Gravity is a force of attraction between two objects. All objects with mass have 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, the stars and the moons of 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. Earth has a stronger pull than the moon because it is larger, so there is more pull on our bodies here on the Earth than astronauts who have gone to the moon would have while they are visiting there. We don't actually "feel" gravity. We only feel the effects of trying to overcome it by jumping or when we fall.
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 stays 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. Earth now has many man-made objects orbiting. Satellites are placed in orbit around the earth to assist with telephone calls, television, 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.
For more information, read about centripetal and centrifugal forces below.
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 topic of a famous story about 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! That is what makes 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 feet/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. You will want to visit NASA's site for additional information on this concept.
To a lesser degree the moon also pulls on the rivers, lakes and land. Find out more about tides by clicking here .
Floating on Water?
When an object floats on the top of water, gravity is still in play. For an object to float, it must displace enough water to make up the same mass as the object itself. Once that happens, the remaining mass then sits on top of the water. For example: if a boat has a mass of 45 grams, it will displace 45 grams of water and if that has happened before the whole thing has sunk below the surface level of the water, the boat floats.
Sir Isaac Newton (1642- 1727) was a scientist who is credited with discovering gravity. You would think that gravity would be a well known concept since all of us have been aware of it since we were little kids dropping our spoons on the floor from our high chair just to make mom come and get it. But it wasn't until Newton published his ideas in a book in 1687 that the scientific world had gravity as a defined term. Newton is well known for his Three Laws of Motion which explain how the physical laws direct the motion of objects.
Helium and Hot Air Balloons
Balloons often seem to defy gravity. We've all lost a balloon when we have accidently let is go and watched it drift away in the sky. These balloons are filled with helium. Helium is a gas that is lighter than the other gases that make up our air. Because this gas is so light, it tries to float to the top of our atmosphere and it takes our balloon with it. The poor balloon is not strong enough to hold on to this helium for the ride to the top and so it ends up popping long before the journey is through.
Hot air balloons work on a similar science. Hot air naturally tends to rise upward because it is less dense than cooler air. A hot air balloon has a heat source that fills the balloon's interior with warm air, allowing it to rise. To come down, the pilot allows the air to escape or to cool down and the balloon will descend.
Two forces work to keep the planets and their moons in orbit around the other object. Centripetal and centrifugal forces are terms scientists use to explain these forces.
The word centripetal is from a Latin word meaning "towards the center." It means that the direction of force is coming from the center. You might think of it like a weight tied to a string that you swing around your head. The force is coming from the string which is pulling on the weight from the center of the large circle you are creating with your motion. Cars driving on a curve and the orbit of the moon around the earth are additional examples of centripetal force as they use gravity to pull them towards the center.
Centrifugal means "fleeing from the center" in Latin. It is the push that a person going around a curve in a car feels toward the outside of the turn.
Flying has been done by birds for millions of years. Yet flight is still an amazing science; the ability to break away from gravity - or at least it appears that way. For birds it is all about the structure of their bones, wings and feathers. For humans that don't have those structural positives, we have to design machinery that mimics the science that allows a bird to fly.
Airplane wings are designed after bird wings with a special curve in an effort for air to "lift" the plane. As a wing moves through the air, the curve of the top of the wing speeds up the air as it flows across. This reduces the pressure of the air over the top of the wing. Air on the underside then pushes upward. This is known as the Bernoulli's Principle, for the famous scientist Daniel Bernoulli, who determined that the higher the velocity of a liquid, the lower the pressure it exerts. This also applies to air and is what allows kites, airplanes, space shuttles, and even plastic discs to fly. It helps fans blow air and propellers to push boats through the water.
The gravity of the earth is intense enough to make escaping its pull a real challenge for scientists when it comes to heading out into space. A rocket or other craft must reach a very high speed or velocity in order to break awayfrom the pull of the earth. Escape velocity is the minimum speed an object must attain in order to leave the gravitational pull of another body. In most cases this is referring to the earth. That was the first body of mass that scientists could learn how to leave. Later on it was the moon. Perhaps some day we will learn to leave other bodies in our solar system or galaxy. For earth, escape velocity is about 7 miles per second or 25,000 miles per hour.