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## Is gravity a result of mass or size?

Hi all. I was hoping someone could explain something for me:

When I was in highschool, we were taught that gravitational force was a result of ridiculous amounts of mass. Like the sun. That high mass meant higher gravitational pull.

But then, I started learning about space time (which I don't fully grasp yet, to be honest)... I learned that gravity is a function of curves or dips in the surface of space time, and that objects in a gravitational field are really just circling around the toilet bowl of a very large piece of whatever.

The issue is, wouldn't that mean that gravity is a result of SIZE (as in, how great a dip it makes in space-time) as opposed to MASS?

Any resources to help me understand would be GREATLY appreciated.

Topics: gravity
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• #### Raj Bhakta

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Aug 10 2011: Well according to General Relativity, mass influences the curvature of space-time. Just like black holes. Black Holes are massive but come in different sizes, but ultimately it's their mass that curves space-time and thus decides the magnitude of gravitational attraction. Black Holes have a LOT of mass concentrated in an infinitesimal region of space aka the Singularity, that's why they pull in everything, including light. Also, massive stars and other celestial objects also curve space time to the extent that light is distorted.

This is an age old analogy that I was taught:
Think of space-time like a bed sheet stretched. Now put a large ping pong ball on it. Most ping pong balls are hallow, therefore have minimal mass. How much do yyou think the ping pong ball will curve the bed sheet? Not much right?
Now put a steel bearing ball . Steel bearing balls are massive and are quite dense. How much did that steel bearing ball distort the bed sheet? By a lot right?

The same principle applies in the General Relativity conception of gravity at macroscopic scales. Of course, I have not delved into the actual mathematical frameworks of all this so cannot say for CERTAIN that I AM ABSOLUTELY RIGHT. I am a Physics student who was taught this conceptually.
• #### Benjamin Torrence

• 0
Aug 11 2011: Thank you very much, your analogy was excellent. I was thinking more in terms of if you placed a ball between two bedsheets, the size would determine how much space rested between them. Thanks :)

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