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With Explore the Planets, investigate the planets, their moons, and understand the processes that shape them. By G. Jeffrey Taylor, Ph.D. See our DVD collection.
An artist's rendering of some of the forces of the universe. The apple falling is of course from the story of Isaac Newton discovering the law of gravity as an apple fell from a tree he was sitting underneath.
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Gravity Definition Page

Gravity is one of the universal forces of nature. It is an attractive force between all matter, and is very weak as compared to the other forces of nature. The gravitational force between two objects is dependent on their masses, which is why we can only see gravity in action when at least one of the objects is very large (like the Earth).

Isaac Newton was the first scientist to define gravity mathematically when he formulated his law of universal gravitation, which says that the force of gravity between two objects (F) equals the mass of one object multiplied by the mass of the second object multiplied by the Gravitational constant (G, equal to 6.67 x 10^-11 Newton meters ^2/kg^2), all divided by the square of the distance between the two objects. This means that gravity is strongest between two very large objects, and gets much weaker as these objects get further apart.

One of the applications of this law is the concept of ‘escape velocity’, which is the velocity an object needs to achieve to escape the gravitational pull of another object (like the Earth). Escape velocity is equal to the square root of the quantity 2GM/R, where M is the mass of the object you’re trying to escape from, G is the Gravitational constant, and R is the radius of the object you’re escaping from. If we plug in the measurements we have for the planet Earth, we see that Earth’s escape velocity is about 11 km/s. This means that if you could throw a baseball at 11 km/s, it would never come down!

The concept of escape velocity is especially interesting when you consider black holes. These objects are extremely dense and very small, so M is huge and R is tiny. When we calculate the escape velocity for these objects, we find that the number is actually the speed of light, so not even light can get out! The size of these objects is almost unbelievable--if the Earth were to collapse so that it was this dense, it would end up being only about 2 cm in diameter!

Last modified May 6, 2008 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA