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I started studying about gravitation recently and I came across the fact that when finding the gravitational force between the earth and some point mass in space, we can consider the mass of the earth to be concentrated in the center.

Now when I read about tidal forces being caused by the difference in gravitational force due to one side of the earth being nearer to the moon than the other, I got confused. If we are considering the distance between the two objects to be from their center of masses, to calculate the gravitational force exerted on each other, then why does it matter that one side of the earth is closer than the other?

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The gravitational pull between two objects is not at the center of those objects but comes from every individual atom which is part of those objects. However, the resulting gravity from all those atoms depends on shape and the variety of different materials and their mass. Earth is quite round in comparison with its size so its point of mass and gravity is in the center.

For distant objects, using the center of mass is good enough as the difference in pull between one part of that object and another is small.

Imagine a black hole pulling on a distant star, the whole star will slowly start moving toward the black hole because the gravitational pull of the star itself is strong enough to keep all the atoms from escaping and keeping them together.

But when the star gets closer to the black hole, the star gets stretched and gas molecules get stripped off, because the gravitational pull on one side is stronger than the other.

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A smaller star has to get closer before getting stretched, so it depends on the distance and the size of your object, and if you are looking at what is happening in general (the star moving to the black hole) or you look what is happening to that object (one part is being pulled on stronger making the star stretch).

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  • $\begingroup$ So can I think of this like this, that when say 2 planets are far away no bulging occurs because the planets attraction with its own particles is greater than the particles attraction to the other planet. But like when they move closer, the particles face greater attraction from the other planet compared to their own pull, thus get like bulged? $\endgroup$
    – Megan mars
    Commented Dec 8, 2019 at 19:12
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The case would be true for rigid bodies (the Earth being an approximate case) but the ocean on Earth's surface doesn't behave as a rigid body (it never can!) and hence such cases arise.

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  • $\begingroup$ So can I think of it like this that the gravitational force between the earth and the moon stays the same, like we consider both of them point masses. But when we look into the more closely we can see that the gravitational force acts on each point of the body (Earth), and so due to the difference in the gravitational force felt by the opposite sides of the earth, tides occur? But then like how am I supposed to know when to treat a body as a point object or as a body with each point facing the same force? $\endgroup$
    – Megan mars
    Commented Dec 8, 2019 at 19:07

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