Gravity

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Gravity is a force. It makes things move toward each other. On the Earth, we see that it makes things fall toward the ground. Gravity may be caused by a theoretical particle known as a graviton.

We say that things that are pulled by gravity have mass. The larger the masses, the bigger the pull of gravity. The farther apart the things are, the smaller the pull of gravity.

On the Earth, we can measure the pull of the Earth's gravity on something as weight. But, the pull of gravity from the Moon is only about 1/6 the pull of gravity from the Earth. So, an object will have the same mass on the Moon as it will on the Earth, but because of the Moon's smaller gravitational pull (that is, its smaller force) the object will weigh less on the Moon.

Things that are falling still have mass, but we cannot measure their weight, so we say that they are "weightless". Astronauts and spacecraft in outer space can be weightless. They look like they are floating. Really they are falling in an orbit around the Earth.

They still have mass, so we still have to push them or pull them to make them go or stop. Astronauts inside spacecraft use their arms and legs to jump or to stop. When astronauts are working outside in space, they use very small rockets to move. Spacecraft use bigger rockets to move.

Sir Isaac Newton is said to have discovered gravity when he saw an apple fall from a tree. He wondered what made this happen.

[change] See also

[change] References

Halliday, David; Robert Resnick; Kenneth S. Krane (2001). Physics v. 1. New York: John Wiley & Sons. ISBN 0-471-32057-9.
Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers, 6th ed., Brooks/Cole. ISBN 0-534-40842-7.
Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics, 5th ed., W. H. Freeman. ISBN 0-7167-0809-4.