Does every object rotate around its center of gravity?

[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.

 

[John Mcrain]

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.

I made mistake ,because I forgeot stick will accelerate in space becasue there is no any drag to counter force ,so question don't make sense at all..

 

[jbriggs444]

A.T. said it will..

I had incorrectly assumed you were firing the engine under the big mass, not the engine at the tip of the rod.

 

[John Mcrain]

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

If engine 2 in ON,stick will accelerate again,because in space drag don't exist.
Stick will accelerate in any combination of engines,both ,only left or only right..

Isnt it?

 

[russ_watters]

I made mistake ,because I forgeot stick will accelerate in space becasue there is no any drag to counter force ,so question don't make sense at all..

You also appeared to locate engine 1 at the COM of the large object, not at the COM of the system.

But anyway, since we don't know where you are going with these questions, it is tough for us to error-check your overall logic. If you only want to know about rotation, it shouldn't matter that the system is also accelerating linearly -- we could simply ignore that. Maybe there's something else on your mind...

If engine 2 in ON,stick will accelerate again,because in space drag don't exist.
Stick will accelerate in any combination of engines,both ,only left or only right..

Isnt it?

Yes. Didn't you do this experiment with a pencil like I suggested?

 

[John Mcrain]

You also appeared to locate engine 1 at the COM of the large object, not at the COM of the system.

But anyway, since we don't know where you are going with these questions, it is tough for us to error-check your overall logic. If you only want to know about rotation, it shouldn't matter that the system is also accelerating linearly -- we could simply ignore that. Maybe there's something else on your mind...

No this is CoM from all system..Forget about it because it doesn't make sense at all,because system is accelerating,so mass show resistance to change speed again

 

[russ_watters]

No this is CoM from all system..Forget about it because it doesn't make sense at all,because system is accelerating,so mass show resistance to change speed again

K

 

[John Mcrain]

Yes. Didn't you do this experiment with a pencil like I suggested?

Yes I know for this test.

I try to prove something else,but now I see that this what I mean is impossible.
Now I realize that for constant speed you must allways have two opposite forces equal in magnitude,so net force again is zero.In space this second opposite force- drag force don't exsit.

 

[A.T.]

Now I realize that for constant speed you must allways have two opposite forces equal in magnitude,so net force again is zero.

This is true for constant velocity. For constant speed (magnitude of velocity) you can have a net force, that is perpendicular to velocity.

 

[Dale]

For rotation as change of orientation of the rigid body the reference point doesn't matter.

Yes, but the term for the rate of change of the orientation of a classical object is "spin", rather than "rotation" which is more general.

 

[John Mcrain]

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.

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 and how would I call this place in space where I am now if I want that place would be my origin of my reference frame?

 

[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

...and how would I call this place in space where I am now if I want that place would be my origin of my reference frame?

The origin of your rest frame.

 

[Richard R Richard]

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

View attachment 276287

Suppose you are in an inertial frame of reference at rest or not with the ship.

Case 1 Both engines off, the ship continues at rest or in uniform rectilinear motion.

Case 2 You turn on motor 1, you apply force 1, on the MC, it accelerates in translation at a rate of $a_1 = \dfrac{F_1}{M}$, but it does not rotate, the angular acceleration is zero $\alpha = 0$ since the lever stroke regarding the MC is null.

Case 3 Turn off engine 1 and turn on 2 The MC accelerates in translation at the rate of $a_2 = \dfrac{F_2}{M}$ in the same direction as the force is applied and in turn rotates with angular acceleration $\alpha = \dfrac MI = \dfrac{F_2L}{KML^2}$ Where K is a value that depends on the design of the ship, which is irrelevant, the important thing is that $\alpha \neq 0$ The speed of the MC in both cases results from $v =\displaystyle \int_0^T \dfrac {F (t)}{M (t)} dt$ only for brief instants $v \cong at$, because both a The force and the mass can be variable in time, because we know that we can vary the thrust by increasing fuel consumption, the mass also varies because we consume the fuel, so these formulas are strictly not true, so it is more I must study what they propose you about the Tsiolkovski rocket.

Considering the same limits, in a few moments you can say that the rotational speed or angular velocity will be $\omega = \displaystyle \int_0 ^ T \dfrac {F (t) L}{KM (t) L^2} dt$ then we can say that $\omega \cong \alpha t$

If you introduce resistive forces, these can be summarized as a single force with direction and direction applied at a point of thrust by modifying the modulus of the translational acceleration, It will also influence the acceleration of the turn, if the axis of the resultant force does not pass just through the MC, since it will have a non-zero lever arm, and will create torque.

 

[Shane Kennedy]

They all try to. Without any other forces, they will, but even a spacecraft will deviate when thrusters are fired,

 

[swampwiz]

It's because the mathematics are such that when dealing with a *rigid* body, the body force - i.e., gravity or centrifugal acceleration (if the radius of the spin is much larger than the volumetric extent of the body) - cancels out at the barycenter, and a net applied load (i.e. force & force couple moment) can be abstracted out to be a force & moment applied to the barycenter, with the inertia-acceleration equations being decouple.

 

[kinsler33]

Turn off the boat's engine and lift it out of the water. Now fling it into the air. It will then rotate around its center of mass.