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Tidal forces, their destructive potential & why are there tides on the Earth ?

Have you ever wondered what causes the tides here on Earth ? Well, you might be glad to hear that it's not too complicated to understand, so let's address that before moving on to the more drastic examples as you'll later find out in this article!

A relationship between 2 massive objects

The tides are effectively a result of the Moon (seriously). The Earth's gravitational influence on the moon is a lot greater than the Moon's gravitational influence on the Earth, simply because the Earth is far more massive than the moon (81 times more massive), hence why the moon orbits around the Earth and not the other way around, but this doesn't mean that the moon has no effect on the Earth at all.

There are 2 main factors to consider here when we think about the tidal forces that are created as a result of the Moon's presence: How close the Moon is to the Earth, and how large in diameter the Earth is.


Perhaps consider this...

Take the Earth as an example. The water, the atmosphere, you and I... everything, is being pulled toward the Earth's centre of mass, because of gravity....

But what if something, an object in space, were to have a strong enough gravitational influence to disrupt that of the Earth's, on say for example, the water? Well, that object is the Moon.

The side that is closest to the Moon, feels a greater acceleration due to gravity towards the Moon than the side that is not. This is because the effect of gravity increases the closer you get to an object's centre of mass. Because of this, one side of the Earth is being pulled toward the moon greater than the other side. This causes the Earth to "stretch" or "bulge" slightly, meaning that the water (and everything else in fact) is pulled to the side that is closest to the Moon, which changes as the Moon orbits around the Earth and as the Earth rotates and pivots on its axis throughout the year.

So the closer the 2 objects are together, the greater this effect will be, as the difference in acceleration due to gravity will be larger, resulting in more of a "stretch". I also mentioned how the diameter of the objects can affect this, although this is broadly the same as the first point, in that the bigger the objects are, the closer they will be together by virtue of the decreased space between them.

I refer to Isaac Newton's Universal law of Gravitation for clarity on this:

The gravitational force between two masses is proportional to the product of their masses and inversely proportional to the square of the distance between them.

That sounds complicated, but to quickly dissect it, this essentially means that if you double the masses of the objects, then you double the "force" of gravity ("force" in quotation marks here as General Relativity tells us that gravity isn't actually a force, but you can read more about that on here), but if you double the distance between the objects, then you decrease the "force" by 4x. Therefore, doubling the distance drastically decreases the force of gravity, and vice and versa.

If you were wondering why exactly the tidal forces increase as the objects become closer together, then definitely check out my separate post (it's only a short read) here, that dives into the mathematics behind this.

So you might be able to see why the moon has such an effect on the Earth; even though the Moon is relatively small, it does orbit quite closely to the Earth, meaning the force of gravity on the 2 objects is quite significant. The moon is only about 250,000 miles away.... which is very small in by the standards of the universe! I mean you might even have more than 100,000 miles on your car...

This mechanism of tidal forces is what creates the tides, and thankfully, just the tides! Both the Earth and the Moon don't have much mass, and they're not close enough with these small masses to create a significant tidal effect, but what happens in more drastic situations, where you have a relatively light object orbiting very closely to something far more massive?

Well, take a look at this - a simulation in Universe Sandbox, a phenomenal simulation game that you should definitely get your hands on if you're interested in Astronomy. Check it out using this link to support the community and our mission at no additional cost to you! (search for universe sandbox and you can get it there and then!)

The moon crumbling next to Earth

Then 15 minutes later (real time)....

The moon breaking apart next to Earth

Hang on.... why is Central Africa now underwater ?!

Here, you can see the Moon orbiting much closer to the Earth than in reality, meaning that the difference in the acceleration due to gravity towards Earth on the side of the Moon that's closest to the Earth and the side that's not, is astronomical (remember halving the distance multiplies the force by 4x)... causing the Moon to a be bit more than "stretched", but rather completely spaghettified (a term that's usually associated with the tidal force from a black hole, but its meaning is the same here).

You might also notice something else from the 2 photos above...

Central Africa has been completely swallowed by the Indian and Atlantic oceans! It's very important to remember that the Moon's tidal force on the Earth would also be drastically increased as the 2 objects orbit very closely to one another, meaning that the "tides" that we would experience on Earth would also be far more significant in size than what we experience now.

The point at which objects begin to strip apart is called the Roche limit of the more massive object (so the Roche limit of the Earth in the simulation).

This effect, although interesting to study, is quite unfortunate for astronomers, as it means getting near a black hole is going to be even harder than ever imagined, as the tidal effect on the spacecraft would cause it to be stretched and stripped down to atoms...

Thanks for reading, and consider checking out some of our other articles here at Expansive!

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