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John Mcrain
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It is said that rocket,plane rotate about center of gravity ,why this is is not case for boats?
Boat pivot point is not in center of gravity.
Boat pivot point is not in center of gravity.
John Mcrain said:It is said that rocket,plane rotate about center of gravity , why this is is not case for boats?
What do you want to say by this?jbriggs444 said:Also, boats have keels, daggerboards and centerboards.
For this reason, planes can be regarded as pitching about a horizontal axis through the wings. And sailboats can be regarded as yawing about a vertical axis through the centerboard.John Mcrain said:What do you want to say by this?
Plane also has rudder,wings,flaps,slats,horizontal stablizer etc etc...
jbriggs444 said:For this reason, planes can be regarded as pitching about a horizontal axis through the wings. And sailboats can be regarded as yawing about a vertical axis through the centerboard.
I do not understand your confusion.John Mcrain said:I don't understand your point.
Does rocket, plane rotate around CG?
Only difference in air and water is fluid density,why then boat don't rotate around CG too?
It's up to you. A plane flying a looping can be described in different ways:John Mcrain said:Does rocket, plane rotate around CG?
Why then center of pressure must be behind CG to have stable rocket?A.T. said:It's up to you. A plane flying a looping can be described in different ways:
1) Plane's CG moves around the center of the looping & plane rotates around its CG
2) Plane rotates around the center of the looping
The point you pick for center of rotation has nothing to do with the question you now ask. The rotation rate of the rocket does not depend on one's choice of reference point.John Mcrain said:Why then center of pressure must be behind CG to have stable rocket?
jbriggs444 said:it is convenient to place the reference axis at the center of mass
The torque around CG determines whether the rocket starts rotating. But that rotation doesn't have a specific "pivot", it's just a change in orientation.John Mcrain said:If you put CG behind center of pressure,rocket will be unstable and crash..So it seems CG is real pivot point for rocket,not just math agreement.
A.T. said:The torque around CG determines whether the rocket starts rotating. But that rotation doesn't have a specific "pivot", it's just a change in orientation.
When gust of side wind hit rocket , will rocket start rotation about CG?hutchphd said:A rigid object set rotating in space far from other influences will rotate about its center of mass. If other "forces" are present it may not.
The instantaneous center of rotation of a rigid object can be pretty much anywhere. Pick a frame. Any frame.hutchphd said:A rigid object set rotating in space far from other influences will rotate about its center of mass. If other "forces" are present it may not.
There is no single answer. It may or may not rotate, in either direction...and if it does, you get to pick the point it rotates about.John Mcrain said:When gust of side wind hit rocket , will rocket start rotation about CG?
Regardless of the choice, the change in orientation of the object is an invariant fact of the matter. It does not depend on which choice you make. [Which I know is what you just got done saying]russ_watters said:There is no single answer. It may or may not rotate, in either direction...and if it does, you get to pick the point it rotates about.
Yes, I didn't mean to imply otherwise. I was mostly trying to say that what happens depends on the specifics of the scenario. I can imagine scenarios where a rocket might rotate toward or away from a wind or not at all.jbriggs444 said:Regardless of the choice, the change in orientation of the object is an invariant fact of the matter. It does not depend on which choice you make. [Which I know is what you just got done saying]
I really don't understand your "mathematical thinking".I am to stupid to understand this abstract thinking..jbriggs444 said:The instantaneous center of rotation of a rigid object can be pretty much anywhere. Pick a frame. Any frame.
If I ask you about what point weather vane rotate,what will be your answer?jbriggs444 said:The instantaneous center of rotation of a rigid object can be pretty much anywhere. Pick a frame. Any frame.
The center of mass and a frame where the object is not translating is a useful choice because it means that the instantaneous center of rotation will not be gyrating along a spiral path.
The pivot point is convenient because that is the point that will remain stationary in the ground frame. In this case, the center of mass choice is inconvenient because it would be gyrating in such a frame.John Mcrain said:If I ask you about what point weather vane rotate,what will be your answer?
1)about pivot point rod
2)earth
3)sun
4)galaxy
5)any point
and what answer has the most physics importnace for human that look at this weather vane?
jbriggs444 said:Nonetheless, one can do physics in any frame of reference. Some choices simplify calculations. Some choices complicate them.
It is the same weather vane regardless of what axis you choose to use for the analysis. It changes its orientation by the same amount regardless of what pair of body-fixed points you choose to use to measure deflection. If one does the analysis either way, the result will still come out predicting that that the center of pressure will equilibriate downwind from the mast.John Mcrain said:But if you know that weather vane is rotating around pivot point rod,then you know you mast design rear plate behind so arrow will pointing into wind.
If you look that this weather vane rotate about some galaxy,what you useful know for design purpose?
jbriggs444 said:I'm will not say that the weather vane "really" rotates about the mast because the norm is to reserve "real" as an adjective for things that are invariant. Not for things that are free choices.
It's a very big mistake to draw such a conclusion about what you see. You're making assumptions that may turn out to be wrong, or are just unstated and may not be agreed upon by all:John Mcrain said:Hmm, that type of thinking confuse me...
Because every human can see with his eyes that wheater vane is rotating about mast.
But for sure with imaginary thinking it can be rotate about any point..
russ_watters said:It's a very big mistake to draw such a conclusion about what you see. You're making assumptions that may turn out to be wrong, or are just unstated and may not be agreed upon by all:
...but more to the point that people have been trying to drive home; there are many different choices you can make, and if the math works they are all equally "real".
That's nonsense. Do you have any videos of objects rotating in the International Space Station?John Mcrain said:If you push stick at one end,it will translate and rotate,but pivot point(position which not change position in space) will be out of CG,can even be at point which is out of stick physical limits..
It's not your just eyes, but also your brain doing a lot of interpretation. But the interpretation your brain chooses doesn't have to be unique.John Mcrain said:Because every human can see with his eyes that wheater vane is rotating about mast.
If I want test aircraft in wind tunnel for wind gust/weather vane effect,where I must placed rod ?jbriggs444 said:It is the same weather vane regardless of what axis you choose to use for the analysis. It changes its orientation by the same amount regardless of what pair of body-fixed points you choose to use to measure deflection. If one does the analysis either way, the result will still come out predicting that that the center of pressure will equilibriate downwind from the mast.
I've already agreed with you that the position of the mast is a convenient choice for the "rotation axis". [It follows naturally from choosing to work in the ground frame]. What more do you wish?
I'm will not say that the weather vane "really" rotates about the mast because the norm is to reserve "real" as an adjective for things that are invariant. Not for things that are free choices.
The point that @jbriggs444 is trying to make is that you are conflating two issues here.John Mcrain said:If I want test aircraft in wind tunnel for wind gust/weather vane effect,where I must placed attached point?
At CG,infront CG,behind CG,at rudder,at propeller...?
Anywhere will do. It's common to see the support under or behind the model.John Mcrain said:If I want test aircraft in wind tunnel for wind gust/weather vane effect,where I must placed rod ?
At CG,infront CG,behind CG,at rudder,at propeller...?
No, the center of gravity and the center of mass are not the same. The center of mass is the point where the mass of an object is evenly distributed, while the center of gravity is the point where the force of gravity acts on an object.
Yes, every object has a center of gravity. It is the point where the force of gravity acts on the object.
Yes, the shape of an object can affect its center of gravity. Objects with irregular shapes or uneven distribution of mass will have a different center of gravity compared to symmetrical objects.
Yes, an object can rotate around a point other than its center of gravity. This is known as off-center rotation and can occur when an external force is applied to the object.
No, the center of gravity can be located both inside and outside of an object. For example, in a hollow object, the center of gravity may be located outside of the object's physical boundaries.