# Torque

Physics

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### #1 Nishan

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Posted 28 June 2019 - 10:22 AM

I wanted to explain torque in the form of fundamental force

I predict that it must be derived from the inverse squared laws of force because of the fact that earth around the sun is acted upon by the torque.

So, I imagine a rod suspended at one end. And in free space I apply force at its free  end perpendicular to its alignment . What I see is that the as the distance from the another ends increases the the molecules at the free end path tend to be in straight line . So I think that the force acted upon there produces small but easy change in the alignment of the whole rod

Similarly if a free rod at rest is pushed with a force at its center of mass the inertia of mass both at its right and left is same but if I apply same force and left or right to the center of mass it inertia at one side is more than that of the other side.  In big picture I think I can imagine someone pushing me when two of my friends holding me from both side and in other case those friends holding me from same side.

Using these proposition how can I derive torque and generalize it. Can anyone help?

### #2 exchemist

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Posted 28 June 2019 - 11:09 AM

I wanted to explain torque in the form of fundamental force

I predict that it must be derived from the inverse squared laws of force because of the fact that earth around the sun is acted upon by the torque.

So, I imagine a rod suspended at one end. And in free space I apply force at its free  end perpendicular to its alignment . What I see is that the as the distance from the another ends increases the the molecules at the free end path tend to be in straight line . So I think that the force acted upon there produces small but easy change in the alignment of the whole rod

Similarly if a free rod at rest is pushed with a force at its center of mass the inertia of mass both at its right and left is same but if I apply same force and left or right to the center of mass it inertia at one side is more than that of the other side.  In big picture I think I can imagine someone pushing me when two of my friends holding me from both side and in other case those friends holding me from same side.

Using these proposition how can I derive torque and generalize it. Can anyone help?

There is no torque acting on the Earth in its orbit around the sun. If there were, the length of the year would be either lengthening or shortening. If a body is in circular motion at constant angular velocity (ω), there is no torque acting.

Apologies if you already know what follows but, from some of the things you say, I can't be sure how much is familiar to you:

ω is the angular equivalent of straight line velocity, v and torque,τ is the angular equivalent of force. If you apply a net torque, you change the angular velocity,  in other words imparting an angular acceleration. So it is just like F=ma, but normally written something like: τ = Iώ. (This applies so long as the rotating object is fixed with respect to the axis of rotation.)

The tricky bit is that the angular equivalent of mass is moment of inertia, I.  Calculating moments of inertia is not straight forward for extended objects, because you need to integrate mass x distance from the axis of rotation, ∫m(r ) dr, across the whole object.

I do not understand your second paragraph at all, so cannot comment on it. I don't understand why you start talking about molecules, in a discussion of mechanics.

Edited by exchemist, 28 June 2019 - 11:11 AM.

### #3 Nishan

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Posted 29 June 2019 - 03:30 AM

There is no torque acting on the Earth in its orbit around the sun. If there were, the length of the year would be either lengthening or shortening. If a body is in circular motion at constant angular velocity (ω), there is no torque acting.

Apologies if you already know what follows but, from some of the things you say, I can't be sure how much is familiar to you:

ω is the angular equivalent of straight line velocity, v and torque,τ is the angular equivalent of force. If you apply a net torque, you change the angular velocity,  in other words imparting an angular acceleration. So it is just like F=ma, but normally written something like: τ = Iώ. (This applies so long as the rotating object is fixed with respect to the axis of rotation.)

The tricky bit is that the angular equivalent of mass is moment of inertia, I.  Calculating moments of inertia is not straight forward for extended objects, because you need to integrate mass x distance from the axis of rotation, ∫m(r ) dr, across the whole object.

I do not understand your second paragraph at all, so cannot comment on it. I don't understand why you start talking about molecules, in a discussion of mechanics.

Sorry I mean I wrongly presumed that,

In orbit around the sun torque on the earth is zero .

Actually I was trying to derive the torque is there any way.

Personally I imagined me with some of my friends aligned in a straight line holding hands strongly. At the same time my another friend ran to us and pushed one at the side of the line. It was easy for him to rotate us at some angle. But next time when he ran in and pushed me in the middle then I was resisted by my friends in my both sides. Can I take derive torque from it at the very fundamental level from this analogy. I was trying to do it in motion of planet when constant force perpendicular to radius was applied but ended up with a lot of variables to be determined .

. Hope you could help . If I am wrong what am I missing.

In short I want to resolve  forces into torque and finally derive formula of torque. If there is any other ways I would love to learn.

Edited by Nishan, 29 June 2019 - 03:50 AM.

### #4 exchemist

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Posted 29 June 2019 - 04:17 AM

Sorry I mean I wrongly presumed that,

In orbit around the sun torque on the earth is zero .

Actually I was trying to derive the torque is there any way.

Personally I imagined me with some of my friends aligned in a straight line holding hands strongly. At the same time my another friend ran to us and pushed one at the side of the line. It was easy for him to rotate us at some angle. But next time when he ran in and pushed me in the middle then I was resisted by my friends in my both sides. Can I take derive torque from it at the very fundamental level from this analogy. I was trying to do it in motion of planet when constant force perpendicular to radius was applied but ended up with a lot of variables to be determined .

. Hope you could help . If I am wrong what am I missing.

In short I want to resolve  forces into torque and finally derive formula of torque. If there is any other ways I would love to learn.

I don't think you can derive anything useful about torque from this example.

And there is a random quality to some of your posts that is making me suspicious.

### #5 Nishan

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Posted 29 June 2019 - 05:45 AM

I don't think you can derive anything useful about torque from this example.

And there is a random quality to some of your posts that is making me suspicious.

So is torque, is just  a correct random assumption or does it have any other proof.

Can't I predict motion other than using torque.

I am just trying to solve my curiosity as we are just taught text not ideas here in Nepal . Until now I had to figure out by myself but now I have at least a place to discuss ideas.

That might be the reason for random quality.

Edited by Nishan, 29 June 2019 - 05:47 AM.

### #6 exchemist

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Posted 29 June 2019 - 11:41 AM

So is torque, is just  a correct random assumption or does it have any other proof.

Can't I predict motion other than using torque.

I am just trying to solve my curiosity as we are just taught text not ideas here in Nepal . Until now I had to figure out by myself but now I have at least a place to discuss ideas.

That might be the reason for random quality.

I am suspicious about who you are and why you are asking these questions. You are, on the one hand, asking questions about differential equations for SHM and about the kinetic theory of gas viscosity, which is quite advanced, while on the other you do not seem to understand some basic mechanics of rotary motion. I ask myself what you are doing and what educational level you have reached.

It seems strange for you to be trying to use complex numbers before you know what torque is. Do you have a tutor or teacher? Or are you just reading bits and pieces out of a physics textbook for yourself?

Edited by exchemist, 29 June 2019 - 11:42 AM.

### #7 Nishan

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Posted 30 June 2019 - 07:47 AM

I am suspicious about who you are and why you are asking these questions. You are, on the one hand, asking questions about differential equations for SHM and about the kinetic theory of gas viscosity, which is quite advanced, while on the other you do not seem to understand some basic mechanics of rotary motion. I ask myself what you are doing and what educational level you have reached.

It seems strange for you to be trying to use complex numbers before you know what torque is. Do you have a tutor or teacher? Or are you just reading bits and pieces out of a physics textbook for yourself?

Well I study bsc physics 1st year . Torque was not just clicking me since childhood. I knew what it is, its formulas but exactly how does it work. I think I have slight idea right now . So that's it.

In case of teachers we don't really motivated teachers.

### #8 VictorMedvil

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Posted 30 June 2019 - 09:21 AM

Well I study bsc physics 1st year . Torque was not just clicking me since childhood. I knew what it is, its formulas but exactly how does it work. I think I have slight idea right now . So that's it.

In case of teachers we don't really motivated teachers.

Okay let me demystify torque, torque is Force at a Radius meaning that it isn't a fundamental force but rather the force rotating the object, how much force to you have to put on the object at what radius to rotate it.

If you decrease the radius you must increase the force to rotate it the same amount by a linear amount.

It is harder to rotate objects at a shorter radius is the fundamental truth of torque.

Edited by VictorMedvil, 14 July 2019 - 07:35 PM.

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### #9 Nishan

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Posted 30 June 2019 - 10:06 AM

Okay let me demystify torque, torque is Force at a Radius meaning that it isn't a fundamental force but rather the force rotating the object, how much force to you have to put on the object at what radius to rotate it.

If you increase the radius you must increase the force to rotate it the same amount by a linear amount.

It is harder to rotate objects at a larger radius is the fundamental truth of torque.

ok thank you for it.
I was overthinking about it .
Can I simply viualise it as in the figure

#### Attached Thumbnails

Edited by Nishan, 30 June 2019 - 10:12 AM.

### #10 VictorMedvil

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Posted 30 June 2019 - 11:25 AM

ok thank you for it.
I was overthinking about it .
Can I simply viualise it as in the figure

Basically, yes there isn't a whole lot to torque other than that figure.

Edited by VictorMedvil, 30 June 2019 - 11:33 AM.