line loads.tanneman wrote:............... looking head on to the beam the twist from an unbalanced load can be classified at torque.
I suppose it is only ever offset that induces twist in any scenario.
I suppose it is the same for any "point"on a scale. The actual position itself doesn't have a dimension. It does on our steel rule or tape, so that we can see it. But in the theoretical sense it doesn't.Corvus wrote:It's a very weird place!tanneman wrote:Infinite, it is used only for calculations. You would not be able to see it with the naked eye, that is why the theory regarding physics, chemistry and maths works.
Dimensionless, except for length (unless it is an infinitely thin disc! )
Going back to my question "is there any place on a spinning disc or shaft where the angular velocity is zero?". If the centre is at infinity then every part of the disc will experience angular velocity! And yet books tell you that the centre is motionless. Go figure!
Is infinity truly the right expression for the centre? We never reach infinity and yet the centre or axis "point" must be there?
Weird!
Fascinating.
The following quote seems typical of the definition of a point (centre point, etc) found on tinternet.
"A point is an exact position or location on a plane surface. It is important to understand that a point is not a thing, but a place. We indicate the position of a point by placing a dot with a pencil. This dot may have a diameter of, say, 0.2mm, but a point has no size. No matter how far you zoomed in, it would still have no width. Since a point is a place, not a thing, it has no dimensions."
I find the idea absolutely fascinating. I have also read that the centre of a spinning disc is motionless. But that seems to contradict the above. The above seems to say that all parts of a spinning disc will experience motion, because the centre point is dimensionless. So how can the centre be motionless?
The thick plottens.
"A point is an exact position or location on a plane surface. It is important to understand that a point is not a thing, but a place. We indicate the position of a point by placing a dot with a pencil. This dot may have a diameter of, say, 0.2mm, but a point has no size. No matter how far you zoomed in, it would still have no width. Since a point is a place, not a thing, it has no dimensions."
I find the idea absolutely fascinating. I have also read that the centre of a spinning disc is motionless. But that seems to contradict the above. The above seems to say that all parts of a spinning disc will experience motion, because the centre point is dimensionless. So how can the centre be motionless?
The thick plottens.