Gravity

[Bill Verburg]

Originally Posted by ckissick

Astronauts orbiting earth are weightless because they are in free-fall while orbiting earth. The spaceship is also weightless for the same reason. Earth is orbiting the sun, just as a spaceship orbits earth. Therefore, Earth is weightless. So if you can be at the exact center of Earth's mass, you are weightless. I'm ignoring other gravitational "forces" - gravity is not a force - like the pull between Earth and Moon, and the fact that orbits decay.

Is this correct? I just came up with this theory. But it makes sense to me.

Weightless is a misnomer

micro gravity is more appropriate

first weight is the pull of gravity, on or near Earth it pulls mass toward the center of Mass for the system,

in orbit that pull still is present it just presents as a constant change in motion(ie acceleration) whose vector carries it in a path around the CoM, In a car going around a corner you inertia wants to keep you and the car in alignment with the motion vector of the car a force applied by the tires to the wheels to the chassis to the seats to your body and eventually to your neck is indistinguishable from the pull of gravity except for it's direction

when falling the same applies, there is a gravitational pull, but here here the motion vector is more closely aligned w/ the acceleration vector, the less alignment the more sideways motion

In a descending elevator the force of gravity is reduced by the acceleration of the elevator, if it accelerated own at 9.8m/s2 then you would feel no weight(till you hit bottom where the neg accel hits all at once making you weigh tons for a micro sec)

similarly when going up on the elevator the acceleration of the elevator adds to the pull of gravity causing increased weight on your legs

These inertial forces are all indistinguishable from the force caused by gravity except for the mechanism and vector

[Bill Verburg]

[QUOTE=island911;11898316]Do I?

.....
Your explanation of how F_g=mg=ma (g=a) "is backwards" seems a bit non sequitur and does not flip these relationships. Not even in Newtonian physics.

A..[/QUOTE}

F_g=mg
is just a mathematical relation between 3 things
F_g is a subset of all other forces in that it specifically names the force caused by gravity, aka weight

mass is the affected mass

g is the local acceleration caused by the local gravitational field, which for a planet varies w/ height and specifics to your location there are many causes for g to vary local mass concentrations contribute as well as the inertial spin forces which lower g at the equator of the Earth to 9.76 m/s² and to 9.83 m/s² at the poles

F = ma is just the more generalized superset that describes the effect of any force on a given mass

It does not specify the source of a force only its effect on one particular thing


it could correctly be written

m = f/a or a = F/m

[island911]

Originally Posted by Bill Verburg

...

g is the local acceleration ..

Wait, you previously (scolding) said it was backwards and that I should go back to school for a refresher for calling gravity (g) an acceleration.

Nice to see you come around on that.

[island911]

Originally Posted by Bill Verburg

...
g is the local acceleration caused by the local gravitational field, which for a planet varies w/ height and specifics to your location there are many causes for g to vary local mass concentrations contribute as well as the inertial spin forces which lower g at the equator of the Earth to 9.76 m/s² and to 9.83 m/s² at the poles...

I get that people call out centrifugal force as a force, even though it is not a force. Yet it is handy to have that relative reference frame.

But what is this about inertial spin at the equator changing the gravitational (or space-time) field?

Seems an asteroid pulled into a pole or the equator would 'see' the same planetary mass-induced acceleration.

No?

[Crowbob]

I set off a maelstrom of nerdiness!

Gravity has always been described as the weak force, IIRC.

Right Bill?

[island911]

That is string theory.

Seems Bill prefers his gravity loopy.

[Bill Verburg]

Originally Posted by island911

Wait, you previously (scolding) said it was backwards and that I should go back to school for a refresher for calling gravity (g) an acceleration.

Nice to see you come around on that.

no sir

Originally Posted by island911 View Post
Gravity is not a force. Gravity is an acceleration that can produce a force. Fg=mg

Gravity, capitol g is a force

little g in Newton's second law quoted in your post is the acceleration caused by Gravity, capitol G

[flatbutt]

This is why I chose chemistry instead of physics.

[herr_oberst]

This is why I chose lithography instead of physics and chemistry.

[Bill Verburg]

Originally Posted by island911

I get that people call out centrifugal force as a force, even though it is not a force. Yet it is handy to have that relative reference frame.

But what is this about inertial spin at the equator changing the gravitational (or space-time) field?

Seems an asteroid pulled into a pole or the equator would 'see' the same planetary mass-induced acceleration.

No?

again don't confuse forces and accelerations

any change of motion can only be caused by a force,

change of motion aka acceleration can be speeding up, slowing down or changing direction

net acceleration is always in the same direction as and proportional to net force applied

net force is the summation of all the forces acting on the body

the force between the asteroid you mention and Earth is defined by this formula
F = Gm₁m₂/r²

F is the force acting to pull the 2 masses together
G is the universal gravitational constant
m₁ is the Earth's mass here or any other generic mass
m₂ is the asteroids mass here or any other generic mass
r is the distance between the gravitational centers of the 2 masses, which in the case of a homogeneous masses is also their geometric center

That formula also applies to the person standing at the N pole or at the equator, but that is not the only force being applied to a person standing on the surface of the Earth, Such a person is also having the direction of his motion continuously being changed by an additional force, in this case the friction( + less important forces like air resistance) between the person and the surface he/she is standing on. The net Force applied is the summation of all the forces being applied and again will be aligned the direction of the summation

In the case of the person on the equator there is a considerable change in the direction of motion, w/o G the person would continue to go in a straight line at ~460 m/s soon leaving the Earth on a tangent, w/o friction the person would be stationary and slam in to any any on coming vertical surface connected to the Earth at ~ 450m/s, The summation of all the forces keep the person planted moving along at ~450 m/s w/ a path the curves parallel to the Earths surface

as one approaches either pole the spin velocity of the Earth's surface asymptotically approaches 0, so the net acceleration asymptotically approaches g caused by G(capitol G, Earths force gravity) and the person's weight adjusts correspondingly

The analogy of a car going around a curve is apt, w/o a force to change direction, the car or you will go straight off the road, the tires generate a friction force(the basis of which is mostly electro-magnetic force, remember there are only 4 fundamental forces) this additional force alters the straight line inertia(aka momentum) to change the direction of motion. You as a passenger cannot sense any difference between this phantom inertial force and gravity other than the direction of action

[Steve Carlton]

Bill- are you a physicist or scientist?

[island911]

Bill, I'm not confusing forces and accelerations. Did you watch the vid I posted?

I also get all that you posted.

BTW, "spin velocity" is not the best wording as the spin at the pole is actually the same as the spin for an Ecuadorian (1rotation/day).

Of course the Ecuadorian has a curvelinear path that contains both translation and angular rotation (spin) whereas the Santa has only spin.

as one approaches either pole the spin velocity of the Earth's surface asymptotically approaches 0, so the net acceleration asymptotically approaches g caused by G(capitol G, Earths force gravity) and the person's weight adjusts correspondingly

IOW- curvelinear translation goes to zero, not the spin.

[Pazuzu]

Originally Posted by herr_oberst

This is why I chose lithography instead of physics and chemistry.

This is why I chose astrophysics instead of lithography, chemistry and physics.


I'm not going to get into the semantic mishmash about forces, but I'll go back to the original question.

There is not a "center" on the Universe, which things are expanding away from. I know that's a bit non-intuitive at first, but think of this...

Imaging a large rubber band. Maybe, 10 miles across? Now, put a few ants on the rubber band. Each can see other ants along the band, maybe they can see how far other ants are because there's pen marks al around the band. So, ants have some sense of space, and distance.

Now, you, as a malevolent god, pull on the rubber band, stretching it. EVERY ant will see EVERY other ant moving away from them. Naturally, EVERY ant will look at that and say "if every single other ant is moving away from me, I must have some special place in the Universe, the center of expansion!". However, every single ant can say that, and therefore, they do NOT have any special place, by definition. The problem comes from the fact that the rubber band is a 1 dimensional world expanding in the second dimension, and that makes it SEEM like every single ant is at the center of expansion. In fact, none are, and the rubber band Universe is far more complex than the ants think it is.


We are in a 3 dimensional Universe which is expanding in the 4th dimension. Every single spec of dust in our Universe sees everything moving away from it, and therefore makes the logical mistake that they are at the center of expansion, which is false.

So, no center of expansion of the Universe, therefore, no point at the center where gravity goes to zero simply because of being at the center.

[ckissick]

Originally Posted by stevej37

No one here knows if it is hot at mid-earth. Sorry excuse.

OK, to answer post #22, it is hot down there. I know, because I'm a geologist.

[island911]

Al Gore says it is millions of degrees.

[Crowbob]

The Big Bang Theory says the universe exploded outward in all directions from a single point, but according to Mike there is no ‘center’ of the universe so the BBT doesn’t hold up?

[Pazuzu]

Originally Posted by Crowbob

The Big Bang Theory says the universe exploded outward in all directions from a single point, but according to Mike there is no ‘center’ of the universe so the BBT doesn’t hold up?

No, the big bang theory says that we are in a 4 dimensional spacetime region, and we are seeing 3 dimensions of it. That region expanded from a quantum fluctuation in the primordial Higgs field, which crested over the Mexican hat potential barrier and then rolled down the potential well, expanding in 4 dimensions. The expansion was rapid (many times faster than the speed of light) for several seconds, which was called the Inflationary Era. Then, the total energy density of the Universe dropped enough for matter decoupling, which was the moment that the energy soup shattered into matter and photons. The photons have been traveling around ever since, and are seen as the Cosmic Background Radiation. Matter has been expanding into the 4th dimensions ever since, which is the Hubble expansion that we see with telescopes.

[techman1]

Wild thought, for brains more informed than mine:
Would the concept of gravity, and all things exhibiting it, be explained if everything was, at one time, a solid mass? Some force blew it apart, and gravity is the force being exhibited because the mass is trying to get back to the form it once had?

[Bill Verburg]

Originally Posted by island911

Bill, I'm not confusing forces and accelerations. Did you watch the vid I posted?

I also get all that you posted.

BTW, "spin velocity" is not the best wording as the spin at the pole is actually the same as the spin for an Ecuadorian (1rotation/day).

Of course the Ecuadorian has a curvelinear path that contains both translation and angular rotation (spin) whereas the Santa has only spin.

IOW- curvelinear translation goes to zero, not the spin.

angular velocity is the same at the poles and equator, instantaneous linear velocity(spin velocity, because the instantaneous v is accelerated into a curve following the arc of the earths surface) is not. At sea level at the equator, your instantaneous linear velocity is ~460 m/s on top of a mountain on the equator it is a bit faster. This is why the preferred orbital launch direction is to the east and the preferred location is a mountain top on the equator, Kilimanjaro or somewhere in the Andes , like Quito

[Crowbob]

Originally Posted by Pazuzu

No, the big bang theory says that we are in a 4 dimensional spacetime region, and we are seeing 3 dimensions of it. That region expanded from a quantum fluctuation in the primordial Higgs field, which crested over the Mexican hat potential barrier and then rolled down the potential well, expanding in 4 dimensions. The expansion was rapid (many times faster than the speed of light) for several seconds, which was called the Inflationary Era. Then, the total energy density of the Universe dropped enough for matter decoupling, which was the moment that the energy soup shattered into matter and photons. The photons have been traveling around ever since, and are seen as the Cosmic Background Radiation. Matter has been expanding into the 4th dimensions ever since, which is the Hubble expansion that we see with telescopes.

Oh yeah! I forgot about the Mexican hat potential. Things are gonna get really weird because we are definitely in another Inflation Era. It’s amazing how everything is connected!