Forces on rotating disk object

[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.

 

[Ibix]

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.

 

[nasu]

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.

 

[Ibix]

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.

 

[TonyStewart]

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..

 

[StoyanNikolov]

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.

 

[Ibix]

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?

 

[StoyanNikolov]

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)?

 

[Ibix]

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?

 

[StoyanNikolov]

Can you state Newton's Second Law?

https://www.physicsclassroom.com/class/newtlaws/Lesson-3/Newton-s-Second-Law
Does somebody at what direction will be the translational motion ?

 

[Ibix]

https://www.physicsclassroom.com/class/newtlaws/Lesson-3/Newton-s-Second-Law
Does somebody at what direction will be the translational motion ?

Did you read that link? The answer is in there at least twice.

 

[StoyanNikolov]

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 ?

 

[Ibix]

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.

 

[StoyanNikolov]

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.

 

[erobz]

 

[nasu]

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

 

[StoyanNikolov]

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

Please forget Moment 1.
Consider only moment 5.
Rotating object at constant angular velocity and
Applied force at the periphery, not the center of the disk
At what direction the center of mass will go?

 

[TonyStewart]

Please forget Moment 1.
Consider only moment 5.
Rotating object at constant angular velocity and
Applied force at the periphery, not the center of the disk
At what direction the center of mass will go?
View attachment 325677

I forget 1 then I don’t know what is the force vector, radius , direction and momentum for initial conditions undefined. It will tend towards stopping but even the slightest error will have residual motion.

will equal and opposite thrust vectors conserve momentum and kinetic energy or are we debating the illegal assumption an impulse may cause rotational motion without linear motion and assuming it can when it cannot. You need to balance the Force on both sides like in the video!!

if I were given this problem , I would reject it , for the reasons given above.

 

[StoyanNikolov]

I forget 1 then I don’t know what is the force vector, radius , direction and momentum for initial conditions undefined

Initial conditions are :
Rotating disk at constant angular velocity.
Applied force is at the periphery of the disk (not the center)
At what direction the center of mass of the disk
will go after the force is applied . Thank you.
Just like on the attached picture.

 

[Ibix]

At what direction the center of mass of the disk
will go after the force is applied .

What does your link say? I've already told you it has the answer. If you aren't sure you've got the correct answer from it, post what you've understood and we can go from there. If you aren't willing to go to the minimal effort of reading a link you supplied, I'm not willing to spend any more time helping.

 

[StoyanNikolov]

What does your link say? I've already told you it has the answer. If you aren't sure you've got the correct answer from it, post what you've understood and we can go from there. If you aren't willing to go to the minimal effort of reading a link you supplied, I'm not willing to spend any more time helping.

I don't find where it is written about the direction.
Please copy the text from the link, where it is said where the center of mass will go.
If you are willing to go to the maximal effort of not helping me,
it is ok.
Just cannot say that i have the answer for the direction after more than
50 replies on this topic.
Thank you.

 

[TonyStewart]

Initial conditions are :
Rotating disk at constant angular velocity.

How much mass do you need for a tangential force to accelerate rotation of a floating mass yet not accelerate center of mass to some motion as stated In a zero-g environment? Consider the solution to deflection massive asteroids from Earth collision! Even though an asteroid has considerable mass, the purpose of the mission is to deflect the path enough, normal, or perpendicular to the impulse or detonation applied. This has been proven to work.

Applied force is at the periphery of the disk (not the center)
At what direction the center of mass of the disk
will go after the force is applied . Thank you.
Just like on the attached picture.
View attachment 325678

 

[Drakkith]

I don't know what happened with this thread. Perhaps people have lost the forest for the trees. I'll reopen this thread but ask that members please give the OP the benefit of the doubt and that everyone, including the OP, please act as if we were all sitting around a table in person and treat others with respect.

Several off topic or non-helpful posts have been deleted. Thread reopened.

 

[Drakkith]

Please forget Moment 1.
Consider only moment 5.
Rotating object at constant angular velocity and
Applied force at the periphery, not the center of the disk
At what direction the center of mass will go?
View attachment 325677

Assuming other forces are either absent or negligible, it will move upwards.

Consider the situation in which the disk isn't rotating, but a fast moving object moves upwards and strikes the disk at the point you've circled (or some pegs sticking out, or however you want to apply the force). Assuming the disk is not attached to anything else, the center of mass will move upwards. This is the same situation, just without the rotation, which I don't believe matters at all for your question.

 

[A.T.]

Initial conditions are :
Rotating disk at constant angular velocity.
Applied force is at the periphery of the disk (not the center)
At what direction the center of mass of the disk
will go after the force is applied . Thank you.
Just like on the attached picture.
View attachment 325678

- Is the center of mass initially at rest?

- Is the force a short impulse or does it act over a certain period? If the latter, is the direction of the force constant in the inertial frame or in the rotating disk frame?

- Are there any other forces acting?

If there are no other forces, and it is a short impulse, the center of mass will accelerate in the direction of the force. So if it was initially at rest, that's where it will go.

 

[jbriggs444]

Newton's second law original wording (after translation to English):

"LAW II: The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed."