Like what IS energy? relative to force

[Malverin]

In your picture(which by the way lacks the force keeping the axle in place), the only force acting on the body is the force provided by the tension of the rope(F_b in your pic). F_r acts on the rope itself, not on the body, and is what keeps the rope taut.

The reaction forces are applied at the same point, but act in opposite directions, so on different bodies. Otherwise you could argue that it's impossible to kick a ball, since the leg-ball system has got two equal and opposite forces exactly cancelling each other at the point of contact.


Back to the picture. You could have zero net forces acting on the body, if you chose to use a rotating reference frame. In which case you'd have to add centrifugal force to the drawing, exactly matching F_b in magnitude.
In an inertial reference frame, the mere fact of there being a rotation makes it clear that there has to be an unbalanced force accelerating the body.

Note that if you'd do what others suggested and simplify things by having the force of gravity acting between two bodies rather than using a rope, it would remove the confusing bits while producing the same results.

I am not arguing about the force acting on the body.
I said that if you change the energy of the body, the energy of the whole system (body, rope, axle) will change.
And any analysis about energy here have to consider them too.

 

[Malverin]

Given that the discussion was about circular motion, where the force is perpendicular to displacement, that is highly dubious.

When you have a rotational motion and energy is changed, it can be due to radial movement and/or change in rotation speed.

 

[russ_watters]

When you have a rotational motion and energy is changed, it can be due to radial movement and/or change in rotation speed.

The uniform circular motion scenario being discussed does not involve a change in any kind of energy. This is the root of the issue: you've made several statements saying or implying you think it does.

 

[Malverin]

The uniform circular motion scenario being discussed does not involve a change in any kind of energy. This is the root of the issue: you've made several statements saying or implying you think it does.

All started when I said that when there is a net(resultant) force it will cause movement/deformation and do work.
Then others gave me an example that there is a net force by circular motion, but no energy change.

My statement was (and still is) that rotating body is a part of the rotational system and when we consider its energy we have to consider all its elements, not only the rotating body alone.

If someone has understand something else, I apologize I have not explained it clear enough.
If you (and the others) think that my statement about energy of rotating system is not correct, please
show(explain) why the change of energy of the rotating body will not affect energy of the rope or axle(or another body) in the rotating system.

I think, disscusion about rotation became so long, because I have not asked my questions clear enough.

 

[russ_watters]

Expanding on the bit about force pairs:

By Newton's third law, all forces applied at a point come in equal and opposite pairs (sum to zero).
http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_third_law

Since it is always true that forces come in pairs, it is redundant - and as we see here often confusing - to draw both halves of the force pair.

An unbalanced "net" force is not referring to such force pairs, but situations where multiple unequal force pairs exist (or just one force pair and nothing opposing it). So when analyzing forces (drawing free body diagrams), only half of each force pair is taken into account; the force that is applied to the object. The force the object applies back is left out.
http://en.wikipedia.org/wiki/Free_body_diagram
http://www.physicsclassroom.com/class/newtlaws/u2l2d.cfm

In the second link, the first diagram describes a box being lifted by a rope. Notice that the tension force is only one force, not two, and it is pointing up. We draw-in the force of the rope acting on the box, but not the equal and opposite force of the box acting on the rope. Same goes for gravity: we draw the force of gravity pulling down on the box from earth, but not the force of the gravity pulling up on the Earth from the box.

So for the revolving tether-ball or circular orbit scenario, there is one force pair, so there is only one vector drawn on a proper diagram; from the object, pointing to the center of the orbit/revolution.

 

[russ_watters]

All started when I said that when there is a net(resultant) force it will cause movement/deformation and do work.
Then others gave me an example that there is a net force by circular motion, but no energy change.

My statement was (and still is) that rotating body is a part of the rotational system and when we consider its energy we have to consider all its elements, not only the rotating body alone.

That is still wrong.

If someone has understand something else, I apologize I have not explained it clear enough.
If you (and the others) think that my statement about energy of rotating system is not correct, please
show(explain) why the change of energy of the rotating body will not affect energy of the rope or axle(or another body) in the rotating system.

I'm sorry, but that makes no sense. In these scenarios, the rope and axle neither have nor apply energy. And if the tethered object is sped-up (like a slapped tether-ball), the forces on the axle and in the rope tension go up, but they still do no work/apply no energy.

 

[Malverin]

That is still wrong.

It can be wrong yes.
Where I can read information that proves it wrong?
Thank you for your answers.

 

[russ_watters]

It can be wrong yes.
Where I can read information that proves it wrong?
Thank you for your answers.

Sorry/see my late edit.

 

[Malverin]

That is still wrong.

I'm sorry, but that makes no sense. In these scenarios, the rope and axle neither have nor apply energy. And if the tethered object is sped-up (like a slapped tether-ball), the forces on the axle and in the rope tension go up, but they still do no work/apply no energy.

If there is a speed-up, then rotational speed Ω is changed.

If the rope has mass, its kinetic energy will change.

If there is a sped-up , there is a torque applied to the axle. It will change its energy either.

Is that so?

 

[Dale]

My statement was (and still is) that rotating body is a part of the rotational system and when we consider its energy we have to consider all its elements, not only the rotating body alone.

This is simply not true. I think this is the 4th time that I have corrected you on this point. There is no requirement that you consider the entire isolated system. In mechanics you are free to define your system boundaries as you desire. You are not obligated to continue expanding your system until it becomes an isolated system.

Please stop repeating this false assertion which I have corrected multiple times already.

 

[Malverin]

This is simply not true. I think this is the 4th time that I have corrected you on this point. There is no requirement that you consider the entire isolated system. In mechanics you are free to define your system boundaries as you desire. You are not obligated to continue expanding your system until it becomes an isolated system.

Please stop repeating this false assertion which I have corrected multiple times already.

OK
I see we will go nowhere here.
So I will not post at this thread anymore

 

[voko]

Much of this discussion could have been avoided, had Malverin just said that "when a force does work, it transfers energy". But I guess that would have been too trivial for him to say :)

 

[russ_watters]

If there is a speed-up, then rotational speed Ω is changed.

If the rope has mass, its kinetic energy will change.

If there is a sped-up , there is a torque applied to the axle. It will change its energy either.

Is that so?

If we consider and we're discussing the mass of the rope and axle, sure. But we were discussing the ball - we don't know anything about the rope and axle, so now you are making up the scenario as you go along. Moreover, the mass of rope is generally much smaller than the ball so it is negligible and the axle is often not moving (such as with a tetherball).

This is goalpost shifting; you are far, far away from what was originally being discussed and your statements about it. From earlier:

there is a force applied to the body, this is the force of the rope, that keeps the body at constant distance from the axis. At the other end of the rope there is the same magnitude force with the opposite direction.

This is only true if you assume the rope is massles. If it is not, the force varies along the rope.

This is a real shame. You had a good learning opportunity here and chose to be argumentative instead.