Complaints or wish list

[gizmo_454]

Sorry Geof. As I said above, from my point of view, I am right, to a degree. I look at it from a math standpoint, and mathematically speaking they are the same. What their traditional meaning is, IS different in how it is WRITTEN. But the math is the same no matter how you look at it.

If I am wrong, I am wrong and I will always be wrong. Why we Americans had to screw everything up with our written words are beyond me. The metric standard has N*m and joules to define the difference. Which makes more sense than just flipping the words around. It's too confusing.

Never the less. I am done with this, whether I am right or wrong, this little matter is not worth the time to debate. There are too many things in life to do to spend the next six weeks debating it.

Have a good weekend

[Geof]

Originally Posted by gizmo_454 ....Never the less. I am done with this, whether I am right or wrong, this little matter is not worth the time to debate. There are too many things in life to do to spend the next six weeks debating it. Have a good weekend

I have taught college level physics, did it for many years actually. You remind me of students I had who were to stubborn to accept when they were wrong in the context of the point upon which they disagreed. Torque and energy are physical concepts, looking at them from a math stand point is wrong, math is merely the tool that is used to manipulate them. It is similar to speed and velocity: To a mathematician they are indistinguishable but to a physicist they are very different and using them interchangeably is wrong.

[HuFlungDung]

And here I thought multiplication was commutative

If the units of an equation work out to lb-ft or ft-lb, there is no difference in the resultant quantity.

Torque is the instantaneous force applied through an arm that has not yet rotated, but if allowed to do so, will begin to perform work, because work is an accumulation of torque applied over time and moving a load. You will not be getting any ft-lbs of work done with zero torque.

The potential to do work is different than actually doing the work, but the electrical energy supplied to the motor will be similar, whether the motor is stalled by a brake, or whether it is successfully moving its load. How would they tell down at the generating station whether that motor is doing work or not? All they know is that they have to input the same amount of energy into the power grid to make that circuit work. Whether the result of the power is heat coming out of the stalled motor, or the load moving is just subjective observation subject to the whim of the observer's attention Thus, the old school teacher's quip (I remember mine saying this) about "no work being done if the load is not moved' is just bull, because some work is being done to create the torque, he's just chosen to ignore that fact.

[gar]

070625-1033 EST USA

Hu:

If I have a cantilevered beam with a weight hanging on the end, then I have torque, and no work being done. There was work done to put the weight on the beam end, but once there we have a static condition.

Also we have many different torque values depending upon where you choose the pivot point. In a statics problem I can choose to put a pivot point anywhere I want and the sum of all torques (moments) are zero and all forces are zero. There is no work done in a static state.

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[HuFlungDung]

Gar,
Isn't that analogous to potential energy? If a rock sits atop a hill and never rolls down, then no energy is released, yet we still say it has potential energy.

The static torque situation is still a potential to do work and is like a battery or other stored energy source.

The unknown quantity is simply the time that you look at the system: before it has moved the load, or after. There is really no difference in what will be, or was, accomplished.

[AMCTony]

Originally Posted by HuFlungDung The potential to do work is different than actually doing the work,

Seems like an employee problem. I can relate. I have had people that have plenty of Torque but NO Horsepower!

[AMCjeepCJ]

I dunno what you're all talking about but my rear end has been static for about fifteen minutes now and absolutely NO work is, was or has been done. BTW, my pivot point is located under the chair and unless I pivot it, I'll stay here all afternoon!

Good luck!

[gar]

070625-2157 EST USA

Hu:

Consider a course in "statics". I do not believe the subject of potential energy, or work done to apply a torque is even discussed. But moments (torques) and forces are the elements discussed. It is a long time ago that I had that course, but some basics remain in my memory.

Consider this example: I have a 300 #-ft full scale torque wrench. This has a long handle, maybe 4 ft. I have a 1500 #-ft clutch that is fully energized. Thus, 300 #-ft won't rotate the clutch. At 4 ft to apply 300 #-ft to the clutch requires 75 # force. Assume that I move 1" with the 75# applied to the lever arm, they usually bend quite a bit. Thus, I have done 75/12 ft-# of work to generate this static 300 #-ft of torque.

Next put a 5 to 1 torque multipler (planetary gear box with no backlash). The torque input to the multiplier is still 300 #-ft and work to get to an output of 1500 #-ft is still the same 75/12 ft-#. Once I achieve that 300 #-ft of torque no more energy is being stored in the system even though I am huffing and puffing holding that torque with my feet slipping on the oily floor.

Torque and work are not the same "animal", even though in some circumstances they are related.

Back to my statics class. This subject is concerned only with non-moving forces and moments and their inter-relationship in structures. I do not need to know anything about work or energy to work in statics. And again I repeat in a static condition the sum of the forces and moments at any point are zero.

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[Geof]

Originally Posted by gar 070625-2157 EST USA Thus, I have done 75/12 ft-# of work to generate this static 300 #-ft of torque....

And when you relax, the system does the same work on you as the lever straightens out. There is no net work done.

[gar]

070626-0719 EST USA

Good point Geof.

A correction on my previous post follows. However, this correction has no effect on the argument.

A 4 ft long torque wrench probably has about 3 to 4 inches of deflection at full scale. At 4 inches the work calculation becomes 75*4/12 ft-#.

(edit)
Consider a more theoretical case. All moment arms have no deflection independent of applied force, and force is a vector with no associated weight (mass). Now T (torque) = perpendicular force * moment arm length. I can apply any amount of torque by a suitable choice of force and moment arm. Apply any amount of force and no work is done since the force vector does not move.

Torque and work are as different as are voltage and current.
(end edit)

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[gar]

070626-1841 EST USA

Another view on the difference between torque and work.

Torque is produced by a force perpendicular to a moment arm (the distance element). While work results from a force moving parallel to a displacement (distance element).

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[HuFlungDung]

So to go off on a side track here (yes, another one ) we've been told that momentum is conserved. I don't disbelieve it of course, but exactly how does the proverbial figure skater speed up in rotation when bringing his/her limbs in closer to the axis of rotation. Yeah, we've heard momentum is conserved, but that is as much a statement of the obvious, rather than an explanation of how the acceleration takes place. Torque arm length must have something to do with it. I've never thought it all through, though. What say ye?