Why does rotation create gravity?

[ydoaPs]

it doesn't matter that it is inconsiquential, it does happen.

 

perhaps that is a reason for the time dilation in special relativity.

[JaKiri]

it doesn't matter that it is inconsiquential' date=' it does happen.

 

perhaps that is a reason for the time dilation in special relativity.[/quote']

 

Time dilation is due to the invariant speed of light for all observers.

[r1dermon]

they spin the space station around an axis. this is called centrifugal force. the outward force of the 2 sides pushes the astronauts against the walls, which are actually the floors inside. artificial gravity at its most basic form. take a jug of water and spin it up over your head really fast, all the water will still be in it if you do it fast enough. i havent read this whole thread, so if im just re-itterating what someone already said, my appologies.

[swansont]

they spin the space station around an axis. this is called centrifugal force. the outward force of the 2 sides pushes the astronauts against the walls, which are actually the floors inside. artificial gravity at its most basic form. take a jug of water and spin it up over your head really fast, all the water will still be in it if you do it fast enough. i havent read this whole thread, so if im just re-itterating what someone already said, my appologies.

 

No, actually, it's not called centrifugal force. The floor pushes the astronauts inward; it's called centripetal force.

 

If the astronauts were floating, and you turned such a mechanism on, nothing would happen. If they were touching the floor, and thus moving, they would feel a force, because the floor's path is curved and the astronaust would have the tendency to move in a straight line (as per Newton's first law). The floor pushing inward is what gives the appearance of gravity in this case. There is no outward-directed force.

[Ophiolite]

If the astronauts were floating' date=' and you turned such a mechanism on, nothing would happen. [/quote']Pedantic thought for the day: wouldn't they smash into the first bulkhead, or rather it would smash into them?

[swansont]

Pedantic thought for the day: wouldn't they smash into the first bulkhead, or rather it would smash into them?[img']http://www.scienceforums.net/forums/images/icons/icon7.gif[/img]

 

If they floating, i.e. weren't moving relative to the bulkhead (in r or ^{[math] \theta [/math]}) then they won't smash into anything.

 

(assuming a vacuum. with an atmosphere, eventually the air will start moving and exert a force)

[r1dermon]

no outward force??? then what's to keep the astronaut from sticking to the "floor" since the floor is stationary, it only pushes on an object that pushes on IT. therefore, there has to be an outward force of the weight of the astronaut.

[Thorium]

Ah but what direction were objects already moving when they received centripedal acceleration? I really don't think there is an outward force (maybe a rod in the ground with something spinning around it, it will feel an outward, but not the thing moving itself)

[Ophiolite]

If they floating' date=' i.e. weren't moving relative to the bulkhead (in r or [sup'][/sup]) then they won't smash into anything.

 

(assuming a vacuum. with an atmosphere, eventually the air will start moving and exert a force)

Let me place my ignorance on display. I envisaged a traditional scifi space station shaped as a torus. Our astronaut is floating a few feet above the 'floor', or what will become the floor once roatation is initiated.

Balanced rockets are fired to initiate the spin. The 'floor' begins to rotate past the floating astronaut, who remains fixed in the same position relative to external points. Now, since I have designed the station with safety high on my list of performance characteristics, it has a number of cross bulkheads oriented radially. It is one of these that will impact our astronaut.

Where did I get it wrong?

[Pete]

WHAT ABOUT ENERGY????????????????????????????

Since mass and energy are related then yes. An increase in the energy of a body will increase the mass of the body and hence increase the strength of the gravitational field.

actually, the effects of acceleration and gravity are indistinguishable. that is the basis for general relativity.

To be precise - The Principle of Equivalence states that a uniform gravitational field is equivalent to a uniformly accelerating frame of reference. This does not mean that all gravitational fields can be replaced by an accelerating frame. When the gravitational field is not uniform (or in the language of general relativity - when the spacetime is curved) then the principle of equivalence applies locally (restrict observations to regions of spacetime where the spacetime curvature, aka tidal forces can be neglected).

Rotation doesn't create gravity. Acceleration and gravity are indistinguishable, according to GR, but not synonymous.

If one is at rest in a rotating frame of reference then according to Eintein there is a gravitational field present. As Einstein wrote in The Foundation of the General Theory of Relativity, Albert Einstein (1916), Annalen der Physik 49

It will be seen from these reflexions that in pursuing the general theory of relativity we shall be led to a theory of gravitation, since we are able to "produce" a gravitational field merely by changing the system of co-ordinates.

 

In Einstein's general relativity' date='

the effects of acceleration and the gravity is the same.[/quote']So long as one understands that the Principle of Equivalence is a local principle if the gravitational field is not uniform. For example; Is it possible for me to determine if I'm in a gravitational field which is not uniform by observing a point charge? The answer is yes. If I'm on Earth and thus in a non-uniform gravitational field and I decided to weigh the charge then the weight of the charge will depend not only on the gravitational acceleration at the location of the weight scale but the weight will also depend on the strength of the spacetime curvature. The reason for this is that the electric field is not local to a charge so this experiment is not a local one. The electric field sort of "reaches out" to feel the tidal gradients in the gravitational field which then effects the weight measurement. The weight would be different if one is in a uniformly accelerating frame in flat spacetime when the gravitational acceleration at the location of the scale has the same value as the previous experiment.

 

whilst this may seem impossible due to lack of gravity (and the fact such a formation is not natural) it is possible due to centrifugal there would appear to be gravity (although not actually).

Newton would say no and Einstein would say yes.

 

Centrifugal force doesn't exist though right? Well it is not a force, we just call inertia of an object having centripedal acceleration...I think

According to Newton it is an inertial force. According to Einstein its a gravitational force. Einstein postulates of GR imply that inertial forces are identical in nature to gravitational forces. He then concluded that since gravitational forces are "real" then so too must coriolis forces and centrifugal forces be real.

 

you do get more gravity for moving, but it is not anywhere near noticeable. kinetic energy is energy(duh!), and mass AND ENERGY both contribute to gravity.

If by this you mean that an increase in energy produces an increase in active gravitational mass then yes, that is correct.

 

they spin the space station around an axis. this is called centrifugal force.

If you're in a rotating frame of reference and there is a particle moving in your frame then it will accelerate. The force related to this acceleration is called centrifugal force. It points radially away from the axis of rotation. There is another force acting on the particle as well - the Coriolis force which is a velocity dependant force (much like the magnetic force). Each of these are inertial forces. If the particles are also acted on by the wall then the it experiences other kinds of forces. But the gravitational forces here are the centrifugal force and the Coriolis force.

 

Mind you - If you're at rest in an inertial frame and someone started spinning a space station around you then no gravitational forces will appear. The reason being that spinning the space station does not mean you've changed your own frame of reference. For gravitational forces to be produced one must change their frame of reference too. When you start to rotate with the space station then, according to general relativity, a gravitational field will be produced.

 

Pmb

[ydoaPs]

Since mass and energy are related then yes. An increase in the energy of a body will increase the mass of the body and hence increase the strength of the gravitational field.

 

and' date=' is kinetic energy not energy? i believe if you put a value close to c in the equation for kinetic energy, then you will get a large energy.(pretty close to half the energy of the object at rest)[math']E_{kinetic}=\frac{1}{2}mV^2[/math]

is it just me or would that added energy increase gravity?

[Pete]

and, is kinetic energy not energy?

Yes. Most certainly.

i believe if you put a value close to c in the equation for kinetic energy, then you will get a large energy.(pretty close to half the energy of the object at rest)[math]E_{kinetic}=\frac{1}{2}mV^2[/math]

The correct expression for the kinetic energy, K, of a particle with proper mass m₀ is

 

[math]K =(\gamma - 1)m_0 c^2[/math]

 

is it just me or would that added energy increase gravity?

Its not just you. As I mentioned above, the increase in energy (regardless of the type of energy) will result in an increase in the strength of the gravitational field. I.e. increase in energy - > increase in active gravitational mass.

 

Hence the widely used term mass-energy.

 

Pete

[The Rebel]

Circular motion to create gravity???

 

Gravity is about the seperation of two particles with mass, not the independant kinetics of one particle.

 

I can only assume you are refering to centripetal force. This is the force that keeps the particle from straying from its orbit, caused by the KE that is the direction of a tangent on the orbit circle.

 

Where I believe the confusion is, is as the KE is pulling us astary from the centre of the orbit, we would seem to weigh less if we standing on a set of scales. Hence on the equator of the earth, we would weigh less as the KE from the spin draws us outward, than in the centre where the orbit speed would be much slower.

 

Circular motion does not give gravity it merely takes up some of the gravitational force via KE.

[Pete]

Circular motion to create gravity???

According to Einstein's General Theory of Relativity (GR)' date=' yes.

Gravity is about the seperation of two particles with mass, not the independant kinetics of one particle.

In Newtonian physics, yes. In Einstein's GR, no. In GR there are no special frames of reference as their are in Newtonian physics. As such all frames of reference are legitimate and as such one needs to explain why a particle is experiencing an inertial force in any given frame. For a rotating frame the cause is that you're rotating with respect to the "distant stars" and as such the matter in the universe which is rotating around you gives rise to a gravitational field. This was demonstrated by Einstein quite early on in GR.

 

E.g. suppose you're inside a hollow massive spherically symmetric shell and the shell is very massive. If you now started to rotate the shell the matter inside would start to rotate as well (something that doesn't happen in Newtonian physics). This is called "frame dragging". If an observer inside decided to stay in your (the outside distant observer's) frame, whereby I mean that he choose a frame in which he is not rotating with respect to you, then to him there will be a measureable gravitational field. This is identical to the situation of a person choosing a frame which is rotating with respect to an inertial frame.

I can only assume you are refering to centripetal force.

No.

Circular motion does not give gravity ..

Newton would agree with you. Einstein would disagree with you. Einstein touched on this point in an essay he wrote in the February 17, 1921 issue of Nature

Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordiantes? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.

 

More on this can be found in The Meaning of Relativity, A. Einstein, pp 100-102

 

Pete

[The Rebel]

Pmb,

 

Firstly apologies I was responding to the 1st page not realising there was a 2nd.

 

I take it you are going along the route of the force is caused by inertia which in turn is caused by the mass around it not the KE it has? What would happen if the outer masses were removed, would this then leave the particle to spiral inward to the centre of the orbit.

 

The shell idea too. If an outer mass is spinning it stands to reason the inner matter will be dragged. Take the arm inside a motor as the fields in the coils fluctuate. Not quite sure what you're getting at.

 

I'm still a little unpersuaded how this is not a confusion between the effects of energy exhibited in a mass having velocity and energy generated from spin.

[Pete]

I take it you are going along the route of the force is caused by inertia which in turn is caused by the mass around it not the KE it has?

I don't understand what that means.

What would happen if the outer masses were removed' date=' would this then leave the particle to spiral inward to the centre of the orbit.[/quote']Outer mases? What ones are those? Center of what orbit?

The shell idea too. If an outer mass is spinning it stands to reason the inner matter will be dragged.

Yep. That's the idea of Mach's Principle.

Take the arm inside a motor as the fields in the coils fluctuate. Not quite sure what you're getting at.

How do you think the gravitational field is defined in Einstein's general relativity?

 

Are you familiar with the principle of equivlence? It states that a uniformly accelerating frame of reference is equivalent to a uniformly accelerating frame of reference. Therefore if you're in such a field and drop a ball you can't tell whether there is a gravitational field present or if you're in an accelerating frame of reference with respect to an inertial frame. The same idea extends to arbitrary inertial frames and to non-uniform gravitational fields.

 

Pmb