A Forces on rotating disk object

AI Thread Summary
The discussion centers on the dynamics of a rotating disk subjected to forces at specific moments. At moment 1, a downward force initiates rotation without translational motion, while moment 5 introduces an upward force. Participants debate whether this upward force will cause translational motion of the disk's center of mass, emphasizing the importance of net forces and the conditions under which the disk operates. The consensus is that if a net force is applied, translational motion will occur, regardless of prior rotational states. The conversation highlights the complexities of applying Newton's laws in scenarios involving both rotational and translational dynamics.
StoyanNikolov
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Forces on rotating disk object
Forces on rotating disk object
Hi. Is it convenient to ask following question.
Suppose we have solid circular object and 5 different moments
like in the picture:
Rotating.jpg
In moment 1 we apply force (downwars direction) so as to start rotating the object around center of
the mass (green dot) , Only rotational motion.

In moment 2-moment 4 the object is rotating around the green dot at the center of the object
(center of the mass), while the center of the mass is not moving.

In moment5 we apply upward same force as force applied in moment1 but in upward direction.
Will the center of mass of the circular object start moving in some direction.

In moment 5
Will there be also translational (not only rotational) motion ?
Will the center of the disk start going in some direction?
Thank you.
 
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What does Newton's second law say about the motion of the center of mass of an object when you apply a force on the object?
 
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The object is located in Perfect vacuum and no Forces except the Applied forces in Moment 1 and Moment 5 . No other forces. No Gravitation .
 
StoyanNikolov said:
The object is located in Perfect vacuum and no Forces except the Applied forces in Moment 1 and Moment 5 . No other forces. No Gravitation .
Ok then not like the video starting with centre thread then unwinding, so there will be motion started in 1 and in 5 the position will have changed, for the force to be applied, this was not stated.

Always state conditions for position, velocity and of forces like gravity or external.
 
TonyStewart said:
Ok then not like the video starting with centre thread then unwinding, so there will be motion started in 1 and in 5 the position will have changed, for the force to be applied, this was not stated.
In this case We apply Equal forces only at moment 1 and moment 5
As shown in the picture.
 
StoyanNikolov said:
In this case We apply Equal forces only at moment 1 and moment 5
As shown in the picture.
- moment of time or moment of inertia are insufficient to properly define a situation. ok?.
 
TonyStewart said:
- moment of time or moment of inertia are insufficient to properly define a situation. ok?.
Will there be motion of the center of mass or the disk simply will stop rotating.
 
also you failed to define F but it seems they want you to assume it is an impulse or a very fast step pulse of F for a very finite time duration before it moves far, otherwise it starts to go down left and your time frame of 3 o'clock or your frame of reference for up may be wrong assumptions in 5. But if assume F is some impulse, it may stop in theory, but in practise, not as 0 risetime impulse forces do not exist except maybe in the big bang theory.
 
  • #10
If you don't want it to translate, then in "moment one" you must apply a force equal and opposite ##F## to the systems center of mass.
 
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  • #11
erobz said:
If you don't want it to translate, then in "moment one" you must apply a force equal and opposite ##F## to the systems center of mass.
So in scenario in picture we will have translational motion.
 
  • #12
StoyanNikolov said:
The object is located in Perfect vacuum and no Forces except the Applied forces in Moment 1 and Moment 5 . No other forces. No Gravitation .
As @erobz points out, this is not consistent with your description of moment 1. If there are no other forces at that time, the disc starts to translate downwards at that time.

What happens at moment 5 depends on how you resolve this discrepancy.
 
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  • #13
Ibix said:
As @erobz points out, this is not consistent with your description of moment 1. If there are no other forces at that time, the disc starts to translate downwards at that time.

What happens at moment 5 depends on how you resolve this discrepancy.
Lets asume we have rotating disk without translational motion (Moment 2 - Moment 4),
In moment 5 we apply Force as shown in attached picture.
Will there appear translational motion ?
 
  • #14
StoyanNikolov said:
We there appear translational motion ?
Depends why the translational motion didn't start in moment 1. What stopped it then?
 
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  • #15
Lets say we rotate the disk in moment 1, so as there
is no translational motion in moment 2 - moment 4.
If we apply in moment 5 Force as shown in picrure ,
will there appear translational motion
 
  • #16
StoyanNikolov said:
Lets say we rotate the disk in moment 1, so as there
is no translational motion in moment 2 - moment 4.
If we apply in moment 5 Force as shown in picrure , will there appear translational motion
Did you remove what was had stopped the translational motion in moment 1?
 
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  • #17
StoyanNikolov said:
Just please consider this situation.
Thank you
No thank you.
 
  • #18
Lets say we rotate the disk in moment 1, so as there
is no translational motion in moment 2 - moment 4.
If we apply in moment 5 Force as shown in picrure ,
will there appear translational motion
 
  • #19
StoyanNikolov said:
will there appear translational motion
What stopped translational movement appearing in monent 1? If you do not answer this question then we cannot answer yours, because there are parts of the scenario that are unknown.
 
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  • #20
Ibix said:
What stopped translational movement appearing in monent 1? If you do not answer this question then we cannot answer yours, because there are parts of the scenario that are unknown.
Just consider the question Without moment 1.
We have rotating disk (only rotational motion) in Moment 2 and in Moment 5 we apply force.
Will in Moment 5 appear translational motion?
Thank you
 
  • #21
StoyanNikolov said:
Just consider the question Without moment 1.
We have rotating disk (only rotational motion) in Moment 2 and in Moment 5 we apply force.
Will in Moment 5 appear translational motion?
Thank you
So long as we are clear that this is a different question from your original scenario, and that the disc is rotating in free space with no axle, then the answer is as in post #2.
 
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  • #22
What is the point of moments 2 to 4 in this new scenario? You have a rotating disk. If you apply any force to it, and this is the only force, the center of mass will accelerate in the direction of the force. What you did (or did not) before is not relevant.
 
  • #23
Thank you.
Please also consider where the force is applied.
In moment 5
The force is applied
Not at the center of the disk (not at the center of mass of the disk)
but at periphery of the disk.
 
  • #24
StoyanNikolov said:
Thank you.
Please also consider where the force is applied.
In moment 5
Not at the center of the disk (not at the center of mass of the disk)
It makes no difference to the question you asked where you apply the force. You can't sneak around conservation of momentum.
 
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  • #25
Ibix said:
It makes no difference to the question you asked where you apply the force. You can't sneak around conservation of momentum.
So in Moment 5 there will appear Translational Motion?
 
  • #26
Another "Reactionless Drive" thread no doubt?
 
  • #27
erobz said:
Another "Reactionless Drive" thread no doubt?
No it is not reactionless drive.
So in Moment 5 there will appear Translational Motion?
 
  • #28
StoyanNikolov said:
No it is not reactionless drive.
So in Moment 5 there will appear Translational Motion?
If there is a net force, it will translate.
 
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  • #29
StoyanNikolov said:
So in Moment 5 there will appear Translational Motion?
Assuming there aren't factors you aren't telling us about, yes.
 
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  • #30
Yes there will always be a change in position from 1 to 5. The mass kinetic energy before and after will both be zero and there is no Potential Energy without gravity.

The magnitude of the translational motion will depend on the mass of the object and the magnitude of the "impulse" forces.

If F is not an ideal impulse then assumptions in 1 ( & presumed in 5) are invalid. "non-translational"
 
  • #31
erobz said:
If there is a net force, it will translate.
That's a good point - if whatever is applying the force is attached to the disc, that would come under "factors the OP isn't telling us about".
 
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  • #32
Thank you all for the reply.
No. I have rotating disk (at constant angular velocity from
Moment 2) and at Moment 5
I have external force applied as shown in the picture (Moment 5).
 
  • #33
Ibix said:
Assuming there aren't factors you aren't telling us about, yes.
Is it convenient to ask (In Moment 5) approximately at what direction will appear the translational motion ?
 
  • #34
StoyanNikolov said:
Is it convenient to ask (In Moment 5) approximately at what direction will appear the translational motion ?
See post #2...
 
  • #35
Ibix said:
That's a good point - if whatever is applying the force is attached to the disc, that would come under "factors the OP isn't telling us about".

erobz said:
If there is a net force, it will translate.
If net (vector) Forces are zero, it will still translate . Because the force acts both on the rotational mass and the linear centre of mass which causes rotation , it must also cause linear displacement.

The impulse force must be an infinitely small rotation for an infinitely small time to cause a step change in velocity and integrated positional changes both in rotation and linear displacement. If the reaction force is not in perfect opposition in amplitude and vector with the linear velocity, there will also be net motion in the new position.
 
  • #36
TonyStewart said:
If net (vector) Forces are zero, it will still translate .
There are at least two different scenarios being considered here, one where a force is required to start the disc rotating and one where it is already rotating. I, and I believe @erobz, are talking about the latter. I think you are talking about the former. I think what happens in the former is indeterminate, because OP says there is no translation after the first force is applied, but does not explain how this comes to be.
 
  • #37
The acceleration of the center of mass is given by the net force. Zero net force means zero acceleration of the COM. What happens to rotation us not relevant.
 
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  • #38
nasu said:
The acceleration of the center of mass is given by the net force. Zero bet force means zero acceleration of the COM. What happens to rotation us not relevant.
I think @TonyStewart is regarding the sum of the forces in "moment 1" and "moment 5" to be the net force, in which case the disc could translate between the two applications of force.
 
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  • #39
Ibix said:
There are at least two different scenarios being considered here, one where a force is required to start the disc rotating and one where it is already rotating. I, and I believe @erobz, are talking about the latter. I think you are talking about the former. I think what happens in the former is indeterminate, because OP says there is no translation after the first force is applied, but does not explain how this comes to be.
Yes that makes the question invalid as these rules are contradictory. You cannot apply a Force with
Only rotational motion when the mass is common to both rotational inertia and linear inertia..
 
  • #40
Lets consider only the scenario , where the disk is already rotating
(with constant angular velocity) and we apply force as shown in moment 5.
Will there be translational motion and in what direction.
 
  • #41
StoyanNikolov said:
Lets consider only the scenario , where the disk is already rotating
and we apply force as shown in moment 5.
Will there be translational motion and in what direction.
What didn't you understand about the answer in post #2, to which I have pointed you twice now?
 
  • #42
Ibix said:
What didn't you understand about the answer in post #2, to which I have pointed you twice now?
Just to clarify the above situation and to ask if somebody can point at what direction will be the translational motion of the center of mass (center of the disk)?
 
  • #43
StoyanNikolov said:
Just to clarify the above situation and to ask if somebody can point at what direction will be the translational motion of the center of mass (center of the disk)?
Can you state Newton's Second Law?
 
  • #46
Ibix said:
Did you read that link? The answer is in there at least twice.
Can I ask to type the direction of the center of mass here ?
I mean in what direction the center of mass will go ?
 
  • #47
StoyanNikolov said:
Can I ask to type the direction of the center of mass here ?
If you can read it here, you can read it at the link you posted.
 
  • #48
Ibix said:
If you can read it here, you can read it at the link you posted.
Is it ok to post the direction of translation of the center of mass from the link or to type it ?
Object is rotating and the force applied is at the periphery of the disk.
The Force is Not applied at the center of the disk.
Thank you.
 
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  • #49
:bow:
 
  • #50
Ibix said:
I think @TonyStewart is regarding the sum of the forces in "moment 1" and "moment 5" to be the net force, in which case the disc could translate between the two applications of force.
It can translate, for sure. But this is because the net force is not zero at point 1, unless there is some non-disclosed interaction. You don't need this made-up definition of net force
 
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