Why do black holes have such strong gravity?
In: Physics
Gravity is stronger the closer you are to the center (§). Quadratically so: twice the distance means four times less gravity. So if you compress Earth by a factor of 100, then standing on it makes you experience 10,000 times what you are used to.
People often say black holes are incredibly dense, but this is not really always the case. The heavier it is, the less dense, and some of the largest known black holes have a density below yours; it can even be less than air! This happens because the mass (~ gravity) of an object made of a single material grows with the third power of the size, while the larger distance from the center only makes the force quadratically weaker as described above.
(§): this only applies as long as you are “outside” the thing, if you enter deep into the Earth, then the entire shell above you won’t actually contribute anymore. You will feel essentially no gravity at all close to the middle.
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