ElI5: Difference between tidal force and gravity.
In: Physics
Tidal forces are the non-uniformity of how gravity feels over a large object.
You can model the Earth-moon system as a point mass at the center of the Earth and another point mass at the center of the moon, and that will describe the orbital mechanics.
But the Earth is pretty big, actually. Big enough that the gravitational force from the moon, which points toward the center of the moon and falls off with distance, feels different on different parts of the Earth. The point closest to the moon will feel slightly more gravity, and the farthest point will feel slightly less. The sides will feel a slight component inward.
Why is this relevant? Mainly because the Earth has a bunch of liquid water on it, and water can move around. The water on the moon side feels the strongest pull, so it gets higher above the Earth’s surface. The water on the opposite side feels the least pull, so it gets higher too, farther from the moon than the Earth below it. Those are the two high tide points.
It also affects the moon. The moon doesn’t have liquid, but even rock can be deformed a little. If the moon was spinning on its axis, it would want its slightly heavier side (no object is perfectly uniform) to point toward the Earth. every time it tried to turn away from that orientation, tidal forces would compress it a little and cost it a little rotational energy, until it would eventually stabilize in that position. That’s called “tidal locking”, and it’s why we always see the same side of the moon.
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