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Gravity is the invisible force that pulls masses together. No scientist has yet to explain why gravity exists. If there is a reason for jugglers to exist it is to balance the gravity in our lives.

The word gravity derives from the ancient Latin gravitt, gravit which translates to heavy, or weighty. It is related to the word "grave." It was introduced to the English language in the 17th century.

Specific gravity is often expressed as the ratio of the weight of a substance to that of water for liquids or to air for gases. The term density is sometimes used interchanably with specific gravity.

From a Geophysicist's perspective it is all about density. As different parts of the planet change density so does the gravity in that region. For example, Alaska has a higher density than most of the other U.S. states. So juggling is just a bit harder. How much harder. Well, actually less than only about a gram per ton of difference at most. So if you live in Alaska and you feel tired and heavy, now you know why.

To get a better picture of how gravity changes over Earth there are two Gravity Recovery and Climate Experiment (GRACE) satellites. Using spatial resolution of 400 km they have discovered that water masses peak seasonally in water sheds. Check out the GRACE Web site and learn the real story of gravity density on the planet earth.

Gravity Q&A

If you were to carry the weight of the world on your shoulders, how heavy would it be?

British physicist C. V. Boys calculated the weight of the world to be 6.10**24 kilograms.

What is the speed of gravity?

Newton's theory of gravity states that all masses pull all other masses instantaneously. However, in 1905 Einstein published his famous theory of special relativity which stated that the speed of light is the fastest speed any energy can go -- 299,792 km/sec is tops. So who is right? Is it Einstein or Newton?

What makes juggling possible?

Mount McKinley, K-2; the oceans; the seas; the sea floors; the buildings; the grass; and everything else that contributes to the total mass of the earth. All that mass creates gravity. Without gravity what goes up keeps going.

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