[alancalverd]

expected rotational radius is not always the same as the real rotational radius.

The fact that you don't understand your own diagram does not indicate a fault in physics.

[Eternal Student]

Hi.

     We don't actually need any notion of Torque or of any other similar thing to do with rotational motion.   I expect you ( @hamdani yusuf ) already know that but it seems worth mentioning again.

     It's OK, it can be a useful concept sometimes and let you solve a problem quite quickly but we do not have any fundamental need for it.   Rotational dynamics is all just stuff that has been developed from Newtonian mechanics.   There's nothing in Newton's basic laws that involves rotational motion or rotational quantities, it's all about idealised particles and their motion.   That's all we need, we can solve every problem involving a rotating body by breaking it down into little particles and small enough chunks of time such that everything is just following Newton's laws.

    Anyway, just thought I'd mention it.   You seem to think that Torque must be some fundamental and physically existant object or entity and as such, there just has to be some appropriate units and meaning for it.   However, it just isn't a required or necessary physical qunatity at all.   The stuff we need for Newton's law - maybe that is and maybe quntities like linear momentum must be physically real quantities but Torque is a long way up the ladder of mechanical concepts built up from Newton's laws.   There's no need to use notions of Torque if you don't want to.

    One of the things I've seen in a few other forum discussions about this sort of thing is that Toque is just another fudge factor some engineers introduced when they were unable to treat a rigid body as a collection of individual particles.   You see, if you know the force on each particle, then who cares where that particle or the force is located?  It can be close to some special place we call a fulcrum or far from it, why would we care?   However, if you can't treat a rigid body as a collection of individual particles then exactly where a force is applied on that body does start to matter.   Torque is a tool of convenience or fudge factor to make some possibly un-real or just totally abstract notions like a "twisting force" work on macroscopic bodies because dealing directly with the much more real forces that exist on each individual particle would be too difficult.

Best Wishes.

[alancalverd]

How tight is tight?

Boeing may agree with you, but most manufacturers and inspectors of vehicles, aircraft, ships,etc.,  think that torque is very important when it comes to screwing doors, wheels, etc to their products.  Or designing the braking system. Or calculating the size of starter motor. Or indeed anything that involves rotation.

[hamdani yusuf (OP)]

Let's say the object is a long thin stick 1 meter long, 1 kg mass, floating in free space. 1 N force is applied to one of its end, perpendicular to the length. What is the torque?

  0.5 Nm, assuming the stick is homogeneous.

What if it isn't?

The stick will rotate about its center of mass, so if you know its mass/length distribution you can calculate the effective torque and subsequent motion. But only if you use the correct definition of torque.

What do you think is the correct definition of torque?
Is the Oxford dictionary's definition correct?

[hamdani yusuf (OP)]

expected rotational radius is not always the same as the real rotational radius.

The fact that you don't understand your own diagram does not indicate a fault in physics.

Can you answer the question?

The question is, what's the torque produced by the force?

[hamdani yusuf (OP)]

Hi.

     We don't actually need any notion of Torque or of any other similar thing to do with rotational motion.   I expect you ( @hamdani yusuf ) already know that but it seems worth mentioning again.

     It's OK, it can be a useful concept sometimes and let you solve a problem quite quickly but we do not have any fundamental need for it.   Rotational dynamics is all just stuff that has been developed from Newtonian mechanics.   There's nothing in Newton's basic laws that involves rotational motion or rotational quantities, it's all about idealised particles and their motion.   That's all we need, we can solve every problem involving a rotating body by breaking it down into little particles and small enough chunks of time such that everything is just following Newton's laws.

    Anyway, just thought I'd mention it.   You seem to think that Torque must be some fundamental and physically existant object or entity and as such, there just has to be some appropriate units and meaning for it.   However, it just isn't a required or necessary physical qunatity at all.   The stuff we need for Newton's law - maybe that is and maybe quntities like linear momentum must be physically real quantities but Torque is a long way up the ladder of mechanical concepts built up from Newton's laws.   There's no need to use notions of Torque if you don't want to.

    One of the things I've seen in a few other forum discussions about this sort of thing is that Toque is just another fudge factor some engineers introduced when they were unable to treat a rigid body as a collection of individual particles.   You see, if you know the force on each particle, then who cares where that particle or the force is located?  It can be close to some special place we call a fulcrum or far from it, why would we care?   However, if you can't treat a rigid body as a collection of individual particles then exactly where a force is applied on that body does start to matter.   Torque is a tool of convenience or fudge factor to make some possibly un-real or just totally abstract notions like a "twisting force" work on macroscopic bodies because dealing directly with the much more real forces that exist on each individual particle would be too difficult.

Best Wishes.

A concept should not be introduced without necessity, as Occam's Razor basically says.
Physics theories are combination of physics concepts and their relationships among one another. They are meant to help us predict the behaviors of physical systems, so we can produce desired results and avoid undesired results by identifying and executing necessary actions.

Rotation is conveniently represented by imaginary number.

In Newtonian mechanics, anytime an object change its velocity, a force must have been applied, assuming the mass doesn't change. In general, a force is what's needed to change momentum of an object.
Likewise, rotational force which is called torque is required to change rotational momentum.

Calculating net forces on individual particles of macroscopic objects isn't practical in most cases. The costs would outweigh the benefits. Moreover, most of those forces will cancel each other. So, you would be better off calculating the most significant forces that affect the results.

[alancalverd]

Right now I have a practical problem, to attach a hose reel to a wall. Problem is that the local bricks are quite soft, so if I only use the bracket supplied with the reel, the screws will pull out. So I need a spreader plate.

I can calculate the dimensions of the spreader plate using the proper definition of torque, with no difficulty. But since the object is to prevent rotation, if I use Hamdani Torque I will need an infinitely long screw at the top of the plate, or an infinitely long plate, since  θ = 0.

[Bored chemist]

Right now I have a practical problem, to attach a hose reel to a wall. Problem is that the local bricks are quite soft, so if I only use the bracket supplied with the reel, the screws will pull out. So I need a spreader plate.

I can calculate the dimensions of the spreader plate using the proper definition of torque, with no difficulty. But since the object is to prevent rotation, if I use Hamdani Torque I will need an infinitely long screw at the top of the plate, or an infinitely long plate, since  θ = 0.

The thing I don't understand is why, when Alan makes it this clear, Hamdani  keeps trying to argue.

Hamdani, what do you hope to achieve here?
(and please don't ask a computer; it can't help)

[paul cotter]

He is likely now to deflect the argument by asking "why a computer cannot help".

[hamdani yusuf (OP)]

Right now I have a practical problem, to attach a hose reel to a wall. Problem is that the local bricks are quite soft, so if I only use the bracket supplied with the reel, the screws will pull out. So I need a spreader plate.

I can calculate the dimensions of the spreader plate using the proper definition of torque, with no difficulty. But since the object is to prevent rotation, if I use Hamdani Torque I will need an infinitely long screw at the top of the plate, or an infinitely long plate, since  θ = 0.

What's your definition, and what do you think is my definition?

[hamdani yusuf (OP)]

Right now I have a practical problem, to attach a hose reel to a wall. Problem is that the local bricks are quite soft, so if I only use the bracket supplied with the reel, the screws will pull out. So I need a spreader plate.

I can calculate the dimensions of the spreader plate using the proper definition of torque, with no difficulty. But since the object is to prevent rotation, if I use Hamdani Torque I will need an infinitely long screw at the top of the plate, or an infinitely long plate, since  θ = 0.

The thing I don't understand is why, when Alan makes it this clear, Hamdani  keeps trying to argue.

Hamdani, what do you hope to achieve here?
(and please don't ask a computer; it can't help)

The finite rigidity of the materials guarantee that the rotational angle will not be zero when a force is applied at some distance away from the supporting bolts not exactly directed at the bolt itself, before it produces reactionary force to cancel the motion.
Here's what will happen.
First, when the force is applied at one part of an object, the object will start to accelerate, and starts to move, no matter how slow it is.
The force is then propagated to the other parts of the object.
At some point, the force is propagated to the wall through the bolt connection.
Before stopping, the rotating object must first decelerate. The deceleration force is produced by the wall as a reactionary force, which must be larger than the acting force.
The plot of angular position will look like an S curve. While the plot of angular velocity is its first time derivative, which is a bell curve. The plot of angular acceleration is its second time derivative, which is a positive bell curve followed by a negative bell curve before getting steady at zero.

[hamdani yusuf (OP)]

Since you asked, perhaps you've missed my previous posts.

According to their awareness of this problem, people can be classified into some categories.
1. Those who are completely ignorant of the problem. Most kids and illiterate people fall into this category. Until high school, I was also included here.
2. Those who are aware of the problem, but haven't found the solution. I was here until a few years ago.
3.Those who are aware of the problem, as well as the solution. Currently, it's the fewest.

By spreading the information about this problem and solution through social media, I hope to change the composition in the classification above to reduce the proportion of people in the first category, and at least increase the proportion of people in the second category.

A new standard needs to exceed some sort of critical mass of public exposure before it can be accepted. By showing the inconsistencies in current standard, I hope to reduce the proportion of first group in our society.

I already quoted previously proposed solutions and why they were rejected. My own proposed solution is built upon one of previous solutions, with a slight modification to eliminate its weakness. I'm still open for a better solution, if there is one.

Anyone who think that there's no problem with current standard units of rotational quantities falls into first category above. I want to inform them that they are violating the principle of general covariance.
Some people may insist that this principle is not important. Some people may even insist that logical consistency is not important either. But they should make decisions after getting necessary information on all possible options.

[Bored chemist]

Right now I have a practical problem, to attach a hose reel to a wall. Problem is that the local bricks are quite soft, so if I only use the bracket supplied with the reel, the screws will pull out. So I need a spreader plate.

I can calculate the dimensions of the spreader plate using the proper definition of torque, with no difficulty. But since the object is to prevent rotation, if I use Hamdani Torque I will need an infinitely long screw at the top of the plate, or an infinitely long plate, since  θ = 0.

The thing I don't understand is why, when Alan makes it this clear, Hamdani  keeps trying to argue.

Hamdani, what do you hope to achieve here?
(and please don't ask a computer; it can't help)

The finite rigidity of the materials guarantee that the rotational angle will not be zero when a force is applied at some distance away from the supporting bolts not exactly directed at the bolt itself, before it produces reactionary force to cancel the motion.
Here's what will happen.
First, when the force is applied at one part of an object, the object will start to accelerate, and starts to move, no matter how slow it is.
The force is then propagated to the other parts of the object.
At some point, the force is propagated to the wall through the bolt connection.
Before stopping, the rotating object must first decelerate. The deceleration force is produced by the wall as a reactionary force, which must be larger than the acting force.
The plot of angular position will look like an S curve. While the plot of angular velocity is its first time derivative, which is a bell curve. The plot of angular acceleration is its second time derivative, which is a positive bell curve followed by a negative bell curve before getting steady at zero.

Have you forgotten that I pointed out it's perfectly possible to build a torque wrench such that the angular deviation is zero or even negative?
Did you not understand it?

Poppycock! The torque wrench measures torque whether or not the bolt turns.

You forget that the wrench lever must turn against its head for any non-zero reading. Except you are using a broken torque wrench.

It's perfectly possible to arrange for that not to happen.

[alancalverd]

Hamdani: You are wasted here!

Your inability to understand simple physics, coupled with your inability to answer a question, and your inability to admit that you are wrong, makes you better qualified for high political office than anyone else on the planet.

I will be pleased to recommend you for an initial post as Minister for Education, for which you do not actually need to be elected. When you have completely obfuscated the national curriculum, I am sure one or other bunch of incompetent parasites will be pleased to find you a safe parliamentary seat from which you can happily talk bollocks into the TV cameras until you have earned an inflation-proofed pension and several influential directorships of companies that sell bullshit to the unwary.

It has been a pleasure to know you.

[hamdani yusuf (OP)]

Right now I have a practical problem, to attach a hose reel to a wall. Problem is that the local bricks are quite soft, so if I only use the bracket supplied with the reel, the screws will pull out. So I need a spreader plate.

I can calculate the dimensions of the spreader plate using the proper definition of torque, with no difficulty. But since the object is to prevent rotation, if I use Hamdani Torque I will need an infinitely long screw at the top of the plate, or an infinitely long plate, since  θ = 0.

The thing I don't understand is why, when Alan makes it this clear, Hamdani  keeps trying to argue.

Hamdani, what do you hope to achieve here?
(and please don't ask a computer; it can't help)

The finite rigidity of the materials guarantee that the rotational angle will not be zero when a force is applied at some distance away from the supporting bolts not exactly directed at the bolt itself, before it produces reactionary force to cancel the motion.
Here's what will happen.
First, when the force is applied at one part of an object, the object will start to accelerate, and starts to move, no matter how slow it is.
The force is then propagated to the other parts of the object.
At some point, the force is propagated to the wall through the bolt connection.
Before stopping, the rotating object must first decelerate. The deceleration force is produced by the wall as a reactionary force, which must be larger than the acting force.
The plot of angular position will look like an S curve. While the plot of angular velocity is its first time derivative, which is a bell curve. The plot of angular acceleration is its second time derivative, which is a positive bell curve followed by a negative bell curve before getting steady at zero.

Have you forgotten that I pointed out it's perfectly possible to build a torque wrench such that the angular deviation is zero or even negative?
Did you not understand it?

Poppycock! The torque wrench measures torque whether or not the bolt turns.

You forget that the wrench lever must turn against its head for any non-zero reading. Except you are using a broken torque wrench.

It's perfectly possible to arrange for that not to happen.

How?

[hamdani yusuf (OP)]

Hamdani: You are wasted here!

Your inability to understand simple physics, coupled with your inability to answer a question, and your inability to admit that you are wrong, makes you better qualified for high political office than anyone else on the planet.

I will be pleased to recommend you for an initial post as Minister for Education, for which you do not actually need to be elected. When you have completely obfuscated the national curriculum, I am sure one or other bunch of incompetent parasites will be pleased to find you a safe parliamentary seat from which you can happily talk bollocks into the TV cameras until you have earned an inflation-proofed pension and several influential directorships of companies that sell bullshit to the unwary.

It has been a pleasure to know you.

Here's another example to show that expected rotational radius is not always the same as the real rotational radius.
It's similar to previous case, but this time a solid object is obstructing the rotation.
The question is, what's the torque produced by the force?

[hamdani yusuf (OP)]

https://farside.ph.utexas.edu/teaching/301/lectures/node155.html

h = l/m
Clearly, h represents the angular momentum (per unit mass) of our planet around the Sun. Angular momentum is conserved (i.e., h is constant) because the force of gravitational attraction between the planet and the Sun exerts zero torque on the planet. (Recall, from Sect. 9, that torque is the rate of change of angular momentum.) The torque is zero because the gravitational force is radial in nature: i.e., its line of action passes through the Sun, and so its associated lever arm is of length zero.

We can't blame ancient people for not understanding the concept of rate of change, nor angular momentum. It's okay for them to try to define torque using simpler concepts which were easier to understand.

But now that most of us have already understood those concepts. Thus the modern definition of torque shouldn't be difficult to explain, at least to those with a decent scientific knowledge.

The reason for choosing a standard is to have a better consistency. Which is exactly what the proposed new standard units of rotational quantities have shown, compared to currently existing standard.

There are many equations relating torque to other physical quantities. But one of them is preferred by most people, especially in ancient times, to determine its standard unit, which is force times rotational radius. It's understandable  because they are relatively easier to measure, compared to other quantities like rotational inertia, angular momentum, angular velocity, angular acceleration, work, and power.

Moreover, the rotational radius can be considered constant in many situations. But that's not necessarily the case in orbital mechanics, as shown in the article above, where it generally changes over time. In a stable orbit where external forces are negligible, the angular momentum is conserved. Torque is the physical quantity which determines the rate of change of the angular momentum.

[Bored chemist]

Re "How?"
like this.
The feedback system ensures that the torque exerted by the weight is countered by that produced electromagnetically, and the needle stays in the same place.
Interestingly, his system would be improved greatly by cutting the bottom of the "flag" that shuts of the light at an angle. That way, there would be a small but definite "linear" range over which the light is (roughly) proportional to the angular displacement.


It possible to replace the simple electronics with and A to D converter, a computer, and a D to A converter.
You can then programme the computer to produce any feedback function you chose, including one which is "slightly more than proportional" resulting in the needle actually rising slightly under load.
You could produce a system with an apparent negative stiffness.
This invalidates your assertion about "The finite rigidity of the materials"

For extra marks, try to think of why it would be sensible to introduce such a non-linearity.

[Bored chemist]

Re "Here's another example to show that expected rotational radius "
That depends whether or not you are sensible in your expectations.
If you are not, that isn't our responsibility.
This "The question is, what's the torque produced by the force?" is a meaningless question.
It's like asking "How far is it to Rome?". There is no "right" answer.

If you don't specify "about such and such a point" then you can't sensibly ask what the torque is.

Pissing about with the units does not alter that.



And, if I have measured the length of the green line in this picture correctly, the answer is about 0.55 N m

Diag 2.png (15.27 kB . 722x588 - viewed 485 times)

[alancalverd]

expected rotational radius is not always the same as the real rotational radius.

Physics has nothing to do with your expectations, however rational or ridiculous.