Try to jump as high as you can. Bend your knees. Now jump! No matter how hard you try, or how high you jump, you always come back down again.
Something called gravity pulls you back down. Gravity holds you down on Earth. Without gravity, you would fly off into space. You would jump up and just keep on going. That might sound like fun, but you could not live very high up. For one thing, you need to breathe air. The higher you go, the less air there is. You need gravity to keep you down on Earth.
WHAT IS GRAVITY?
Gravity is a force that pulls two objects toward each other. The force is also called gravitation. The bigger an object is, the stronger its gravity. Small objects, and even cars and buildings, have so little gravity that you can’t feel their pull. Huge objects like Earth, however, are a different story.
Earth is so much bigger than you that its gravity makes you “stick” to the ground. Earth is bigger than the Moon. Earth’s gravity pulls on the Moon. That is why the Moon orbits, or goes around, Earth. The Sun is bigger than Earth. Its gravity makes Earth go around the Sun.
Earth’s gravity pulls toward the center of Earth. Gravity holds the oceans and the atmosphere on Earth. The Sun’s gravity pulls toward the center of the Sun. It holds the Sun together.
WHO DISCOVERED GRAVITY?
Gravity has always been around. In ancient times, people tried to explain why things fall toward Earth. An English scientist named Sir Isaac Newton came up with a great idea about gravity in 1687. He thought about how an apple falls and wondered how far gravity went. He came up with the idea that gravity does more than hold people on Earth.
Newton thought of gravity as a kind of mysterious force pulling objects together. He said that gravity holds the Moon in orbit around Earth. It holds the planets in orbit around the Sun. Newton’s ideas on gravitation explained many things about how apples fall and how stars and planets move.
A DENT IN SPACE-TIME
In the early 1900s, another scientist named Albert Einstein came up with a very different idea of what gravity is. Einstein had some pretty amazing ideas. He said there was something called space-time. He thought of gravity as a dent in space-time.
This is not as hard to imagine as it might seem. Think about placing a bowling ball on a soft mattress. The mattress is like space-time. The bowling ball is like a star. The bowling ball makes a dent in the mattress. This is sort of how a star dents space-time. If you try to roll a tennis ball past the bowling ball, it curves around the bowling ball as it passes through the dent.
The planets in our solar system roll around the dent in space-time made by the Sun. The dent keeps planets from going off into space. The dent in space-time keeps the planets going around the Sun.
Scientists are still not sure what causes gravity. Finding out more about gravity is an important problem for the future.