Gravity represents the attraction between objects. All objects with mass are affected by gravity. Gravity acts like a magnet — pulling objects together. What causes gravity is not really known.
The Earth has gravity. Gravity holds everything close to this planet. Trees, water, animals, buildings, and the air we breathe are all held here by gravity. All of the planets, their moons, and the stars in the universe have gravity. Even our own bodies have gravity. The Earth's gravity is far stronger than our own so we don't notice the gravity our bodies possess.
Gravity is affected by the size and proximity of objects. The Earth and the moon have a stronger pull on each other than the Earth and, say . . ., Jupiter because the Earth and moon are closer to one another. Likewise, Earth exerts a stronger pull than the moon because it is larger, so there is more pull on our bodies here on the Earth than we would feel on the moon if we were astronauts visiting there. We don't actually "feel" gravity. We only feel the effects of trying to overcome it by jumping or falling.
Mass or Weight?
Mass is the "stuff" that matter is made of. People often confuse mass with weight. But weight is actually the result of gravity pulling on the mass. We measure mass in grams. We measure weight in ounces and pounds. Your mass would stay the same if you could travel from planet to planet, but your weight would vary depending on how the gravity of that planet pulls on you. Check out how much you would weigh on another planet.
When one space object revolves around another, it is referred to as an orbit. The earth orbits the sun. Our moon orbits the earth. Many of the other planets in our solar system also have moons that orbit them.
Earth now has many man-made objects orbiting it. Satellites are placed in orbit around the earth to assist with telephone calls, television broadcasts, and other forms of communication. Astronauts have sometimes left debris orbiting our earth too. Gloves, tools, and other junk continuously orbit our earth, floating for an unknown period of time in space.
Two forces — centripetal and centrifugal — work to keep the planets and their moons in orbit around each other. Learn more about these forces .
If a person drops a 10 pound ball and a 5 pound ball off a building at the same time, which one will hit the ground first? Will gravity pull harder on the 10 pound ball?
This was the subject of a famous story about the Italian scientist Galileo Galilei, who is said to have tried dropping two objects from the Leaning Tower of Pisa to find the answer to this question. You might be surprised to find out that the two objects hit the ground at almost the same time.
Gravity works the same on all objects unless wind resistance gets in the way. So if one of those balls had been attached to a parachute, it would have slowed the ball down. Amazing science! Check out Galileo's story here.
How strong can gravity get? Imagine gravity pulling on everything and not letting it escape — not even light! This is the essence of a black hole. The gravity in a black hole pulls everything back in toward its center which may be as small as a single atom. This makes a black hole invisible because even light is unable to break away from its effects. Scientists believe there may be millions of black holes in the Milky Way Galaxy.
Floating in Space?
So how do astronauts float in space if the pull of gravity is so strong? Astronauts don't actually float. They are being pulled by the Earth's gravity just the same as the rest of us.
But they are also orbiting the Earth, or moving sideways. This is known as centrifugal force. This sideways movement actually is pulling them away from the Earth at the same time that the Earth is pulling them down, so it appears as if they are floating.
Microgravity — is that like small gravity? Not really. Let's explain it this way. If something is falling near the earth, or normal gravity, it accelerates at 32.2 feet per second per second or 32.2 ft / sec².
Objects in space not only fall toward the earth, but also sideways or around the earth. So if you see a video of an astronaut using a tool and that tool seems to float, it is because the astronaut and the wrench are both falling at the same time. This is a free fall or microgravity. Visit NASA's site for additional information on this concept.
The Earth's tides are caused by the moon's gravitational pull on the oceans. Tides are the rise and fall of the ocean level as related to the shoreline. High tides occur when Earth and moon are facing each other, and the moon is exerting its greatest pull on the ocean waters.