Does every object rotate around its center of gravity?

[Dale]

How would you explain why object rotate when force is act anywhere that is not at CG?

I assume that you mean “How would you explain why object rotate about the CM when force is act anywhere that is not at CM?” We have already mentioned the importance of specifying what it is rotating about, so it is a little disheartening to see you continue to not include that.

As I said above, that is because the torque about the CM is zero when the force acts at the CM and nonzero when it acts elsewhere. Are you familiar with the concept of torque?

So inertia(m x a) is reason why plane start rotating upwind..

I have never seen inertia expressed as ##ma##, which is force. I have seen it expressed as ##m## which is mass or as ##mv## which is momentum. Please be aware that we don’t permit just making things up here at PF. All posts must be consistent with the professional scientific literature. Inertia as ##ma## is not consistent.

 

[A.T.]

How would you explain why object rotate when force is act anywhere that is not at CG?

The above is not correct:
1) It's not the center of gravity but rather the center of mass
2) A force doesn't have to act at the center of mass to avoid angular acceleration, as long it acts through the center of mass.

Why? Because the center of mass is defined such that the above is true.

 

[John Mcrain]

As I said above, that is because the torque about the CM is zero when the force acts at the CM and nonzero when it acts elsewhere. Are you familiar with the concept of torque?

But this senteces don't explain why torque is non zero when force act elsewhere out of CM.

How can I call fictitius force what I feel as something push me to seat,when car accelerate?

How can you explain how I know that board will turn left,when accelerate in my post #64 ,without any calculation?

(Yes it is better to say about CM...Yes I know what is torque, force x lever arm...)

 

[A.T.]

...why torque is non zero when force act elsewhere out of CM ...Yes I know what is torque...

If you know what torque is you should know why it is non zero.

 

[John Mcrain]

A force doesn't have to act at the center of mass to avoid angular acceleration, as long it acts through the center of mass.

What is this?if force act CM -angular accelration is zero
if force act elsewhere out of CM- angular accelaration is non zero

 

[A.T.]

What is this?

What is what?

 

[jbriggs444]

But this senteces don't explain why torque is non zero when force act elsewhere out of CM.

If there is a non-zero net force on the object then the torque will depend on the chosen reference axis.

However, if one is interested in rotation, torque is only part of the answer. Torque only tells you how angular momentum changes over time. It does not tell you how the object rotates.

One can decompose angular momentum into two pieces. One piece accounts for the rotation of the body about is center of mass. The other piece accounts for the linear motion of the center of mass about the reference axis.

If we choose to put the reference axis at the center of mass then we eliminate one piece. This is very convenient.

 

[John Mcrain]

What is what?

I don't understand this construction.

 

[Dale]

Now I am thouroughly confused about what level to address my replies. First, you say this

But this senteces don't explain why torque is non zero when force act elsewhere out of CM.

Which makes me think, that I need to explain torque. But then you say this.

Yes I know what is torque, force x lever arm...)

Which makes me unsure how you can know what torque is and yet claim that my previous comment doesn't explain why torque is zero. I am not sure why you don't see that since you know the definition.

What is the lever arm about the CM for a force that goes through the CM?

Spoiler

lever arm = 0

Plug that into your definition and then what is the torque?

Spoiler

force x 0 = 0

How can I call fictitius force what I feel as something push me to seat,when car accelerate?

How can you explain how I know that board will turn left,when accelerate in my post #64 ,without any calculation?

I have no idea why you are bringing fictitious forces into the discussion. It seems wholly irrelevant. Regarding post #64, I don't know how you would do it without calculation. With calculation you simply calculate the torque about the CM and find that it is the torque for a left turn.

 

[A.T.]

I don't understand this construction.

I don't understand your question.

 

[PeroK]

What is this?if force act CM -angular accelration is zero
if force act elsewhere out of CM- angular accelaration is non zero

https://www.cambridge.org/gb/academ...nics-2nd-edition?format=HB&isbn=9780521198110

 

[Richard R Richard]

Any force that is not applied to an axis that does not pass through the MC creates a torque, when the force passes through the MC the length of the lever arm is zero and there is no torque, nor rotation.

In the video that you present, the force at the different ends of the boat creates different movements, as none passes through the MC, they create a moment or torque, which makes the boat rotate through its MC in the instant, but like the resistive forces of the water does not pass through the MC also create a moment with respect to the MC.
In case 1 the resistive forces oppose the turn but move the ship more, in the second case the resistive forces collaborate with the moment and the ship turns faster and moves slower.
When the ship has rotated, the resistive forces do not have to oppose the force pushing the ship in direction and direction, rather it will have a direction very close to the direction perpendicular to the ship's axis of symmetry, the unbalanced components also accelerate at ship in the ##x## direction, even though the force was applied in the ##y## direction.

I mean at accelerating phase,from time zero when force start to act to time when object stop accelerating..

Imagine we have boat with very very heavy metal ball at left side.Neglect aerodynamic forces at ball..
When boat is going at constant speed ,nothing happend,he is going in straight line.
But if you increase throttle,increase engine thrust,boat start accelerate,boat will turn to the left because mass of ball show resistance to change speed/position..

View attachment 276218

Check the drawing, the MC of a boat is always on or over the boat, (it is not a catamaran that is a special case of design), if the Cm is outside the boat it will lie down. Even if this does not happen, in a weak balance, accelerating the boat from the point of view of its occupants, has the same effect as changing the modulus and the direction of gravity (you can see the effect on the free surface of a glass of water). the resulting acceleration as a function of the acceleration with respect to the water ## a_x ## and its angle ## \theta ## remain
$$ a_R = \sqrt {g ^ 2 + a_x ^ 2} $$
$$ \tan \theta = \dfrac {g} {a_x} $$
In this way the direction of the resistive forces and the weight of the elements of the boat with respect to the MC changes, so the boat can capsize backwards if it accelerates against the position of the extra weight you have put on.

 

[John Mcrain]

Engines are same,have equal thrust...
Stick is is deep space,engine 1 is act at CM, engine 2 is turn OFF,stick travel with constant speed v.
In time t1 I turn OFF engine 1 and turn ON engine 2.

Will stick start rotating counterclock wise or it will keep going in straight line without rotation?

 

[A.T.]

Stick is is deep space,engine 1 is act at CM, engine 2 is turn OFF,stick travel with constant speed v.

If only rocket 2 is OFF, the stick will not travel with constant speed.

In time t1 I turn OFF engine 1 and turn ON engine 2.

Will stick start rotating counterclock wise

Yes.

 

[John Mcrain]

If only rocket 2 is OFF, the stick will not travel with constant speed.

I forget that in space there is no drag.
What then determine final speed if there is no drag and my engine has finite thrust?
stick accelarate to infinity?

 

[A.T.]

I forget that in space there is no drag.
What then determine final speed if there is no drag and my engine has finite thrust?
stick accelarate to infinity?

No, it will approach c.

 

[John Mcrain]

No, it will approach c.

¸What determine real rocket speed in space,how fast they move in space,if drag is zero?

 

[A.T.]

¸What determine real rocket speed in space,how fast they move in space,if drag is zero?

The speed of everything is determined by the reference frame you choose to describe its motion.

 

[John Mcrain]

The speed of everything is determined by the reference frame you choose to describe its motion.

imagine space is fill with fluid,what is speed in relation to fluid?similar as airspeed in planes

 

[A.T.]

imagine space is fill with fluid,what is speed in relation to fluid?

Look at:
https://en.wikipedia.org/wiki/Terminal_velocity
Just replace the constant gravity with your constant thrust.

 

[John Mcrain]

Look at:
https://en.wikipedia.org/wiki/Terminal_velocity
Just replace the constant gravity with your constant thrust.

But space don't have fluid,I just say fluid as example for reference point ..Fixed point in space

 

[A.T.]

I just say fluid as example for reference point ..Fixed point in space

There is no absolute rest, if that's what you mean.

 

[John Mcrain]

There is no absolute rest, if that's what you mean.

What frame use ussualy when people talk about rocket speed in space?

 

[John Mcrain]

There is no absolute rest, if that's what you mean.

I am in rocket when engine is turn off.I get out of rocket and rocket turn ON engine and going away from me..

What is rocket speed in relation to me?

 

[A.T.]

I am in rocket when engine is turn off.I get out of rocket and rocket turn ON engine and going away from me..

What is rocket speed in relation to me?

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
https://en.wikipedia.org/wiki/Relativistic_rocket#Relativistic_rocket_equation

 

[jbriggs444]

Engines are same,have equal thrust...
Stick is is deep space,engine 1 is act at CM, engine 2 is turn OFF,stick travel with constant speed v.
In time t1 I turn OFF engine 1 and turn ON engine 2.

Will stick start rotating counterclock wise or it will keep going in straight line without rotation?

View attachment 276287

The stick will not rotate in this case. From the problem statement, we are given that the engine acts at the center of mass. That means that the engine is just a bit right of center on that huge massy blob. Just enough right so that things remain balanced.

Please do not present drawings that disagree with the accompanying text.

 

[jbriggs444]

What frame use ussualy when people talk about rocket speed in space?

We pick one that is convenient.

For a rocket on a sub-orbital trajectory aimed at a target, we might use the rotating frame in which the surface of the Earth is stationary.

For a rocket in orbit around the Earth, we might use the inertial frame in which the center of the Earth is stationary.

For a rocket making a trip to the moon, we might switch frames in the middle of the trip. [It would be foolhardy for Neil Armstrong to be using the earth-centered inertial frame when descending the last 30 meters to land on the moon].

For a rocket making a slingshot maneuver, we can shift frames and observe how a trajectory which conserves spacecraft mechanical (kinetic + potential) energy in one frame increases it in another. This might confuse us until we realize that energy is conserved but is not invariant.

 

[Dale]

Will stick start rotating counterclock wise or it will keep going in straight line without rotation?

Again, you need to specify the point of rotation you are asking about. “Will stick start rotating counterclock wise about the CG or it will keep going in straight line without rotation?” If you find that too cumbersome then the word “spin” refers to “rotation about the CG” in classical physics.

 

[jbriggs444]

Again, you need to specify the point of rotation you are asking about. “Will stick start rotating counterclock wise about the CG or it will keep going in straight line without rotation?” If you find that too cumbersome then the word “spin” refers to “rotation about the CG” in classical physics.

I am inclined to let this one pass.

The rate of change of orientation of a rigid object is an invariant. It does not depend on a choice of reference axis.

If we were asked about the rate of change of angular momentum, then the reference axis would [usually] matter.

 

[Dale]

I am inclined to let this one pass.

I am not. His questions are exceptionally sloppy. This is one topic that he has been specifically instructed about from the very first page. That this far into it he still is being sloppy indicates an unwillingness to learn or improve.

 

[A.T.]

The rate of change of orientation of a rigid object is an invariant. It does not depend on a choice of reference axis.

Exactly. For rotation as change of orientation of the rigid body the reference point doesn't matter. It matters for translation, torques and angular momentum.

 

[John Mcrain]

The stick will not rotate in this case.

A.T. said it will..

 

[russ_watters]

A.T. said it will..

I think @jbriggs444 was thrown by the errors in the description and diagram. If only engine #2 is firing, which is not at/through the COM, it rotates.

 

[John Mcrain]

I am inclined to let this one pass.

You know what I am talking...

 

[jbriggs444]

You know what I am talking...

Actually, not usually. Care and clarity helps both the poster and the responder.