Gravity

Gravity is the force that draws an object or living thing towards another. The word comes from the Latin gravis, meaning "heavy".

Objects that have mass pull on each other. We call this 'pull' gravity. The strength of the pull between two objects depends on two things. The first is the mass of both of the objects. The larger the masses the stronger the pull of gravity. The second is the distance between the objects. The larger the distance the weaker the pull of gravity. Both of the objects will feel this pull equally. The formula for the strength of this pull is:

$\text{Gravity} = \text{Universal gravitational constant}\times\frac{\text{First mass}\times\text{Second mass}}{\text{Distance}^2}$

You and the Earth have mass, so the Earth pulls on you and you pull on the Earth. This pull is what we call weight. The Moon has 1/6 the mass of the Earth. So if you were on the moon the pull between you and the moon would be six times smaller. This means your weight on the Moon would be six times smaller. Your mass is always the same. It does not matter if you are on the Earth, or the Moon, or anywhere else, your mass does not change.

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. Spacecraft use rockets to move.

History

Sir Isaac Newton is said to have discovered gravity. He worked out the laws that gravity seemed to follow. These rules are simple but surprisingly accurate; they have been used to work out how to get space probes to planets across billions of miles.

In 1915, Albert Einstein showed that gravity could be explained as the bending of space (and time) by mass. A planet makes a dent in space that makes things fall towards it or curve around it. The moving object is passing through more space, in the same amount of time, at the same speed. In fact there is also an effect on time: Einstein predicted that time slows down near a large mass, and this has been verified using very accurate clocks on satellites. Some scientists have looked at Einstein's theories and found ideas in them that gravity could not actually be a force. However, they have not yet come up with a more precise definition.

Recent studies suggest that frame-dragging may be responsible for gravity. Frame dragging is a theory which states that particles interact with space, and are constantly changing energy levels. They are performing tests such as Gravity Probe B to see if this theory is correct.

Some people think that gravity is carried by a particle called the graviton, but so far no evidence for this has been found.

Other pages

• Gravitation
• General relativity
• Mass
• Newton's law of universal gravitation
• Newton's laws of motion
• Weight
• Physics

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. ed.). Brooks/Cole. ISBN 0-534-40842-7. 
• Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed. ed.). W. H. Freeman. ISBN 0-7167-0809-4. 

Other websites

• Gravity - a chapter from an online textbook
• Newton's Law of Universal Gravitation on Project PHYSNET
• Gravity Probe B Experiment The Official Einstein website from Stanford University
• Center for Gravity, Electrical, and Magnetic Studies
• Alternative theory of gravity explains large structure formation -- without dark matter PhysOrg.com