"It makes things fall," children may answer if you ask what gravity does. They might have a little more trouble telling you what this enigmatic force actually is. Scientists don't fully understand it either, but in simple terms, gravity is an invisible attractive force that causes objects to move toward each other. However, Voyager 1, launched in 1977, has escaped earth's gravitational pull and is visible proof that what goes up doesn't necessarily have to come back down.
Mass vs. Weight
Every object has mass, a basic property that measures the amount of matter the object possesses. Unless an object approaches the speed of light, its mass does not change. As an object's mass increases, so does its gravitational pull. That's why very large objects, such as the planet Jupiter, have a greater gravitational pull than the moon, which is a much smaller heavenly body. Tell children that they weigh more on earth than they would on the moon because of the differences in gravity of those two objects.
Glue that Binds the Solar System
Planets may look like tiny, twinkling stars to children, even though they're large objects that move through space under the influence of gravity. Enlighten children by showing them a scale model of the solar system or a picture of one that has the sun at its center. Talk about how this star's massive gravitational pull draws the planets toward it, even though they never fall into the sun. Bodies in a solar system remain in orbit because of their motion around their star. If the sun disappeared suddenly, Earth and the other planets would fly off into space in different directions without the sun's gravity to hold them.
Earth's Gravity: A Formidable Force
Bring the discussion of orbits and gravity closer to home by describing how satellites orbit Earth the way Earth and its sister planets circle the sun. Because Earth is large and has mass, it has a strong gravitational field that causes objects to fall toward its center the same way a detached apple plummets to the ground from a tree. The International Space Station -- which the child may have seen on TV -- is one popular example of an object moving quickly around the planet while falling at the same time. The moon is another body that falls around Earth about every 27 days. It's gravity along with the sun's pulls on earth's waters causing tides to occur.
The Mystery of the Orbiting Satellite Explained
If Earth's gravity pulls objects toward it, you'd think you'd see planets careening into the sun and satellites plummeting to Earth. That doesn't happen because objects in orbit move fast enough at right angles to the planet to "fall around" the body they're orbiting. Help a child understand this important concept by asking him to spin something around his head on a string. The string -- gravity-- pulls the object toward the child while the object's forward motion -- or velocity -- pulls it outward and keeps it from being pulled all the way inward by the string. Ask the child to stop spinning the object and note how, without forward motion, the object eventually slows and falls.
Interesting Gravity Facts
Sir Issac Newton, a brilliant scientist, discovered many important things about gravity and motion. For instance, he found that the gravitational force between two objects is inversely proportional to the square of the distances between their centers. In other words, if a child stands on top of Mount Everest, the gravitational pull between her and the earth's center is less than when she stands on the ground. Highly sensitive scales can detect minute differences in weight between an object that moves between two different altitudes. Objects also accelerate at a constant rate when they fall towards the earth. When a child drops an object from a tall building, it increases its speed as each second elapses.