Earth’s Systems


When you jump up, you come back down to earth again. Did you ever wonder why you don’t float off into space? The answer is gravity. The Earth’s gravitational force pulls you down so you don’t go up and up forever.

What is gravity

What is gravity?

Gravity is a force of nature that pulls objects together. The Earth’s gravity pulls you toward the ground. The Earth’s gravity also pulls on the moon. Gravity is what makes the moon orbit around the Earth. Your body has gravity, too. But, because you are much smaller than a planet, your gravitational force is much smaller. You probably don’t have any moons orbiting around you!

To understand gravity, we need to understand a little bit about the difference between mass and weight. It’s pretty simple, really. Mass is how much material, such as atoms, an object contains. Weight is how much gravity pulls at that object.

The size of a planet is an important part of how much gravitational pull it exerts. If you went to a smaller planet, you would weigh less than you do on Earth, even though your mass would be the same. On a bigger planet, you would weigh more.

The moon is a good example. It has gravity, but because it is so much smaller than Earth, its gravity is only one-sixth as strong. No wonder that when astronauts walked on the moon, they took big, bouncy leaps. They weighed less there, so they could go farther with each step.

The force of gravity also decreases with distance. For example, when a rocket takes off, it needs a great deal of force to break away from Earth’s gravity. Once it gets farther from the surface of the Earth, the pull of gravity is weaker and it is easier to continue into space. Similarly, Earth can keep the moon in orbit around it because the moon is nearby. But more distant objects, such as the moons orbiting Saturn, are much too far away for Earth to have much gravitational effect on them.

Here at home, Earth’s gravitational pull is very strong. Not only does it hold our moon and all us humans in place, it also keeps our precious atmosphere from drifting off.

Isaac Newton Albert Einstein

Sir Isaac Newton and the law of gravity

The person who first explained gravity was Sir Isaac Newton in 1687. He developed an equation to measure gravitational force. Newton’s equation explained mathematically how the gravitational pull between two objects was directly related to how much mass those objects have, and the distance between them.

Newton called this the Law of Gravity. It is one of the laws of physics. In science, a law is different from a theory. A scientific law means something not only has been proven, but can be explained with a mathematical equation. That’s gravity!

Albert Einstein and a new theory of gravity

In the early 1900s, about 250 years after Newton described the Law of Gravity, Albert Einstein came up with a theory that added a lot to our understanding of gravity.

Einstein realized that the force of gravity even affects things that don’t have mass, such as light. Einstein also determined that space and time were interwoven. He called this spacetime, and he figured out that gravity pulls at spacetime, too.

Before Einstein, scientists had observed phenomena that couldn’t be explained by Newton’s equation, like why light always moved at the same speed, and how gravity seemed to warp light. For example, astronomers had noticed that light from far away stars seemed to bend as it passed by large objects in space. Einstein’s theory explained these and other observations.

Einstein also predicted things that no one had yet observed. His most mysterious and exciting prediction: black holes.

A black hole is a spot in the universe where gravity is so strong that nothing can resist it. Even light can’t escape from a black hole. Today, scientists are able to see where there are black holes in space. The black hole doesn’t emit light, but astronomers can observe gases and objects being pulled in by the strong gravitational pull toward the edge of the black hole.

Black holes

Life without gravity

What would life be with little or no gravity? The astronauts who live aboard the International Space Station can tell you all about it. Sure, it’s fun to spin and float around in midair. But the astronauts deal with plenty of challenges, too.

Eating is one challenge. Food and liquids can float away. Imagine chasing your pizza around for your next bite! That’s why, on the International Space Station, many foods are packaged in squeeze tubes. Other foods and plates are held onto the table with sticky tape. Salt and pepper come in liquid form so the little grains can’t float away while being sprinkled.

To sleep, astronauts strap themselves down. If they didn’t, the expression “drift off to sleep” would actually be true! And because living with low gravity makes everything lighter, they also have to spend a lot of time exercising so their muscles don’t get too weak.

When the astronauts return to Earth, gravity takes some getting used to again. No more floating. Instead, they have to walk like the rest of us. Everything seems heavy at first, too. But at least they don’t have to chase their pizza through the air.

Written by Laura McCamy

Edited by Beth Geiger, MS Geology

Illustrated by Renee Barthelemy