Forces on rotating disk object

[StoyanNikolov]

TL;DR Summary

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:

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.

 

[kuruman]

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?

 

[TonyStewart]

You forgot to show direction of gravity and if centre is fixed.

In the Space Lab with 0g, interesting things happen when you spin a tool with some wobble.


The same was predicted to occur in earth's liquid magcore. (fact-check) https://astronomy.com/news/2021/09/when-north-goes-south-is-earths-magnetic-field-flipping

 

[StoyanNikolov]

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 .

 

[TonyStewart]

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.

 

[StoyanNikolov]

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.

 

[TonyStewart]

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?.

 

[StoyanNikolov]

- 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.

 

[TonyStewart]

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.

 

[erobz]

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.

 

[StoyanNikolov]

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.

 

[Ibix]

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.

 

[StoyanNikolov]

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 ?

 

[Ibix]

We there appear translational motion ?

Depends why the translational motion didn't start in moment 1. What stopped it then?

 

[StoyanNikolov]

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

 

[erobz]

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?

 

[erobz]

Just please consider this situation.
Thank you

No thank you.

 

[StoyanNikolov]

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

 

[Ibix]

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.

 

[StoyanNikolov]

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

 

[Ibix]

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.

 

[nasu]

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.

 

[StoyanNikolov]

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.

 

[Ibix]

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.

 

[StoyanNikolov]

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?

 

[erobz]

Another "Reactionless Drive" thread no doubt?

 

[StoyanNikolov]

Another "Reactionless Drive" thread no doubt?

No it is not reactionless drive.
So in Moment 5 there will appear Translational Motion?

 

[erobz]

No it is not reactionless drive.
So in Moment 5 there will appear Translational Motion?

If there is a net force, it will translate.

 

[Ibix]

So in Moment 5 there will appear Translational Motion?

Assuming there aren't factors you aren't telling us about, yes.

 

[TonyStewart]

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"

 

[Ibix]

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".

 

[StoyanNikolov]

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).

 

[StoyanNikolov]

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 ?

 

[Ibix]

Is it convenient to ask (In Moment 5) approximately at what direction will appear the translational motion ?

See post #2...

 

[TonyStewart]

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".

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.