Isolated rotating unbalance. Why is there a vibration?

[John2020]

Hi everybody!

What is the cause of vibration on the above system (isolated), from the moment there are no external forces?

[exchemist]

48 minutes ago, John2020 said:

Hi everybody!

What is the cause of vibration on the above system (isolated), from the moment there are no external forces?

I'm not with you. There is obviously an unbalanced rotating mass. What's the issue? 

Later: Or are you asking how a system with no - apparent - exterior force acting on it can vibrate? If that is what you mean, I think the thing to consider is the centre of gravity of the system. If it were not constrained by the setup shown, the system would move such that the CG did not accelerate. Because it is asymmetric, keeping the CG in one place would involve parts of the system moving as the rotating mass rotates. But this is prevented by the side guides, springs and damper constraining it. So these guides will exert forces on it as the eccentric mass rotates. So exterior forces do act on it after all.    

 

P.S. Are you training to be a washing machine designer or something? 😀

Edited May 5, 2021 by exchemist

[John2020]

Hi exchemist!
I am not speaking about the obvious things we all know. Regarding the exterior forces, they will not act along the vertical axis of motion of the system. They will not influence system's motion on the vertical axis. Even if the system was not constrained, it would vibrate horizontally and vertically.

What I am asking is: How an isolated rotating unbalance may vibrate from the moment Newton's 3rd law holds? In other words, vibration in any direction will automatically violate Newton's 3rd law. Vibration presupposes center of mass redeployment, however this cannot be conducted using internal forces because it would violate Newton's 3rd law.

So, how classical mechanics justify the vibration of an isolated rotating unbalance within Newton's laws of motion? 

Just for the record,  I am not training to be a washing machine designer.

Edited May 5, 2021 by John2020

[swansont]

The CoM doesn’t move if it starts at rest and there are no external forces.

[studiot]

7 minutes ago, swansont said:

The CoM doesn’t move if it starts at rest and there are no external forces.

Simple and obvious. +1

@John2020  Since you say there are no external forces, is there no gravity on your planet ?

[John2020]

Hi swansont and studiot!

Let me clarify this: The system is isolated (in absence of external forces -> no gravity) and has on board power. Moreover, the eccentric mass has a constant tangential speed. Under these conditions, the system will start vibrate along the vertical axis, right?

[exchemist]

5 hours ago, John2020 said:

Hi exchemist!
I am not speaking about the obvious things we all know. Regarding the exterior forces, they will not act along the vertical axis of motion of the system. They will not influence system's motion on the vertical axis. Even if the system was not constrained, it would vibrate horizontally and vertically.

What I am asking is: How an isolated rotating unbalance may vibrate from the moment Newton's 3rd law holds? In other words, vibration in any direction will automatically violate Newton's 3rd law. Vibration presupposes center of mass redeployment, however this cannot be conducted using internal forces because it would violate Newton's 3rd law.

So, how classical mechanics justify the vibration of an isolated rotating unbalance within Newton's laws of motion? 

Just for the record,  I am not training to be a washing machine designer.

You don't seem to understand mechanics, certainly.

You appear to have got hold of a wrong idea, namely that no freely floating system can vibrate. But of course it can. If you have a spring, in free fall in space, say, that has been set vibrating by some means, it will continue to vibrate. Most molecules of a gas at normal temperature are free floating and vibrating. In the case of non-rigid bodies, Newton's Laws only tell you that the centre of gravity (centre of mass) will continue at constant speed in a straight line if no external forces operate.

In this case, if your rotating system, with its eccentrically mounted mass, were freely floating, the frame would describe a circular oscillation, opposite to that of the eccentric mass, such as to keep the CG motionless.

By attaching springs, a damper and a vertical guide with rollers, you are preventing it from doing that freely. Constraining it involves applying forces, which accelerate the CG first in one direction and then in another. The rollers, the springs and the damper all exerts forces on it. If they did not, there would be no point in them being there.

[swansont]

It will vibrate along both axes.

A conceptual simplification would be to assume everything is massless except this one mass. The chassis moves in a circle around the stationary mass.

[John2020]

25 minutes ago, exchemist said:

By attaching springs, a damper and a vertical guide with rollers, you are preventing it from doing that freely. Constraining it involves applying forces, which accelerate the CG first in one direction and then in another. The rollers, the springs and the damper all exerts forces on it. If they did not, there would be no point in them being there.

Let's simplify the system by removing the damper and the springs by keeping just the rollers.

The constraining forces are not responsible for the vibration along the vertical axis, they just cancel out the horizontal vibration. Let me reformulate the question to show where my point is: The system vibrates upwards and downwards. Question: From the moment the system is isolated (no gravity, nothing), how the system as a whole can move upwards (let's consider just the upwards motion for a moment) and Newton's third law still holds? Normally, it shouldn't vibrate at all since the momentum of the eccentric mass along the vertical axis would be cancelled by the momentum of the rest of the system at any instance. Consequently, no vibration should occur. 

Edited May 5, 2021 by John2020

[exchemist]

17 minutes ago, John2020 said:

Let's simplify the system by removing the damper and the springs by keeping just the rollers.

The constraining forces are not responsible for the vibration along the vertical axis, they just cancel out the horizontal vibration. Let me reformulate the question to show where my point is: The system vibrates upwards and downwards. Question: From the moment the system is isolated (no gravity, nothing), how the system as a whole can move upwards (let's consider just the upwards motion for a moment) and Newton's third law still holds? Normally, it shouldn't vibrate at all since the momentum of the eccentric mass along the vertical axis would be cancelled by the momentum of the rest of the system at any instance. Consequently, no vibration should occur. 

This makes no sense. If as you say "the momentum of the eccentric mass along the vertical axis would be cancelled by the momentum of the rest of the system at any instance", then what you have is precisely a vibrating system. 

Every vertical motion of the eccentric mass is compensated by an opposite motion of the frame. So when the mass moves up, the frame moves down, and when the mass moves down, the frame moves up. What is that if not vibration?  

Edited May 5, 2021 by exchemist

[John Cuthber]

In the absence of any constraints (for example, if it was floating in space), if you start the system up (say, by remote control) the rotor will spin one way, and the stator (and power supply etc) will spin the other.

 

An ant sat on the battery will see the stars moving and will deduce that he's spinning.

Another ant on the mass "m" will make the same observations and deductions.

If you let them communicate they will be able to establish that they are rotating in opposite directions around the same axis.

Neither of them will think they are vibrating.

 

 

Meanwhile back on Earth I can watch the tip of the second hand of the clock as it goes round.
It's going more or less horizontally left to right

15 seconds later it's going straight down. Another 15 seconds and it's moving right to left and another 15 seconds and it's  going vertically up.

 

But it isn't normal to think of that as the sum of a left to right, and up and down motion.

It's going round in a circle.


On the other hand, going round in circles is just one option. if you can vary the phase and frequency of the L-R and U-D vibrations independently, you acn get pretty patterns.

 

https://en.wikipedia.org/wiki/Lissajous_curve

None of this is rocket science.

[John2020]

6 minutes ago, exchemist said:

So when the mass moves up, the frame moves down, and when the mass moves down, the frame moves up. What is that if not vibration?  

So here is exactly what I pointed out (after your words): According to Newton's laws of motion, there is no way the frame to move in any direction (up or down) because of an internal rotating mass. We speak about internal forces/momentum that means the frame should never move in any direction. Since the experiment says "it vibrates" then, what Newton has to say about all these?

[exchemist]

21 minutes ago, John2020 said:

So here is exactly what I pointed out (after your words): According to Newton's laws of motion, there is no way the frame to move in any direction (up or down) because of an internal rotating mass. We speak about internal forces/momentum that means the frame should never move in any direction. Since the experiment says "it vibrates" then, what Newton has to say about all these?

That's rubbish and I've already explained why. Twice. If you still haven't seen the point, I'm not going to spend more time on this. 

[swansont]

21 minutes ago, John2020 said:

So here is exactly what I pointed out (after your words): According to Newton's laws of motion, there is no way the frame to move in any direction (up or down) because of an internal rotating mass.

No. That’s not what anybody has said, and Newton’s laws say the chassis must move

21 minutes ago, John2020 said:

We speak about internal forces/momentum that means the frame should never move in any direction. Since the experiment says "it vibrates" then, what Newton has to say about all these?

If the internal mass moves, the rest of the system has to move as well, according to Newton’s 3rd law.

[John2020]

28 minutes ago, swansont said:

If the internal mass moves, the rest of the system has to move as well, according to Newton’s 3rd law.

This is correct in terms of momentum conservation, however since the cause of this motion comes from internal forces, the CoM of the system as a whole will not change or better saying the CoM will not accelerate by means of internal forces. Again, not within a cycle but at any instance. Newton does not say the chassis will move (see internal forces and Newton's third law).

[studiot]

Gosh my head hurts.

What do you have against poor old gravity.

First you say there is no gravity

But you retain terms which only have meaning in the presence of gravity

To whit 'up' and 'down' and 'vertical' and 'horizontal'.

Then you say you can apparantly switch gravity on and off at will.

1 hour ago, John2020 said:

Question: From the moment the system is isolated (no gravity, nothing),

 

[Ghideon]

16 minutes ago, John2020 said:

since the cause of this motion comes from internal forces, the CoM of the system as a whole will not change or better saying the CoM will not accelerate by means of internal forces.

The CoM of the system as a whole will not move. The CoM of the frame will move which means vibration.

  

[swansont]

23 minutes ago, John2020 said:

This is correct in terms of momentum conservation, however since the cause of this motion comes from internal forces, the CoM of the system as a whole will not change or better saying the CoM will not accelerate by means of internal forces. Again, not within a cycle but at any instance. Newton does not say the chassis will move (see internal forces and Newton's third law).

Yes, it does. If you treat the mass as one system and the chassis as another, they must each have equal and opposite momentum at all times.

!

Moderator Note

The larger issue here is you can’t ask the question and then give the answer; that’s a violation of our good faith rule (advancing an ideology or agenda at the expense of the science being discussed). Your question has been answered, and we’re not going to entertain yet another discussion involving your fanciful notions of Newton’s laws.