# Gravity in the Solar System: Shaping Planets & Stars

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• 0:02 What is Gravity?
• 1:38 Why are Planets and…
• 3:45 Lesson Summary

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Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain what gravity is, and the role it plays in forming the shapes of planets and stars. A short quiz will follow.

## What is Gravity?

Gravity is the reason you can throw a ball up and catch it again. It's the reason you sometimes fall and hurt yourself. And why we can't fly around on brooms. But what is gravity?

In physics, gravity is one of the fundamental forces of the universe, one that causes every object in the universe to attract every other object. In fact, anything that has a mass, anything that contains physical matter, will be attracted to every other object with mass. Objects fall because they are attracted to the Earth (and the Earth is attracted to them).

So why doesn't your mug fly off your desk and hit you? Well, gravity is actually very weak. It might seem strong when you fall out of a tree house and hurt yourself, but that's only because the Earth is so huge. Everyday objects have such weak gravitational forces that you can't really notice them. Only on the scale of planets, the Moon, and the Sun is gravity the incredible force that shapes the universe.

Distance also has a big effect on gravity. If you double the distance between two objects, you cut the force of gravity between them to a quarter. So you don't feel a strong force of gravity between, say, you and the planet Mars, for example, not because Mars is small, but because it's just so far away.

Gravity causes the motions of planets, stars, and galaxies. It's why the Moon orbits around the Earth, and the Earth orbits around the Sun, and the solar system orbits around the galaxy. It's all because of gravity.

## Why Are Planets and Stars Round?

Gravity also has a big impact on the shape of objects. Have you ever wondered why the Earth is round? In fact, lots of things are round. The Sun and other planets are also round. So is the Moon. They're all roughly spherical. Doesn't that seem a bit odd?

Well, gravity is the reason they're spherical. In the early solar system, it was very...messy. There were millions of bits of rock flying all over the place. Small ones to huge ones. Things were always hitting each other, breaking into pieces, and combining together with other pieces. It was ridiculously chaotic.

In that early mess of the solar system, the objects tended to be gravitationally attracted to each other and moved together. Things combined more than they broke apart. The heaviest elements like iron had the strongest forces of gravity, so they attracted together quickly, forming what would become the core of planets like the Earth. Then lighter layers formed on top as objects gradually got bigger through all the collisions.

Since gravity is strongest when you're close to an object, it could be described as a ''radial'' force. That means it's like the radius of a circle. The longer the radius, the weaker the force. This effect, combined with the fact that most objects would have been rotating super fast, meant that as planets gradually grew in size, they tended to do it in a really even way. As the matter was pulled in, the rotation only increased, just like how spinning ice skaters will spin faster when they move their arms closer to their body. This rotation helped smooth out the surface. It was for these reasons that the planets over time became more and more spherical.

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