Silas
Quote:
================================================== ======================
------------------------------------------------------------------------
The center of mass the black "+" is one thing, and
the center of gravity the red "x" is another.
------------------------------------------------------------------------
Unfortunately, it's wrong. An ellipsoid does not have a "center of [the
pull of] gravity." Yes, certainly, it has a center of mass. (The black
'+') But it does not have a fixed point that acts as if the body's mass
were concentrated there.
================================================== ======================

Silas please,

You are not hearing me. The red "x" center of gravity is only at that spot for the instant that the Moon is above that spot.
The center of gravity moves at the speed of light, or the speed of gravity if that speed is different from the speed of light.
We don't care where it is as long as we know that all the forces act as if they pass through a single point. That is why I went out and screwed the tire to the fence-post. The shape and distribution of mass change the intensity and location of the center of gravity. But there is the one per customer rule. Every body in the universe has as many centers of gravity as there are bodies in the universe, but only one per customer so to speak.


Quote:
------------------------------------------------------------------------
Spheroids do not. The equatorial bulge has its own properties, and the
attraction from one side does not "cancel out" the attraction from the
other side.
------------------------------------------------------------------------
When I screwed the old tire to the post I was thinking bulge. I could have just as well have turned it 90 degrees and put it in the center of the post. One mass one center of gravity, the distribution of mass just changes the strength and location of the center.
I hope you remember my following statement.
"As the Earth rotates under the Moon, and as the Moon falls around the Earth the intensity of the field strength may very and their centers of mass/momentum may bounce up and down and jiggle around."
Normally my bounce and jiggle is called perturbation.

What you just tried to say in a very complex way with spheroids and bulges, is the exact same thing I said with the bounce and jiggle.

What I'm saying is that at any instant there is one and only one center of gravity in the Earth caused by the Moon.
Furthermore during that instant all the weight of the Earth as the Moon sees it, is balanced equally at that point therefore there is no torque. Yes, the density and shape of the Earth causes more, or less gravity, something like the dimples on a golf ball, but the direction is always down (so to speak). Einstein spoke to this, and explained how a gravitational field warped space. As the Moon goes around the Earth it moves in, around, up, and down those gravitational dimples, speeding and slowing accordingly. But, there is no torque. The Moon can not and does not torque the Earth. Yes, the Earth perturbs the Moon the Moon perturbs the Earth. These motions being internal to a system come in equal and opposite pairs, the net result is zero.


Gary

We hear you, but you don't seem to hear us. We disagree and backup our arguments with known and proven science and mathematics. Silas is right. A non-spherical object does not act (gravitationally) as if its entire mass is concentrated at the center of mass. Are you aware of how the gravitational constant ("G") is determined? Two masses are attached to a stick (like weights on a barbell). This is then suspended at its center of mass by a very fine wire. A large mass is brought close to first one mass then the other and the angular deflection is measured. From the known torsional strength of the wire, the "torque" is calculated. From this the value of "G" can be derived (to several decimal places). This would not work if the "barbell" acted (gravitationally) as if its mass was concetrated at the center of mass.

_________________
When all is said and done - sit down and shut up!

<font size=-1>[ This Message was edited by: Kaptain K on 2002-07-01 11:24 ]</font>

Kaptian K

My English must be terrible you did not understand me either.

The center of gravity is seldom if ever in the center of a mass.

When you say "This would not work if the "barbell" acted (gravitationally) as if its mass was concentrated at the center of mass."
You are absolutely correct. What I'm saying is the barbell only has one center of gravity if you only bring one large mass near it. If you bring two large masses near it then it has two centers of gravity.

It has mostly to do with distance. From the large mass to the far end of the barbell is perhaps 1000 times as far, as to the near end. Therefore the single center of gravity is only displaced a microscopic amount from the center of the near end's center of mass.
Let me quote the physics book.

Real bodies consist of large numbers of particles. If the body is near the earth, (another body) the attractive forces will be nearly parallel. The line of action of this resultant force, or weight, always passes through a single point in the body, a point called the center of gravity of the body.

The Earth gives the Moon one center of gravity.
The Moon gives the Earth one center of gravity.

The location and magnitude are dependent on the distance between, and the distribution of particles in, the masses.

As I repeatedly have said those centers of gravity are free to move and they do so at the speed of light or gravity if that speed is different.


Gary

Geo3gh started this thread on 3/12. Gary hi-jacked it 6 days later and has spent the last three and a half months telling us that not only we but every astronomer, physicist and mathematician of the last 400 years is wrong and only he is right. I give up. I shall take my own advice and sit down and shut up. I have nothing more to say here.

<font size=-1>[ This Message was edited by: Kaptain K on 2002-07-02 07:53 ]</font>

I gave up hope -- twice -- but I returned, largely because I know the answers to *some* of the questions being posed here.

Also, Gary, bless him, is being polite. If he'd descended into some of the name-calling we've seen from other people in other threads, that would be a problem, but the worst he's ever expressed is frustration.

I'll keep trying as long as our good host doth permit.

Gary: no hard feelings, okay?

Silas

Gary,
Take 2 objects (A and B) and draw a line between their centers of mass.

If the center of gravity on A in relation with B is somewhere on the line, there should be no torque.

But if the center of gravity on A is not on the line, there should be a torque.

Does this make sense?
--Tommy

Thanks Silas.


Kaptian K,

I'm sorry you feel that way.

As you may recall I was polite and asked Jeff if I could continue. This is a free country, and you don't have to just "sit down and shut up", you can also get up and leave. But please don't! I truly believe that I'm correct and without help (both positive and negative) the world will never know the truth.

As near as I can figure out the "Tidal Forces" hypothesis was conceived to explain the "Leap Second" and the "Regression of Nodes".

Isn't that correct?

If you think that the "Tidal Forces hypothesis" is correct and that it answers all the questions, then that is your choice. As I said this is a free country.

I say the "Tidal Forces" explanation is wrong.
I say that the source of the torque is never given.
I continue to try to get you to be a good scientist and question it as well.
I don't feel that it is me that is on trial.
I feel it is the astronomy books on trial, the "Tidal Forces hypothesis" specifically.

In my last post, I went and got out the physics book and explained to you how gravity works. Those laws are not something I just conjure up. Those laws were written by the physicists and mathematicians that you say I disagree with. I do not disagree with the physicists and mathematicians.

I have stated over and over, that it is the astronomy books, which are wrong, the "Tidal Forces" hypothesis portion of it, in particular.

It appears to me, that the things I said were starting to make sense, and you got frightened by the truth.

If you live with a lie all your life, the truth is scary.

The truth is:
The Earth and Moon fall around each other.
They do so because they are gravitationally bound together.
The gravitational field of each body, gives weight to the other one.
The physics book says this:
"The line of action of this resultant force, or weight, always passes through a single point in the body, a point called the center of gravity of the body."

That physics book goes on to explain how to determine or find that center of gravity. The methods given involve finding the one place within the body where all the weight is perfectly balanced and there is no torque. I went out and screwed a tire to a fence post. I then determined the center of gravity. If we rotate that junk around that point it will continue to rotate until stopped by friction not gravity. It can be stopped and it will not restart because of gravity. Gravity has no torque unless a secondary offset force is applied.

I'll admit that I take the liberty to assume the center of gravity is free to move at the speed of light or gravity if that speed is different. No one seems to object to that.

Thus the Earth and Moon are held together and fall around the points within each other, which cannot be torqued by the force that ties them together.

If you say I can't prove that you are correct.
If you say the Moon torques the Earth and slows it down all I ask is that you prove it. Show me how it can happen. DoctorDon used the term "R cross F", but he never came up with the "R". I have tried for 12 years to figure it out I can't do it. I just showed you that gravity in and of itself can't do it. There would need to be a second force. Show me the second force.

If the "Tidal Forces" explanation is wrong, then as Einstein said "what is the alternative"?


Gary

traztx
Quote:
----------------------------------------------------------------------
Gary,
Take 2 objects (A and B) and draw a line between their centers of mass.
If the center of gravity on A in relation with B is somewhere on the
line, there should be no torque.
But if the center of gravity on A is not on the line, there should be a
torque.
Does this make sense?
--Tommy
--------------------------------------------------------------------
That would make perfect sense if the centers of mass were a point of suspension. This is where the problem is. The Earth and Moon are not suspended from their centers of mass. They are suspended from their centers of gravity.

The Earth and Moon are trying to fly off in straight lines according to Newton's first law, but they can not they are suspended or retarded from doing so because of gravity. The point of suspension is the point around which there is no torque.

Gary

Yes.

Yes.

Nope. Once again, you're using a technical term improperly.

"Weight" is a force I feel when standing on the earth (or moon.) Gravity pulls me downward (and, yes, to a very minor degree, I pull the earth upward) and this attraction is opposed by the interceding matter (such as the chair I'm sitting on.)

The earth and the moon, in orbits, don't have any "weight" for us to speak of. They do have mass, which is quite different.

The earth, not being a sphere, has parts that pull more strongly on the moon, and other parts that pull less strongly. These forces, being unequal and unbalanced, apply torque to the moon's orbit.

I thought we'd covered this adequately back in the example of the plank on the ice...

You don't have to prove it... A really good start would just be if you could describe it using standard terminology and with sufficient clarity as not to leave us baffled and perplexed.

The "other force" is the gravitational attraction from the earth's equatorial bulge.

You keep saying that you can factor this attraction into the overall attraction of the earth; that is simply not correct. You can't.

Do you have a computer that is capable of executing BASIC programs? Or do you have another favored programming language? All we need is something that can cope with sines and cosines. In two dimensions, and with various simplifying assumptions, the math is wholly tractable.

Full three-d orbital mechanics are fun. They're also tough. But until you catch on to the basics of ordinary mechanics, we're going to have problems...

Get your abacus handy!

Silas

Just a note on what is meant by "R cross F." A longer version of the statement is "The torque vector (tau) is equal to the cross product of the radius vector (R) and the force vector (F)."

For every point on the Earth, you measure its distance from the Earth's center of gravity. That's R for that point. Then you measure the force of the Moon's gravity on that point. That's F for that point.

To find the cross product, you do two steps. The magnitude of the resulting vector (in this case, tau) is F*R*(sin A) where A is the angle between R and F. You find the direction of tau by using the "Right Hand Rule." (If we put our fingers in the direction of R, and curl them to the direction of F, then the thumb points in the direction of the tau vector.)

Now, you do this for each point, then add up all the taus. For a perfect sphere, this will add up to zero. There is no torque in this case. For anything else, this will add up to something non-zero.

Again, this is how my Physics book, not my Astronomy book, does it. If the Astronomy books are "on trial" so are the physics books.

__________________
Jeff Schwarz
__________________________________________________
Argh!! They booby-trapped their sun!!****--Invader ZIM

I don't think so. I'm looking right now for info to back me up. Newton himself was looking at the tidal system, and I don't think that leap seconds were on his mind.

__________________
Jeff Schwarz
__________________________________________________
Argh!! They booby-trapped their sun!!****--Invader ZIM

I believe somewhere in this thread GR acknowledges that a mountain will cause a plumb bob to diverge a small amount from vertical. The effect is caused by the fact that a massive object perched on the surface of a spherical body causes a local variation in the gravitational field. At a greater distance, the effect diminishes. At a very great distance, the gravitational effect of the mountain is smoothed out to the point that it would be undetectable. A "lumpy" spherical body has a lumpy gravitational field. At great enough distances, this lumpiness is smoothed out.

I hope there's no argument about the moon's gravity raising a bulge in the Earth's oceans and in the body of the planet itself. Taking just the oceanic tidal bulge, you have a mass of water elevated to an average height of meters; it is a very substantial body of water. It represents a very substantial 'lump' in the earth's gravitational field...a low, immensely broad mountain. Because of the earth's rotation, this bulge leads the centerline of the moon by a small amount.

As with the plumb bob, this mass of water produces a distortion in the earth's gravitational field. It has not been utterly smoothed away at the distance of the moon. It acts slightly off the line of centers of the earth/moon system. A very small component of this force acts along the moon's velocity vector. The moon is accelerated in that direction. As it is an orbiting body, this acceleration appears as a steadily widening spiral, the rate of widening decreasing with time.

If we replaced the earth and its tidal bulge with a stick with two heavy objects on either end and set this rotating past a third object farther away, what would happen?

There would be a net attraction between the third object ("moon") and the center of the two masses on a stick ("earth + bulge"). But the masses are separated rather widely. As the stick spins, a mass approaches the moon and the effect of its gravitational field begins to increase (inverse square). An off-line force vector begins to develop. As the lump overtakes the moon, it would seem that it would retard it, then, as it swept by, it would tend to speed it up. If the geometry were such that the passage of the lumps was very close to the moon, the effects would be very pronounced.

Slow the spinning down to keep one lump leading the line of centers by a small angle. The slight gravitational pull off-line tends to accelerate the moon.

Momentum is transferred. The moon swings wider, the earth slows.

To the point. Do you agree that mountains can deflect plumb bobs? If you do, your theory is in trouble, I think.

To the real astronomers, I hope I haven't misinterpreted anything too badly.

<font size=-1>[ This Message was edited by: roidspop on 2002-07-02 20:36 ]</font>

Silas
Quote:
================================================== ================
--------------------------------------------------------------------
The gravitational field of each body, gives weight to the other one.
--------------------------------------------------------------------
Nope. Once again, you're using a technical term improperly.
"Weight" is a force I feel when standing on the earth (or moon.) Gravity pulls me downward (and, yes, to a very minor degree, I pull the earth upward) and this attraction is opposed by the interceding matter (such as the chair I'm sitting on.)
The earth and the moon, in orbits, don't have any "weight" for us to
speak of. They do have mass, which is quite different.
================================================== ================

If I have a bowling ball, it has a gravitational field of its own. But the Earth gives it weight.

The Moon has a gravitational field of its own, but the Earth gives it weight.
The Earth has a gravitational field of its own, but the Moon gives it weight.

Those two weights are counter balanced by the inertia that is trying to cause them to fly off in a straight line.
The effect is the go around and around. The force or weight caused by each one on the other one still follows the same rules and works through the single points called centers of gravity.


Quote:
-----------------------------------------------------------------------
The earth, not being a sphere, has parts that pull more strongly on the
moon, and other parts that pull less strongly. These forces, being
unequal and unbalanced, apply torque to the moon's orbit.
-----------------------------------------------------------------------
There is no torque, let me clarify that by saying there is no net gain or loss due to torque. The force is always down. Yes, the magnitude or strength may very as the Moon passes over, that is the bounce and jiggle, I talked about. If there were a gain or loss, it would mean the conservation of momentum was in trouble.


Quote:
-----------------------------------------------------------------------
I thought we'd covered this adequately back in the example of the plank
on the ice...
-----------------------------------------------------------------------
And I thought I had proven that, that example was impulse and momentum and had nothing to do with gravity. Did you ever put the meter stick on the floor as I suggested? If not, you really should.


Quote:
-----------------------------------------------------------------------
You don't have to prove it... A really good start would just be if you
could describe it using standard terminology and with sufficient clarity as not to leave us baffled and perplexed.
-----------------------------------------------------------------------
This really makes me feel bad. Every time I reply here I run my response through my grammar checker. Seldom does it say it would take more than an eighth grade education to understand what I say. Yes, I read the Scientific American a few months back when it talked about how you can tell a crackpot. So I can understand how I could be considered such.


Quote:
------------------------------------------------------------------------
If you say the Moon torques the Earth and slows it down all I ask is
that you prove it. Show me how it can happen. DoctorDon used the term "R cross F", but he never came up with the "R". I have tried for 12 years to figure it out I can't do it. I just showed you that gravity in and of itself can't do it. There would need to be a second force. Show me the second force.
-----------------------------------------------------------------------
The "other force" is the gravitational attraction from the earth's
equatorial bulge.
-----------------------------------------------------------------------

Silas,
The Earth spins on its axis once a day the Moon goes up and down that bulge twice almost every day. First it speeds up then it slows down. Up and down, up and down, day in day out. The speed up is the same as the slow down. I suppose I could use some technical gobbledygook (jargon) to explain that, but why.


Gary

<font size=-1>[ This Message was edited by: Gary Redmond on 2002-07-03 00:46 ]</font>

"Weight" is resisted acceleration. To use the terms in the standard physics way, you shouldn't use "force" and "weight" interchangeably. It just leads to confusion. If the Moon were sitting on the Earth, it would be reasonable to talk about its weight. But in orbit, not.

Take a mental snapshot; a frozen moment in time. Add up all the instantaneous torques. Since the Earth is not a sphere, they don't all cancel out. Most of them do. But there is a small amount left.

I don't have a quibble with your grammar or spelling. But you are trying to use specific physics terms not as they are defined in physics. You are rather using their common, non-scientific counterparts. You are using your machinist's definition of "torque" not the physics definition. You are using "man on the street" concepts of weight vs. force.

Your grammar checker will not balk at these errors, but we will. You also shouldn't rely on Grolier's or Webster. Use the glossary in the back of your physics book. Generally there will be one and only one term that fits what you are trying to say.

But the tides are always in the same place in relation to the Moon. As the Earth spins through the tidal bulge, it is constantly feeling the tug of torque. It's a constant braking maneuver.

__________________
Jeff Schwarz
__________________________________________________
Argh!! They booby-trapped their sun!!****--Invader ZIM

By the way, I'm going on vaction for a week, so don't take my absence in the discussion the wrong way. I'm looking forward to seeing where we are when I return next week.

__________________
Jeff Schwarz
__________________________________________________
Argh!! They booby-trapped their sun!!****--Invader ZIM

The "weight" of a bowling ball exists only when it is prevented from falling freely.

The moon is falling freely.

It has no weight.

The earth is falling freely.

It has no weight.

A 40 lb rock is placed on a scale sitting on the earth. According to the scale, a force of 40 pounds is required to prevent the rock from falling. The force exists between the two objects. From the point of view of the rock, the earth weighs 40 lbs. Or, from the point of view of the earth, the rock weighs 40 lbs. Drop the rock and the scale...as far as the scale is concerned, the rock has no weight, nor does the earth.