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Gravity

The universe is unbelievably large. Despite this fact, the vast majority of it is empty space! All of its matter exists in dense spheres and clouds. How could this be? Simply put, our Universe is ordered by gravity. Gravity is a force which pulls things together based on their mass. It builds on its own power, allowing for objects with extreme gravity. Black holes are one example which we’ll look at here.

 

An illustration of the solar system, held together by gravity.

Planets orbit stars because of the stars’ strong gravitational force.

Newton’s Law of Gravity

This force has existed since the Universe formed. That said, we weren’t sure how it worked until Isaac Newton came around. Newton produced a formula which allowed us to say how strong it is. It predicts how strongly two objects are pulling on each other.
 

To understand Newton’s law of gravity, we first have to look at another one of his laws. Newton’s third law tells us that for every force, there’s an opposite force of equal strength. For example, let’s say you pick up a rock. You exert a force on the rock to move it upwards. At the same time, the rock pushes down on your hands.

An image of a child holding a rock in her hand.

Rocks don’t make for very good pets. They’re always trying to get back to the Earth.

This relationship also applies to gravitational forces. Clearly, the Earth is pulling us down. But, we’re also all pulling back on the Earth. Nothing happens, though, because we’re so much smaller than the Earth.
 

Distance in The Law of Gravity

As we’ve seen, the basic idea behind this force is that two objects pull on each other. That said, it’s a little more complicated than that.
 
A central thing to understand about gravity is that it relies heavily on distance. As objects get further apart, their attraction weakens greatly. Every time their distance doubles, the force between them gets four times weaker! This tells us that objects which are very far apart should feel no real tug from each other.
 

That is, unless, one of them is VERY massive. Interestingly, it’s a black hole’s gravitational pull that keeps our galaxy together. Our Sun orbits this black hole despite being over 25,000 light years away from it. That’s because it’s roughly 4,310,000 times more massive than the Sun.

An artists illustration of a black hole in space.

A black hole. Their extreme gravitational pull even captures light!

How Does Gravity Work?

This force guides all large-scale motion in our Universe. But if it’s so central, we have to ask: How and why does it work?
 
The answer – and this isn’t fun to hear – is that we don’t really know. There is a lot of complicated physics showing how it behaves. But, we don’t exactly know how or why it happens in the first place.
 

Here’s an example to make this clear. Apples sometimes fall from trees. They do so because the Earth’s mass causes it to pull things close to it. This pull is gravity. But why is it that things with mass exert a force at all? The answer to this question isn’t clear. 

An image of apple trees in an orchard.

Apples fall from trees, that’s gravitational force. What’s that, you ask? We’re not wholly sure!

This is not a satisfying answer, of course. It’s just the best we can offer right now. Maybe one day, scientists will figure out what, exactly, this force is.
 

Gravity in Astronomy and Space Exploration

Centers of Gravity

When we talk about this force in astronomy, we’re usually discussing centers of gravity. This is a point which pulls in equal amounts in all directions. 
 
The stronger the center, the more it can pull towards it. Planets and dwarf planets are some of the most basic (in astronomy). They might have one or two moons, but they generally don’t pull that much in. Oppositely, stars are extremely strong. Our Sun’s gravitational force is what keeps the entire solar system together!
 

Zero Gravity

When we talk about this force in space exploration, we’re usually referring to its effect on humans. In outer space, there isn’t any. This leads to a sensation known as weightlessness. It’s difficult to get used to and requires a special living environment.
 

Additionally, other planets have different gravitational force from that of Earth. On each mission, space scientists have to prepare astronauts to deal with this.

An illustration of an astronaut floating in the atmosphere.

Who is supposed to pull this astronaut back in? Space is the other way! 

Other Great Resources

Understanding Gravity (with Interesting Facts): https://www.theschoolrun.com/homework-help/gravity

(Video) Defining Gravity – Crash Course Kids: https://www.youtube.com/watch?v=ljRlB6TuMOU

Physics for Kids: Gravity – Ducksters: https://www.ducksters.com/science/gravity.php

 

Written by: Noah Louis-Ferdinand