Do objects always rotate around center of mass?

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    Center of mass Rotation
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Discussion Overview

The discussion centers on whether objects, such as airplanes and boats, always rotate around their center of mass when forces like a rudder are applied. Participants explore various conditions and definitions related to rotation, including the implications of external forces and the nature of rigid body motion.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question if objects always rotate around their center of mass, suggesting there may be specific conditions where this is not the case.
  • One participant describes an example with a disk nailed to a wall, indicating it does not rotate around its center of mass when spun.
  • Another participant notes that for an SR-71 blackbird, the aircraft rotates about an axis significantly distant from its center of mass when the rudder is turned.
  • There is a discussion about the definition of rotation and how it can depend on the assigned axis of rotation, especially in non-inertial frames.
  • Participants mention that while a rigid body can be said to rotate about any point, the center of mass plays a crucial role in the dynamics of motion.
  • Some argue that the term "rotate" should be distinguished from "revolve," particularly in the context of celestial bodies like the moon.
  • There is a mathematical perspective presented, stating that any rigid body motion can be decomposed into rotation about an arbitrary point and translation.
  • Participants express differing views on the empirical convincingness of various points of rotation, particularly regarding the center of mass versus other points.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether objects always rotate around their center of mass, with multiple competing views and definitions of rotation being discussed. The discussion remains unresolved regarding the conditions under which rotation occurs.

Contextual Notes

Participants highlight the complexity of defining rotation, especially in the presence of external forces and non-inertial frames. The discussion includes various interpretations of motion, including instantaneous centers of rotation and the distinction between rotation and revolution.

  • #121
user079622 said:
stick in free space,circle is com,all forces are equal in magnitude and act all the time, they are rocket engines connected to stick.

Top= pure rotation in clockwise direction around com, com zero translation
middle=net force is non zero so we have acceleration,stick rotate around point P like I draw at the right side? in com we have force=ma in opposite direction from right force ?
bottom=net force is zero so com dont accelerate in any direction, stick rotate clockwise around com because right force has longer lever arm?View attachment 333245If middle case has just impuls force ,hit with hammer instead const force ,com will translate up at my graph, and stick will rotate around com.
top & bottom are OK, if you are asking about the instantaneous center of rotation in the initial rest frame of the stick (or the center of rotation that results in zero translation in that frame).

middle: the picture on the right is wrong. There is no centripetal force to create that circular motion.
 
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  • #122
A.T. said:
top & bottom are OK, if you are asking about the instantaneous center of rotation in the initial rest frame of the stick (or the center of rotation that results in zero translation in that frame).

middle: the picture on the right is wrong. There is no centripetal force to create that circular motion.
I dont know what to do with middle case, net force is non zero that mean com must accelerate, it cant move in circle because we dont have centripetal force for do this, but how will com accelerate in straight line if force act all the time on stick, force change orientation all the time as stick rotate..
?
 
  • #123
user079622 said:
I dont know what to do with middle case, net force is non zero that mean com must accelerate, it cant move in circle because we dont have centripetal force for do this, but how will com accelerate in straight line if force act all the time on stick, force change orientation all the time as stick rotate..
?
 
  • #124
@Lnewqban

Hhahaha nice one!

So what is your answer?
It cant just rotate around com, because net force non zero, must somewhere accelerate
 
  • #125
That video is m
user079622 said:
@Lnewqban

Hhahaha nice one!

So what is your answer?
It cant just rotate around com, because net force non zero, must somewhere accelerate
That video is my answer.
Yest, it can and it does.

Any other point of the machine describes a circle at the same time that rotates as much as the CM does.
That is why others have explained that not only the CM rotates.

The only reason the thing is landing some distance from where launched is that the plane of rotation was not perfectly vertical.
There was a little component of the tilted lift force that was forcing the CM to move sideways.

There is initial rotational acceleration, but only until the drag force equals the thrust of the propeller.
After that point, the rotational velocity is constant.
Then, that thrust decreases when the engine runs out of fuel, and sufficient lift is gone.

One question: Was your original question general or based on flying objects?
 
  • #126
Lnewqban said:
That video is m

That video is my answer.
Yest, it can and it does.One question: Was your original question general or based on flying objects?
I have lots of questions, every post other question fall out...

But how my middle case can do pure rotation if I have just one force?
 
  • #127
user079622 said:
I have lots of questions, every post other question fall out...

But how my middle case can do pure rotation if I have just one force?
I have edited my previous post, sorry.
What about the inertia (ma) or natural resistance of the CM to be moved in the same direction in which the off-center force is initially applied?

In the video, the man-stick is helping to prevent the CM from any initial horizontal movement during the first instants.
 
  • #128
Lnewqban said:
What about the inertia (ma) or natural resistance of the CM to be moved in the same direction in which the off-center force is initially applied?
I dont know ,but I think one force(even it is continuos) cant cause pure rotation.
 
  • #129
user079622 said:
I dont know ,but I think one force(even it is continuos) cant cause pure rotation.
 
  • #130
Dale said:
Yes, that is all good
Monocopter prove that there is no translation of c.g. he rotate around c.g. like it has couple forces(two engines)
How can one continuos force only rotate him around c.g. if net force is non zero?
 
  • #131
Around which point a rigid body rotates is up to your choice, and this choice must be well thought about. For a free rigid body (or a body freely falling in the constant gravitational field of the Earth) the choice of the center of mass of that body is the best choice though. For details, see

https://itp.uni-frankfurt.de/~hees/pf-faq/spinning-top.pdf
 
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  • #132
user079622 said:
Monocopter prove that there is no translation of c.g. he rotate around c.g
No it doesn't: you do not know where the center of mass is.

user079622 said:
he rotate around c.g. like it has couple forces(two engines)
There is a couple of forces: what stops the monocopter from rotating faster and faster?

However the only way to find the instantaneous center of rotation is to draw a free body diagram containing all the forces and calculate it. You will need to know where the centre of mass is, and also the centre of lift which will vary according to the rate of rotation and the position of the centre of rotation (although if the 'copter's motion is in equilibrium there is something you know about the position of the centre of lift relative to the centre of mass).

This thread is just going in circles (pun intended), if you want to understand complicated motion you need to do the calculations (which start with a free body diagram), not just keep making guesses.
 
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  • #133
pbuk said:
No it doesn't: you do not know where the center of mass is.
There is a couple of forces: what stops the monocopter from rotating faster and faster?
Aerodynamic drag force stop to going faster.
Don understand how they set center of rotation into CoM with just one force?
Maybe with shifting the drag force

At 5:20 he must put it on his c.g.

pbuk said:
n (although if the 'copter's motion is in equilibrium there is something you know about the position of the centre of lift relative to the centre of mass).
I think gyro effect stop "roll" because here lift force is fore sure not align with c.g.

pbuk said:
This thread is just going in circles (pun intended), if you want to understand complicated motion you need to do the calculations (which start with a free body diagram), not just keep making guesses.
I agree with you, I find I better learn if solve tasks then just talk about theory.
In future I will switch to homework questions.
 
Last edited:
  • #134
user079622 said:
Monocopter prove that there is no translation of c.g. he rotate around c.g. like it has couple forces(two engines)
How can one continuos force only rotate him around c.g. if net force is non zero?
I don’t know what a monocopter is, but it for sure doesn’t break the laws of physics as they have been taught to you here. And in this context “prove” would require detailed careful measurements and detailed careful math. Not vague handwaving assertions.
 
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  • #135
  • #137
vanhees71 said:
For a free rigid body (or a body freely falling in the constant gravitational field of the Earth) the choice of the center of mass of that body is the best choice
@user079622 I think that this is the best summary. You can choose any point you like, the math does not constrain your choice. But for a free body the com is usually the best choice and for a body attached to a hinge or an axis the hinge or axis is usually the best point.
 
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  • #138
Dale said:
See post #121
Sorry I made mistake, I mean middle case, other cases are solved...
 
  • #139
user079622 said:
Sorry I made mistake, I mean middle case, other cases are solved...
You will have to solve that one mathematically. It doesn’t have an obvious simple motion. That is the whole reason that the math was developed.
 
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  • #140
user079622 said:
Don understand how they set center of rotation into CoM with just one force?
The video says the exact opposite of what you claim. The center of rotation is not the CoM when airborne (at 5:33):

 
  • #141
A.T. said:
The video says the exact opposite of what you claim. The center of rotation is not the CoM when airborne (at 5:33):
Yes they are separated by 2cm, so he has problem with flight/shaking,he made him so he must do some fine tunning.
Wait a second, that mean CoM move in circle, where is centripetal force to do that?

If we say, rotate around CoM then we brake net force non zero rule.
If we say com move around center of rotation, then we dont have centripetal force to do this.
Paradox :)
 
  • #142
user079622 said:
If we say, rotation is around CoM then we brake net force non zero rule.
Or perhaps your analysis of the forces is wrong. Let’s see, which is more likely? The established laws of physics are wrong or a novice missed something important.

Anyway, this is getting both frustrating and pointless. I am going to close this thread.

You are welcome to start another thread on the topic under the following conditions:

1) it contains a clear description of the scenario(s) that you want to analyze

2) it shows your personal effort in analyzing it

3) you state clearly what you need help with to proceed

4) you make no claims that it only rotates about some axis and not any other axis (claiming that is does rotate about some axis is fine, only the claim that it does not rotate about another is problematic)

5) you make no more claims about violations of the laws of physics

Posts that do not meet those requirements will not be accepted.
 
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