# Tides question

Hi all! I was just wondering if anyone can explain me why the tides (created by moon effect) are created on opposite sides.

(image is on attachment)

Thank you.

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sylas
Hi all! I was just wondering if anyone can explain me why the tides (created by moon effect) are created on opposite sides.

(image is on attachment)

Thank you.
In simple terms... the bit closest to the moon gets a bit more pull than the rest of the Earth, and so there is a hump towards the Moon.

The bit furthest away from the moon gets a bit less pull than the rest of the Earth, and so there is a hump away from the Moon.

Cheers -- Sylas

gravity and centrifugal force

russ_watters
Mentor
....centrifugal force
Since that is an insignificantly small fraction of what is going on (not sure how small, but certainly less than 1%), it is best not to bring it up when someone is looking for the broad overview of tides.

chmate, I tend to word it slightly differently than cylas: The hump nearest the moon is caused by that ocean being pulled away from the earth. The hump furthest from the moon is caused by the earth being pulled away from that ocean.

D H
Staff Emeritus
gravity and centrifugal force
What centrifugal force? In what rotating frame? Centrifugal forces only arise in rotating reference frames. You should be able to explain this phenomenon from any reference frame, and there is no centrifugal force in a non-rotating frame. Some phenomena (e.g., hurricanes) are much easier to explain in terms of a rotating frame than a non-rotating frame. This is not one of them.

IMO, the easiest frame to use as a basis for this explanation is an Earth-centered inertial frame. This is an accelerating frame, so a fictitious inertial force arises from the gravitational acceleration of the Earth toward the Moon. The apparent gravitational force exerted by the Moon at some point on the surface of the Earth is the gravitational acceleration toward the Moon at this point less the acceleration of the Earth as a whole toward the Moon. This apparent force is
• Directed toward the Moon (and away from the Earth) on the side of the Earth facing the Moon,
• Directed away from the Moon (and away from the Earth) on the side of the Earth opposite the Moon, with nearly the same magnitude as the apparent force at the sub-Moon point, and
• Directed toward the Earth (but at half strength compared to the sub-Moon point) at the points where the Moon is on the horizon.

The result: Bulges. Looking at just the sub-Moon and anti-Moon points omits what is happening elsewhere. The squeeze in the middle is just as important as those outward forces at the nodes.

gravity attracts the bodies toward one another and centrifugal force pushes them away from their center of mass (around which they orbit). there is an excess gravitational force on the moon side of the earth and an excess centrifugal force on the opposite side of the earth.

at the center of the earth the 2 forces are balanced.

D H
Staff Emeritus
gravity attracts the bodies toward one another and centrifugal force pushes them away from their center of mass (around which they orbit). there is an excess gravitational force on the moon side of the earth and an excess centrifugal force on the opposite side of the earth.

at the center of the earth the 2 forces are balanced.
In what rotating frame? Whenever you invoke centrifugal force you need to specify what your frame of reference is. There are no centrifugal forces in a non-rotating frame.

gravity attracts the bodies toward one another and centrifugal force pushes them away from their center of mass (around which they orbit/rotate).

DaveC426913
Gold Member
The hump nearest the moon is caused by that ocean being pulled away from the earth. The hump furthest from the moon is caused by the earth being pulled away from that ocean.
This is the explanation I have found to be the most clarifying.

D H
Staff Emeritus
gravity attracts the bodies toward one another and centrifugal force pushes them away from their center of mass (around which they orbit/rotate).
The Earth and Moon don't orbit in the rotating frame used to explain tidal effects from the perspective of centrifugal force. They are stationary in such a frame.

DaveC426913
Gold Member
The Earth and Moon don't orbit in the rotating frame used to explain tidal effects from the perspective of centrifugal force. They are stationary in such a frame.
I dunno if you're beating him up excessively.

Inasmuch as centrifugal force can explain to the layperson why water stays in a swung bucket on a string, surely this is also an acceptable answer for the tides.

Not that I think it's the best explanation...

D H
Staff Emeritus
If you do the math properly, the Earth's ocean tides can be explained from the perspective of a Mars-centered, Mars-fixed reference frame. Just because it can be done doesn't mean it is the best choice. Explaining the tides from the perspective of a rotating frame is not the best (i.e., easiest) choice, either. If you pick a frame rotating with the Earth and Moon and with origin at the Earth-Moon barycenter, the sub-Moon point is a lot closer to the origin than is its antipodal point. If you pick a Moon centered or Earth centered frame, you have the fictitious inertial force plus the fictitious centrifugal force to deal with. Any way you cut it, the math is messy. There is no reason to invoke a rotating frame when a non-rotating frame gives a clearer and cleaner explanation.

Staff Emeritus
I dunno if you're beating him up excessively.
I think he is not beating him up excessively. If anything, not enough.

The answer provided was simply "gravity and centrifugal force". Now, I think we can all agree that "gravity" by itself is a poor explanation. While gravity is the cause, the word "gravity" alone is not descriptive. So therefore, it is natural to conclude "and centrifugal force" means that centrifugal force plays a major role in the explanation. While it may be possible to twist things so this is technically correct, as an explanation, it's poor.

Russ' answer is, in my mind, the clearest. The moon's gravity pulls the near-side ocean away from the earth and the earth away from the far-side ocean.

I know some textbooks explain tides by centrifugal force with mass center of the two bodies.
But i myself agree with " The moon's gravity pulls the near-side ocean away from the earth and the earth away from the far-side ocean"

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russ_watters
Mentor
gravity attracts the bodies toward one another and centrifugal force pushes them away from their center of mass (around which they orbit). there is an excess gravitational force on the moon side of the earth and an excess centrifugal force on the opposite side of the earth.

at the center of the earth the 2 forces are balanced.
That effect is so small as to be unimportant. Perhaps you could actually calculate its magnitude and compare it to the magnitude of the tidal force....
The fact that someone in another forum said it means exactly the same as when someone (you) in this forum said it: they got it wrong. Not sure what the point of the other link was.

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russ_watters
Mentor
I dunno if you're beating him up excessively.

Inasmuch as centrifugal force can explain to the layperson why water stays in a swung bucket on a string, surely this is also an acceptable answer for the tides.

Not that I think it's the best explanation...
No, it is not an acceptable explanation. In the swinging bucket, if the bucket isn't swinging, the water falls to the ground. In the tides example, if the earth and moon are not in orbit around each other, the tides are still there. What he is describing is not tidal force. It is a different effect entirely, and a miniscule one at that.

russ_watters
Mentor
I know some textbooks explain tides by centrifugal force with mass center of the two bodies.
If they do, they are wrong - as I said before, the tides exist even in a non-rotating system. What granpa is describing is not tidal force. Are there terms in the tidal force equation for location of the barycenter and period of rotation?

Since it is still too early in the morning for math, for now just consider this problem:
The Earh has no moon, but a moon-sized asteroid is on a direct collision course with earth. Are there two tidal bulges or one?

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LURCH
No, it is not an acceptable explanation. In the swinging bucket, if the bucket isn't swinging, the water falls to the ground. In the tides example, if the earth and moon are not in orbit around each other, the tides are still there. What he is describing is not tidal force. It is a different effect entirely, and a miniscule one at that.
So, if the Earth and Moon were not rotating around a common center of gravity, and were stationary relative to one another, the oceans would still bulge out on the far side? What would cause this bulge?

sylas
So, if the Earth and Moon were not rotating around a common center of gravity, and were stationary relative to one another, the oceans would still bulge out on the far side? What would cause this bulge?
The bulge occurs when the Earth, and its oceans, are all in freefall. If you actively hold the Earth stationary somehow, it's no longer the same situation.

DaveC426913
Gold Member
So, if the Earth and Moon were not rotating around a common center of gravity, and were stationary relative to one another, the oceans would still bulge out on the far side? What would cause this bulge?
The key is, if they were not rotating around each other, they would not be stationary relative to each other. They would accelerate toward each other. What would happen to the oceans in this case?

sylas
You'd still get the bulge, on both sides. Briefly. Then the ocean would be vaporized in the collision.

D H
Staff Emeritus
I know some textbooks explain tides by centrifugal force with mass center of the two bodies.
If they do, they are wrong - as I said before, the tides exist even in a non-rotating system.
Russ is exactly right. The tides exist, so one had better be able to explain them from any frame of reference. Some choices are good in the sense that the explanation is clear and concise. Other choices, such as a frame with origin at the center of Mars and rotating with Mars' rotation rate (1 revolution per 1.026 days), are not so good in the sense that the explanation is anything but clear and concise.

Here is what one of my profs has to say:
http://www.math.nus.edu.sg/aslaksen/teaching/tides.html
From the link, a bit out of order,

Some argue in the same way as above, but in order to get an outward pointing force at the far side, they consider a centrifugal force caused by the Earth's rotation around the Earth-Moon center of mass. This is not correct.
Hmm. That's just what Russ said.

This explanation seems to be popular among oceanographers. All the sites below use a centrifugal force.
It appears oceanographers have been infected by a bad meme.

They argue that the centrifugal force is the same at every point of the Earth, and that it must be equal to the gravitational pull at the center of the Earth. I find it too much of a “coincidence” that the centrifugal force happens to equal the gravitational pull at the center (or the center of mass).
This is an incredible hand-wave that is an aberration of the concept of centrifugal force. More below.
Some people, including Aslamazov and Varlamov in the book “The Wonders of Physics” use a centrifugal force, but they let the force depend on the distance from the center of rotation, which is clearly wrong. If we ignore the rotation of the Earth, then all points on the Earth describes circles with the same radius, but different centers, in the course of the month.
I disagree with the author of that web site here. The centrifugal force on some object of mass m located at some point r from the origin of a rotating reference frame is defined as $m{\boldsymbol{\omega}}\times{\boldsymbol r}$. If one insists on using a rotating frame, one should at least do it right.

The underlying assumption needed to arrive at the conclusion that "all points on the Earth describe circles with the same radius" is that the Earth is not rotating with respect to inertial space. This in turn means that the Earth is rotating with respect to the rotating Earth-Moon frame, and hence one has to consider Coriolis force as well as centrifugal force in explaining the tides. To make things worse, the authors of the sites erroneously use the term "centrifugal force" to describe what is really the sum of the centrifugal and Coriolis forces. Why do that when all one has to do is invoke the concept of tidal gravity and be done with it?

But i myself agree with " The moon's gravity pulls the near-side ocean away from the earth and the earth away from the far-side ocean"
Personally I like tidal gravity (or gravity gradient) as the explanation. The explanation pops out clear and concise from the math, and this also explains why the squeeze at the middle. Words don't do as much for me as does mathematics.

russ_watters
Mentor
So, if the Earth and Moon were not rotating around a common center of gravity, and were stationary relative to one another, the oceans would still bulge out on the far side? What would cause this bulge?
Sorry, I'm not sure if I said it(i was thinking it) but the earth and moon can't be held stationary with respect to each other: if a hypothetical support was constructed to keep them there, the regular gravitational force would swamp the tidal force and all of the water would rush to the side of the earth that the moon is on, so it isn't good for visualizing the issue. So consider instead the scenario I described before:
Me said:
The Earth has no moon, but a moon-sized asteroid is on a direct collision course with earth. Are there two tidal bulges or one?
There are two tidal bulges.

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