If gravity pulls everything towards it, why isn’t the Earth getting smaller by being pulled into the middle?
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Because gravity isn’t the only force at play here. There’s also various inter- and intramolecular forces acting on each particle, pushing them apart. The harder you try to push things together, the higher the force gets you need to exert on something in order to continue compression. This eventually reaches a point where these internal forces overcome the pull of gravity, at which point, you reach an equilibrium. Lowering the internal forces trying to push Earth apart causes gravity to push things back together, thus increasing the forces trying to push things apart. Trying to push things closer together in contrast causes the internal forces to get stronger and overcome gravity again, thus pushing things apart again. This is what constantly happens in nature. Stability essentially means that all the forces trying to do one thing and all the forces trying to do the other thing balance out, and thus nothing happens.
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