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### Course: Cosmology and astronomy>Unit 2

Lesson 4: Cepheid variables

# Why gravity gets so strong near dense objects

Why Gravity Gets So Strong Near Dense Objects. Created by Sal Khan.

## Want to join the conversation?

• If you were in the middle of the earth (theoretically), would you be pulled apart by the gravity in every direction? (serious answers please)
• Nope. If you were in the center of the earth, you would have only a part of the earth's mass in any given direction, so the pull of gravity in any direction would be less than what we experience on the surface, since here all of earth's mass is laying in just one direction from us. And since the stronger gravity we experience here isn't anough to cause us any damage, neither would the gravity in the center.

In fact, I'd figure (I'm no expert though) that the roughly equal amount of gravity coming from every direction in the center would kinda cancel each other out, and if there was a cavity in the very middle of the earth you could just sort of float around in it. Just guessing about this though.
• so then if it was possible to put a tube through the exact center of the earth and you jump in it, would you be pulled back and fourth and eventually stop in the center, and while in the very center of the earth, still in the tube would you float, because the earth is pulling evenly in all directions?
• Theoretically yes.
• If you were just inside the event horizon of a black hole would the gravity pulling you in be the same as you being twice as close to the black hole?
• The closer you get, the stronger the gravitational pull is because you are getting closer to the big massive object. That means the bigger the black hole, the stronger the pull is.
• what is the oldest star that has been found?
• Hey, how can HD 140283 can be 14.5 billion years if the age of the Universe is 13.7 billion years? That's what I never understood.
• The earth revolves round the sun because the sun attracts the earth. The sun also attracts the moon and this force is about twice as large as the attraction of the earth on the moon. Why does the moon not revolve round the sun ?
• Because it doesn't want Earth to be lonely.
• If the net gravitational force gets smaller as you get nearer to the center of the mass, does that work with a smaller star too? And would it work with the Earth, like the closer you get to the center of the Earth, the less gravitational force, so you wouldn't be crushed? Or do you get crushed because of the pressure? Thanks!
• Gravity operates in the same manner for all objects, including the Earth and stars. If you could somehow travel to the center of the Earth, you'd get crushed by the pressure, not gravity.
• What does Relativistic mass mean?
• When an object accelerated to near the speed of light, its mass changes, increasing as the object gets closer and closer to the speed of light. Taking this effect into account gives you the relativistic mass, which at high speeds becomes very, very different from the rest mass.
• Most of the videos on cosmology and Astronomy, are very much informative and descriptive about the science behind the infinite universe.... but how can I understand exactly 'Gravity' where does it come from.. and how is created? what mechanism make it be what it is from an atomic level if in fact that is how is originates?
• In fact, one of the bigger questions in science right now is what powers gravity. Gravity just seems to happen, while the 3 other fundamental forces have justifications with it.
• Well if the black hole is just a point in three-dimensional space then as per the video, shouldn't "r" be equal to zero, making the gravitational force equal to infinity?
This seems paradoxical to me. Another result would be that all light and matter in the universe would be sucked into the black hole "instantaneously"and would therefore violate the universal speed limit. Please help.

Also, how exactly is the gravity proposed by general relativity different from Newtonian gravity.
• That is why we refer to the black hole as a singularity. Our current physical models break down as some value approaches infinity. We really can't say what happens to matter in the black hole beyond the event horizon.
To understand why, we can use Newton's law of universal gravitation. It states that `F = (G * M1 * M2) / r ^ 2`. G is the gravitational constant, 6.67 * 10^-11, M1 and M2 are the masses of both objects, and r is the distance between the objects.
The gravitational force between two people weighing 70 kg each and standing a meter away can then be calculated as `F = (6.67 * 10^-11 * 70 * 70) / 1`. This comes out to be about 3.26957e-7 newtons, which is not a lot.