Why exactly does the ocean bulge on both sides of the Earth?

In summary, the mean force, F_mean, is defined as the average force acting on the Earth from the Moon. F_close is the force on the side of the Earth closer to the Moon, and F_far is the force on the side of the Earth furthest away from the Moon. This means that the net force on the closer side is greater than the mean force, while the net force on the further side is less than the mean force. This difference in force leads to a difference in acceleration, causing two bulges on the Earth's surface. It may seem counterintuitive that there would be two bulges instead of just one on the closer side, but this is due to the difference in acceleration, not just the force. This
  • #1
vector
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Here's what my prof says:

"Define F_{mean} to be the mean force, F_close to be the force on the side of the Earth closer to the moon, and F_far to be the force on the side of the Earth furthest away from the moon.

On the closer side the net force is F_close - F_mean > 0
On the further side the net force is F_far - F_mean < 0

So on the far side, the pull is weaker than it is on the rest of the Earth, hence the second bulge."


But I don't understand this explanation, because if the forces on the closest side of the Earth and the farthest side of the Earth are both pulling forces, then it doesn't make sense that the bulge should occur on both sides, but, rather, intuitively, it would seem that there should only be one bulge, that is on the closest side of the Earth to the moon, not two sides.
When I reviewed the process as described mathematically, it made sense, but I would like to understand this on the level of physical intuition. Would appreciate your input.
 
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  • #2
It's all about POV. If you just have the Moon - and you put two pebbles in space at different radii from the Moon, on the same radial line, then the close pebble will accelerate towards the Moon faster than the far pebble. An observer floating half way between them will accelerate slower than the near pebble and faster than the far pebble. To that observer, then, it will look like the two pebbles are moving away from him.

For more details in a pop-sci format:
 
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  • #3
Gravity pulls on the near side harder than it does on the Earth and both the near side and the Earth are pulled harder than the far side. This leads to a difference in acceleration, and, thus, two bulges. If it weren't for the fact that Earth's own gravity pulls on the water on the far side, it would get whipped off into space. Or rather it and the rest of the Earth would wind up in different orbits owing to their different orbital radii. For small objects like asteroids, or large tidal forces like what are generated from being close to stars or other massive objects, this difference in acceleration between the near and far sides can easily pull them apart.
 
  • #4
Drakkith said:
Gravity pulls on the near side harder than it does on the Earth and both the near side and the Earth are pulled harder than the far side. This leads to a difference in acceleration, and, thus, two bulges.
Isn't that an oversimplification?... if that was all there was to it then wouldn't you get noticeable tides in a glass of water (near the equator)? Isn't the acceleration difference across the Earth really small - too small to account for the size of the tide? (The first question would occur to my Y7-8 students and Y10-11s would come up with the second one.)

I think the original question was more about how the Moon's gravity doesn't just pull all the water more to one side than the other.
I was going to go into more detail but found the video so posted than instead. It's worth the watch.
 
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  • #5
Simon, I don't see how your explanation and Drakkith's differ: you said acceleration and he said force, but both were pointing out the same phenomena; the difference in force or acceleration between the three points (near, far and center). Afterall, for a unit mass, f=a
 
  • #6
My explanation included a video which elaborated on the description I gave in text.
The text is not the whole story. Please see the vid.
(I'd rather not have to type it out...)
 
  • #7
Simon Bridge said:
Isn't that an oversimplification?

No, I don't think it is.
 
  • #8
Simon Bridge said:
Isn't the acceleration difference across the Earth really small - too small to account for the size of the tide?
Depends on how you do the "accounting for" bit. Real world tides water are complicated by many factors, but the OP question is quite basic about the tidal deformation of an idealized object.
 

1. Why does the ocean bulge on both sides of the Earth?

The ocean bulges on both sides of the Earth due to the gravitational pull of the Moon and the Sun. This is known as the tidal bulge. The gravity of these celestial bodies pulls at the Earth's oceans, causing them to bulge towards the direction of the Moon and the Sun.

2. Does the ocean bulge equally on both sides of the Earth?

No, the ocean does not bulge equally on both sides of the Earth. The side of the Earth that is facing the Moon or the Sun experiences a greater gravitational pull, causing a larger tidal bulge, while the side of the Earth facing away from the Moon or the Sun experiences a smaller bulge.

3. How does the rotation of the Earth affect the tidal bulge?

The rotation of the Earth causes the tidal bulge to move around the Earth's surface as the Earth rotates. This is why we experience two high tides and two low tides each day, as the tidal bulge passes over different parts of the Earth's surface.

4. Is the tidal bulge the same in all bodies of water?

No, the tidal bulge can vary in different bodies of water depending on their shape and location. For example, the tidal bulge in a narrow inlet may be more pronounced than in a wide open ocean.

5. Can the tidal bulge cause dangerous ocean conditions?

Yes, the tidal bulge can cause dangerous ocean conditions such as high tides, storm surges, and strong currents. These conditions can be particularly hazardous during extreme weather events, such as hurricanes, which can amplify the effects of the tidal bulge.

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