Torque question

[Relative]

 

I was starting simple. Just the first part of that example, where there is indeed no rotation.

Errrmmm there is always rotation if force is applied in rotation direction to the linear, angular momentum can only be achieved by angular force,

 

Torque is linear twist.

if Faraday who I absolute love has a scientist made rotation from a small electrical force, the sun is huge what rotation would the Faraday experiment have if he used the sun and the Earth, yes or no, would it rotate, would there be an energy torque, an energy friction?

Photons have angular momentum , they spin, why do they spin, what torque is transferred, what force make them spin?

And why do I know all this stuff,

[Strange]

Errrmmm there is always rotation if force is applied in rotation direction to the linear

 

As pointed out, if you apply a force to the centre of mass, there will be no rotation.

As pointed out, you can try this yourself with a ruler.

 

if Faraday who I absolute love has a scientist made rotation from a small electrical force, the sun is huge what rotation would the Faraday experiment have if he used the sun and the Earth, yes or no, would it rotate, would there be an energy torque

 

 

As previously explained, the electric and magnetic fields associated with the Sun are very, very small at the distance of the Earth. Immeasurably small, I suspect. So there would be, effectively, no force. Does your compass rotate to follow the Sun during the day? No.

 

Photons have angular momentum , they spin, why do they spin, what torque is transferred, what force make them spin?

 

They have angular momentum. This is (confusingly) called "spin". They do not spin. No torque is transferred. No force makes them spin.

[Relative]

 

As pointed out, if you apply a force to the centre of mass, there will be no rotation.

As pointed out, you can try this yourself with a ruler.

 

 

 

As previously explained, the electric and magnetic fields associated with the Sun are very, very small at the distance of the Earth. Immeasurably small, I suspect. So there would be, effectively, no force. Does your compass rotate to follow the Sun during the day? No.

 

 

They have angular momentum. This is (confusingly) called "spin". They do not spin. No torque is transferred. No force makes them spin.

if you apply a force to the centre of mass, there will be no rotation.- Depends on what angle the force is coming in at, a static body in space for example, lets say a cube for easy assumptions.

 

 

We all know F=ma, for the cube to travel vertical, with no rotation, the force would have to be at a precise central point, for the cube to accelerate away , any offset to that force, from the point X,

 

the cube will be forced linear, and to rotate, by angular momentum of force,

 

When considered Torque, I remove the thought of gravity, mass having effect on any movement, two equal bodies, lets say m1 and m2, and they both weight 5 kilo,

 

the net force between them is zero, they are static in position, however the force is zero because an attractive force is opposed by a repelling force ,

 

 

An axle if you like,

 

 

I add twist to the center of the axle, both m1 and m2 will rotate the axle.

 

 

The rotation been equal V

 

 

The only force/torque involved is the central twist,

[Fuzzwood]

You cannot add a rotational force to the exact center of a circle.

[Relative]

You cannot add a rotational force to the exact center of a circle.

you can if you put an axle through it

or if you add isotropic centripetal force with a slight offset...

[Fuzzwood]

The axle has a certain thickness thus you are putting a force on its outside.

 

Slight offset = not going exactly through the CoM

[Relative]

The axle has a certain thickness thus you are putting a force on its outside.

 

Slight offset = not going exactly through the CoM

See Plasma model in speculation.

[Strange]

We all know F=ma, for the cube to travel vertical, with no rotation, the force would have to be at a precise central point

 

Yes, that is what I said. If you apply the force to the centre of mass, there is no rotation. As your later digram shows. The force has to be offset from the centre of mass.

[Fuzzwood]

And for a homogeneous cube (the density is equal in every part of the cube) the CoM happens to be at the precise central point.

[swansont]

Photons have angular momentum , they spin, why do they spin, what torque is transferred, what force make them spin?

You don't have a grasp of the basic, Physics-101-level material, which is a prerequisite to discussing quantum physics.

 

And why do I know all this stuff,

Easy: you don't. You've failed every quiz so far.

Errrmmm there is always rotation if force is applied in rotation direction to the linear, angular momentum can only be achieved by angular force,

 

I have no idea what you mean by "applied in rotation direction", but I'm sure it does not count as "applied to the center of mass" because the CoM is not rotating. (technically the whole object is not rotating in the example given)

 

 

Torque is linear twist.

That's a poor way of explaining it.

 

if Faraday who I absolute love has a scientist made rotation from a small electrical force, the sun is huge what rotation would the Faraday experiment have if he used the sun and the Earth, yes or no, would it rotate, would there be an energy torque, an energy friction?

 

Without reference to the actual example you're discussing I can't comment in detail, but one thing I'm sure of is that you have somehow misunderstood the experiment if you think it's an example of a force acting on the CoM.

[studiot]

 

Studiot - I have not followed closely enough to find out if this is covered by your use of older UK terms and not the US terms I am familiar with - but the above is non-standard if using modern terminology

 

In the first example the screwdriver as a system goes from a situation in which it has zero angular momentum to a second state in which it has angular momentum - the net torque (in modern US terminology) is the change in angular momentum in time; there is an external net torque in the first exampe

 

In the second example - again taking the screwdriver as the system - there is no change in angular momentum (I am presuming the screw isn't turning) and by definition there is no external net torque.

 

 

I suggest you look at posts 20 and 21 for definitions of terminology.

 

I think this thread is not the place to discuss Us v Uk terminology, although I would be happy to in a new thread set up for that purpose.

 

Unfortunately your comments on the screwdriverd are unsound.

 

( as a matter of interest the most recent US authority on the subject, Sokolnikoff, late professor of mathematics UCLA uses the term couple when describing this situation, p107 of his book)

 

With the screwdriver located into the screwhead, the screwdriver itself is subject to torsion when a couple is applied to the handle, whether the screw moves or not.

The difference is that if the screw moves it is obviously not in equilibrium, whilst moving.

 

Torque and tosion and the relationships between them were defined by engineers for sound engineering reasons to distinguish them from ordinary couples and other turning effects and many engineering relationships and formulae have been developed that just will not work using the looser definition of a torque being a single couple around and axis.

 

An unrestrained 'torque' situation with the unengaged screwdriver cannot work for much the same reason as others have been telling Relative you cannot apply a torque at the centre of rotation.

Edited October 10, 2014 by studiot

[imatfaal]

 

Studiot - I have not followed closely enough to find out if this is covered by your use of older UK terms and not the US terms I am familiar with - but the above is non-standard if using modern terminology

 

In the first example the screwdriver as a system goes from a situation in which it has zero angular momentum to a second state in which it has angular momentum - the net torque (in modern US terminology) is the change in angular momentum in time; there is an external net torque in the first exampe

 

In the second example - again taking the screwdriver as the system - there is no change in angular momentum (I am presuming the screw isn't turning) and by definition there is no external net torque.

 

 

 

I suggest you look at posts 20 and 21 for definitions of terminology.

 

I think this thread is not the place to discuss Us v Uk terminology, although I would be happy to in a new thread set up for that purpose.

 

Unfortunately your comments on the screwdriverd are unsound.

 

( as a matter of interest the most recent US authority on the subject, Sokolnikoff, late professor of mathematics UCLA uses the term couple when describing this situation, p107 of his book)

 

With the screwdriver located into the screwhead, the screwdriver itself is subject to torsion when a couple is applied to the handle, whether the screw moves or not.

The difference is that if the screw moves it is obviously not in equilibrium, whilst moving.

 

Torque and tosion and the relationships between them were defined by engineers for sound engineering reasons to distinguish them from ordinary couples and other turning effects and many engineering relationships and formulae have been developed that just will not work using the looser definition of a torque being a single couple around and axis.

 

An unrestrained 'torque' situation with the unengaged screwdriver cannot work for much the same reason as others have been telling Relative you cannot apply a torque at the centre of rotation.

 

"is non-standard if using modern terminology"

"(in modern US terminology)"

 

Did you deliberately ignore my provisos above? I made it quite clear

 

And basis those provisos my physics is completely sound - and moreover its description follows that of most modern teaching of physics (even in the UK). This is a physics forum and we have a lot of member who might read only your pronouncements and take away a profoundly unusual concept - the bulk of our readers expect modern terminology not old engineering terms.

 

I think it is unwise to use dated terminology - matters move on and it will not be the first time science and technology have used terms differently - but to then incorrectly criticise the use of more modern terms with an implication that I am failing to understand the problem seems deliberately misleading.

[studiot]

 

Did you deliberately ignore my provisos above? I made it quite clear

 

Look at section 3.2.1 (Mechanics) of the current (2014 and beyond) A level syllabus for Physics in the UK

 

http://filestore.aqa.org.uk/subjects/specifications/alevel/AQA-2450-W-SP-14.PDF

 

It would seem the UK education authorities disagree with you.

 

and here is the Further Mathematics syllabus (applied mathematics section)

http://filestore.aqa.org.uk/subjects/specifications/alevel/AQA-6360-W-SP-14.PDF

 

Once again torque is not discussed under rotational statics or dynamics.

 

So UK educational specifications are in step with each other.

 

 

 

 

 

Just as a matter of interest I have seen a number of modern textbooks draw a single curved arrow around an axis to denote turning moments of some sort.

 

Ask yourself what would happen if this was actually applied in the fashion indicated to a real physical object?

 

However I could not find any UK textbooks, serving the above syllabus, mentioning torque or offering such a description.

Edited October 11, 2014 by studiot

[studiot]

This was all rather diverting, there is much more to be said about moments in general and torque in particular, and why we should make a distinction if anyone is still interested.

[Relative]

This was all rather diverting, there is much more to be said about moments in general and torque in particular, and why we should make a distinction if anyone is still interested.

I am listening still, you went a bit into the complexity side for me to follow sorry.