Errata ?

[Gary Redmond]

Another way of saying that the oceans attract the Moon, is that the Moon is attracted by the oceans. It's the passive voice construction. Same thing though. Passive voice is sometimes frowned upon, but it's not illegal.

That is correct, but that is not what the encyclopedias say. They say that the attraction is "by" or "of" the Moon and Sun. As I keep saying, they have the direction of the force wrong.
The Physics book is correct; 'The direction of the force is somewhat hidden in the word "attract" '.
The English language seems to cause some major problems, and this is no exception.
A mass or particle has the property of gravity. Gravity is a force of attraction only. Mass attracts itself to other mass. A mass is not attracted by another mass. If a mass seems to be gravitationally attracted to another mass it is a relative motion towards the body of most inertial resistance to motion.
Thus, the oceans appear to be attracted "to" or "by" the Moon and Sun, but the Moon and Sun are like immoveable objects to which the oceans attract themselves.

[kilopi]

Another way of saying that the oceans attract the Moon, is that the Moon is attracted by the oceans. It's the passive voice construction. Same thing though. Passive voice is sometimes frowned upon, but it's not illegal.

That is correct, but that is not what the encyclopedias say. They say that the attraction is "by" or "of" the Moon and Sun. As I keep saying, they have the direction of the force wrong.
The Physics book is correct; 'The direction of the force is somewhat hidden in the word "attract" '.
The English language seems to cause some major problems, and this is no exception.
A mass or particle has the property of gravity. Gravity is a force of attraction only. Mass attracts itself to other mass. A mass is not attracted by another mass.

I thought you just said that "the Moon is attracted by the oceans" is a correct way of phrasing it?

[Gary Redmond]

I thought you just said that "the Moon is attracted by the oceans" is a correct way of phrasing it?

I'm sorry, and must apologize. ops:
I quoted you as you have, and unfortunately I either didn't read thoroughly or misunderstood your comment, then I answered much too quickly.
Your statement "the Moon is attracted by the oceans" is not correct. Yes, it is the passive voice construction. It appears that gravity maybe one place where the passive voice construction is illegal.
Newton's law of gravitation says that "any two bodies attract each other". A body is not attracted by the other body; it attracts itself to the other body. We know that mass is passive from the standpoint of Newton's first law, it either just sits there or keeps moving. To change its speed or direction it must be acted on by a force. When the force is gravity the force comes from within, not from the other body.
So to use the term "attracted by" reverses the direction of the force and makes the statement incorrect.

[kilopi]

Your statement "the Moon is attracted by the oceans" is not correct. Yes, it is the passive voice construction. It appears that gravity maybe one place where the passive voice construction is illegal.

Interesting take on that. You'd also object to "the oceans are attracted by the moon," right? It seems you have a problem with the language--not the science.

A body is not attracted by the other body; it attracts itself to the other body.

Now that's a misuse of language! The other body has to be involved somehow.

[Gary Redmond]

It seems you have a problem with the language--not the science.

At this point yes, you are absolutely correct.
If anything exists, occupies space and has weight, it is composed of mass, mass emits or exudes gravity continuously. Therefore gravitational mass is never passive, it always "attracts" it is not "attracted". Perhaps the inertial mass is attracted.

A body is not attracted by the other body; it attracts itself to the other body.

Now that's a misuse of language! The other body has to be involved somehow.

Perhaps the inertial mass is involved.
If you have a ship tied to a dock with a long line and then cause the ship’s capstan to start reeling in the line, the ship is "attracting itself" to the dock, it is "self attracted" "it attracts" the dock. Just because the dock is there, the ship is not "attracted by" the dock. Yes the dock is there but it has no force it can‘t change the speed or direction.

The mass of the ocean exudes gravity it has the force. That force "attracts" the Moon, the oceans "attract themselves" to the Moon, the Moon is “attracted by the oceans“.

The oceans cause the tides, the oceans "attract" themselves to the Sun and Moon.
The Sun and Moon do not cause the tides. Yes, it is true there would be no tide if the Sun and Moon weren't there, but the oceans are the cause.

Once again I say the encyclopedias are technically WRONG.

[swansont]

The oceans cause the tides, the oceans "attract" themselves to the Sun and Moon.
The Sun and Moon do not cause the tides. Yes, it is true there would be no tide if the Sun and Moon weren't there, but the oceans are the cause.

I think you're torturing the physics a bit, here. Objects are accelerated because of forces exerted on them, not forces exerted by them. Bootstrapping doesn't happen.

If the tides wouldn't exist in the absence of the sun and moon, then they're involved somehow.

The oceans aren't the only thing to experience tides.

[Cougar]

The oceans aren't the only thing to experience tides.

Correct!

Tree stem diameters fluctuate with tides.

This was the title of a bit of scientific correspondence from Zurcher and Cantiani in the April 16, 1998 issue of Nature. They said "The diameter of tree stems... undergoes rhythmic fluctuations. We find a strong correlation between these fluctuations and the timing and strength of tides. This correlation suggests that the Moon is influencing the flow of water between different parts of the trees."

Of course, they must mean "the Moon and the Sun..." It's also interesting to note that the sun-moon tidal effect on any spot on the surface of the earth is different than on any other spot (at that point in time.)

[Gary Redmond]

I think you're torturing the physics a bit, here. Objects are accelerated because of forces exerted on them,

Both those parts of your statement are true.

not forces exerted by them. Bootstrapping doesn't happen.

That part I can't agree with. An apple falls to and lies on the ground, is it because of the apple or the ground?
Does the apple attract or does the ground attract, actually it is both, correct? The apple and the ground both have the property of gravity, they both attract.

The oceans aren't the only thing to experience tides.

Correct, even the accelerator at CERN has a problem with it. Einstein said: "In gravitational fields there are no such things as rigid bodies with Euclidean properties; thus the fictitious rigid body of reference is of no avail in the general theory of relativity."

As I said the encyclopedias are WRONG, when they say the "Moon and Sun" or the "Sun and Moon" attract the oceans. The gravitational attraction between two bodies is mutual, not one way or the other.
What that does, is bring up the question of degree. Is one or the other more responsible?
I say yes!
I say the oceans are the most responsible for their own motion. Earlier I showed the Earth's mass to be over 98.78 % responsible for the total force holding the moon in orbit.
Gravity diminishes with distance, and with quantity of mass, because the distance factor in the formula squares, the closer to the source the stronger the force, the greater the effect. Does the ocean move the stones on the Moon, or distort a solar flare I don't think so. Does the ocean attract itself to those two bodies and distort its own shape, yes I think it does. The ocean is very close to itself, its internal force is very strong compared to the external forces of the Sun and Moon.

Any mass in the universe attracts or pulls itself toward every other mass in the universe. The oceans do bootstrap. It is their bootstrapping to the rest of the planet which keeps them within their shores.

[kilopi]

The gravitational attraction between two bodies is mutual, not one way or the other.

When you say the attraction is mutual, by that do you mean that the moon attracts the oceans, and the oceans attract the moon?

[Gary Redmond]

When you say the attraction is mutual, by that do you mean that the moon attracts the oceans, and the oceans attract the moon?

I believe so, unless you have some better way of saying it.
You know the law it says any two bodies attract each other.

[kilopi]

When you say the attraction is mutual, by that do you mean that the moon attracts the oceans, and the oceans attract the moon?

I believe so, unless you have some better way of saying it.
You know the law it says any two bodies attract each other.

So, the moon attracts the oceans then.

But in the sentence just before that you wrote: " As I said the encyclopedias are WRONG, when they say the 'Moon and Sun' or the 'Sun and Moon' attract the oceans." That seems to be a contradiction on your part. I'm not sure why you say they're wrong then.

[Cougar]

If you have two bodies, A and B, the total effect of (newtonian) gravity on B is determined solely by the mass of A and the distance between A and B (that is, the distance between the center of mass of A and the center of mass of B). B's mass does not enter into the equation (or more precisely, it cancels out of the equation)

No, that's definitely not true. It's only an approximation, even under Newtonian mechanics, when B is much smaller than A. The more massive B is, the more it is not true. JS Princeton and I had a go around on that a few months ago.

Well, I feel somewhat vindicated by this obscure book I found entitled Bad Astronomy, Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax" by the Bad Astronomer himself, Philip Plait, wherein Dr. Plait says in plain English....

"The force of gravity, the amount that it pulls on an object, depends on only two things: the mass of the object doing the pulling, and how far away it is."

What can I say? Am I right or am I right?

[Cougar]

By the way, that quote was from the 5th paragraph of Chapter 7 - "The Gravity of the Situation: The Moon and the Tides."

[kilopi]

"The force of gravity, the amount that it pulls on an object, depends on only two things: the mass of the object doing the pulling, and how far away it is."

What can I say? Am I right or am I right?

Looks like you're both wrong (good thing this is the Errata? thread). Even under Newtonian gravity, the force is equal to GMm/r^2, so it depends on the mass of the two objects, and how far apart they are. So, it's close.

[Changed "away" to "apart"]

[Cougar]

"The force of gravity, the amount that it pulls on an object, depends on only two things: the mass of the object doing the pulling, and how far away it is."

Even under Newtonian gravity, the force is equal to GMm/r^2, so it depends on the mass of the two objects, and how far apart they are.

Well, I think the difference is just in identifying which force is being discussed. Phil clarifies "The force of gravity, the amount that it pulls on an object..." so he's talking about either the force of M on m or the force of m on M, but not the combined, total force involved.

To conclude, if body A has mass M and body B has mass m, then the force of A's gravity on B is GMm/r^2 divided by m.

[kilopi]

To conclude, if body A has mass M and body B has mass m, then the force of A's gravity on B is GMm/r^2 divided by m.

Eh?

[Eroica]

To conclude, if body A has mass M and body B has mass m, then the force of A's gravity on B is GMm/r^2 divided by m.

No, no, no, no! I protest. I cannot allow that. In Newtonian mechanics, where mass is constant, "GMm/r^2 divided by m" is the acceleration which A imparts to B, not the gravitational force it exerts.

kilopi is right. The force is GMm/r^2.

[Cougar]

To be a bit more clear, I should say (GMm/r^2) all divided by m.

"GMm/r^2 divided by m" is the acceleration which A imparts to B, not the gravitational force it exerts.

kilopi is right. The force is GMm/r^2.

So that means the force that body A imparts changes drastically if there is a small satellite orbiting it as opposed to a large moon, even though the mass of A stays the same?

[Eroica]

So that means the force that body A imparts changes drastically if there is a small satellite orbiting it as opposed to a large moon, even though the mass of A stays the same?

Yes. The force changes, but the acceleration is the same.

Remember Galileo dropping balls of various masses from the top of the Leaning Tower of Pisa (well, let's pretend he did)? They took the same time to reach the ground because the acceleration due to the Earth's gravity was the same for all of them. But the force was different, depending on the mass of each ball.

[Celestial Mechanic]

[Snip!]Remember Galileo dropping balls of various masses from the top of the Leaning Tower of Pisa (well, let's pretend he did)?[Snip!]

Actually, his graduate students did. 8)

[Cougar]

Remember Galileo dropping balls of various masses from the top of the Leaning Tower of Pisa?

Like it was yesterday.

They took the same time to reach the ground because the acceleration due to the Earth's gravity was the same for all of them.

No argument there.

But the force was different, depending on the mass of each ball.

So the gravitational mass of the earth reaches out and thinks, "Whoa, now here's a ball with twice as much mass as this other ball, so I've got to apply twice as much force - but just on it - so as to counteract its doubled inertial mass so that it and anything else this Galileo chap wants to throw at me will accelerate at the exact same rate." Am I the only one that this strikes as rather odd? Is this one of the reasons that Einstein discarded the idea of gravitational force altogether?

[russ_watters]

Am I the only one that this strikes as rather odd? Is this one of the reasons that Einstein discarded the idea of gravitational force altogether?

What is odd about it? I weigh half as much as someone with twice as much mass. Maybe I'm not understanding your objection though.

Well, I think the difference is just in identifying which force is being discussed. Phil clarifies "The force of gravity, the amount that it pulls on an object..." so he's talking about either the force of M on m or the force of m on M, but not the combined, total force involved.

?? Are you saying there are two force? The force of me pushing down on the earth and the force of the earth pulling me down? Of course the ground could be said to be pushing me up while the earth is pulling me down... But these are all just different ways of describing the same force.

In physics class, we did a little demonstration with the biggest guy and littlest girl on skateborads about 10' apart with a rope between them. Both pulled as hard as they could at the same time. Who pulled harder?

They pulled with the same force. See, force occurs between two objects. There is only one force, not two. The force of you pushing down on the ground is the same as the force of the ground pushing up on you.

[Eroica]

So the gravitational mass of the earth reaches out and thinks, "Whoa, now here's a ball with twice as much mass as this other ball, so I've got to apply twice as much force - but just on it - so as to counteract its doubled inertial mass so that it and anything else this Galileo chap wants to throw at me will accelerate at the exact same rate." Am I the only one that this strikes as rather odd? Is this one of the reasons that Einstein discarded the idea of gravitational force altogether?

You took the words right out of my mouth. I was just going to make that point about Einstein. I think you're absolutely right. And I think Einstein was right too. The Earth doesn't really do anything to the balls: it merely distorts spacetime, and the amount it distorts it by is completely unrelated to the mass of the balls. It's the distorted spacetime which then accelerates the balls.

If you retain the old Galilean-Newtonian sense of force, I don't think there's any way of avoiding your paradox (and I accept that it is a paradox). I wonder did anyone ever bring this objection to Newton's attention. And if they did, what was his response?

[Cougar]

What is odd about it?

That the strength of the gravitational attraction of the earth changes according to what it is attracting!

Are you saying there are two forces? The force of me pushing down on the earth and the force of the earth pulling me down?

Not those two forces, but yes, two forces: the force of the earth pulling me down and the force of my mass pulling UP on the earth. Each body attracts the other with a particular force. I mean, do I at least have that right? :-?

[kilopi]

Not those two forces, but yes, two forces: the force of the earth pulling me down and the force of my mass pulling UP on the earth. Each body attracts the other with a particular force. I mean, do I at least have that right?

russ_watters point is that they have the same value--irrespective of any relativity "paradox"

[Cougar]

Thank you, Eroica! Yes, Einstein and I are tight. :roll:

So are you associated with this outfit, by any chance?


My first album

[Cougar]

russ_watters point is that they have the same value--irrespective of any relativity "paradox"

Wow, I would have thought that the gravitational attraction of the earth on me would be considerably more than the attraction that my meager mass has on the earth!

[Eroica]

So are you associated with this outfit, by any chance?

No. Just a longtime fan of Beethoven.

[Musashi]

I think the attractions are the same. But since the Earth has a much greater mass than you, you are affected by that attraction more.

[informant]

What is odd about it?

That the strength of the gravitational attraction of the earth changes according to what it is attracting!

I think the phrase "gravitational attraction of the earth" can be misleading in this context. To have gravitational attraction, you need two bodies; the intensity of the force is a property of them both. It's the manifestation of a simultaneous interaction between Earth and the satellite. It takes two to tango, if you will.

In classical mechanics, forces are assumed to act instantaneously. So, gravity does not "start" at the centre of Earth, and then "travel" outwards to the satellite. It "starts" in both at the same time. At least, this is my layman's understanding of Newton's theory.

I seem to recall, however, a concept in the classical theory of gravitation which only depends on the mass of the "source": g, gravitational acceleration, which is related to the gravitational field. Place a mass in space, and it will create a field around itself (in fact, everywhere in space). But g is an acceleration, not a force!

Edited to correct, and add: At least it seems to me that you are confusing the force of gravity with the gravitational acceleration. See also this and this.