Have you ever wondered why you always fall back to the ground with a bump, no matter how high you jump? Many years ago people asked the same question. Then a British scientist named Isaac Newton discovered the force of gravity.
A popular story, or legend, says that Isaac Newton was in a garden when an apple fell on his head and he began to wonder why the apple fell down and not shot upwards instead. He came up with the idea that some unseen force must be attracting the apple towards the Earth. He named this force “gravity” – from the Latin word “gravitas” meaning “weight”.
Newton realized that every object in the Universe attracts every other object in the Universe. Even an apple tugs slightly at everything around it. The tiny apple’s gravity is too weak to overcome the attraction of the huge Earth, so it falls towards the center of the planet.
Gravity is a force that tries to pull two objects toward each other. Anything which has mass also has a gravitational pull. The strength of gravity depends on the size and density of an object – what we call “mass”. The more massive an object is, the stronger its gravitational pull is.
The more matter something has, the greater the force of its gravity. That means really big objects like planets and stars have a stronger gravitational pull. Earth’s gravity is what keeps you on the ground and what causes objects to fall. It is gravity that holds the Moon in orbit around the Earth, and it is the force of gravity that keeps all of the planets in orbit around the sun.
Gravity’s pull by an object decreases with distance from it. The closer you are to an object, the stronger its gravitational pull is. Gravity is what gives you weight. A climber on top of Mt. Everest weighs a little less than at sea level. If a spaceship travels far enough from Earth, it will eventually escape the planet’s pull completely.
The gravitational pull of an object depends on how massive it is and how close it is to the other object. For example, the Sun has much more gravity than Earth but we stay on Earth’s surface instead of being pulled to the Sun because we are much closer to Earth.
Without gravity, we wouldn’t be able to stay put on Earth’s surface. Objects would simply float away if Earth's gravity didn’t exist.
Gravity is also the force that keeps the Earth in orbit around the Sun, as well as helping other planets remain in orbit.
Even, the weight of an object is based on gravity. Weight is actually the measurement of the force of gravity pulling on an object. For example, your weight on Earth is how hard gravity is pulling you towards the Earth’s surface. If we travel to other planets, we would weigh more or less depending on if those planets have more or less gravity than Earth. Since gravity is related to mass, we would weigh less on smaller planets and more on larger planets.
For example, the moon’s gravity is 1/6 of Earth’s gravity, so objects on the moon will weigh only 1/6 of their weight on Earth. So, if you weigh 60 pounds here on Earth, you would weigh about 10 pounds on the moon.
High and low tides in the ocean are caused by the moon’s gravity. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides. As the Earth rotates, your region of Earth passes through both of these bulges each day. When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.
There is zero gravity in outer space, so we would be weightless if we were floating out in space.
Objects weigh a little bit more at sea level than they do on the top of a mountain. This is because the more distance you put between yourself and Earth’s mass, the less gravitational force Earth exerts on you. So, the higher you go, the less gravity pulls on you, and the less you weigh.
If you wanted to escape Earth’s gravitational pull, you would have to travel seven miles (about 11 kilometers) per second. This number is called Earth’s “escape velocity.”
Gravity is what holds the planets in orbit around the sun and what keeps the moon in orbit around Earth. The gravitational pull of the moon pulls the seas towards it, causing the ocean tides. Gravity creates stars and planets by pulling together the material from which they are made.
Black holes are the strangest objects in the Universe. A black hole does not have a surface, like a planet or a star. Instead, it is a region of space where the matter has collapsed in on itself. This catastrophic collapse results in a huge amount of mass being concentrated in an incredibly small area. The gravitational pull of this region is so great that nothing can escape – not even light.
