By Hobie Thompson and Sarah Havern

The History of Gravity

• Humans only recently (like in the last 300 years) realized what Gravity is all about.
• Greek philosophers thought that the planets and stars were part of the gods' realm and followed a "natural motion." They did not realize that Gravity is involved. The Greeks' ideas stuck around until the 16th century.
• Beginning in the 1500s, though, astronomers like Galileo and Brahe discovered that the earth and other planets revolved around the sun. Kepler showed that they moved in an eliptical orbit, not a circle. The question was why.

Sir Isaac Newton -- The Discoverer of Gravity!

• Sir Isaac Newton was an English mathematician and mathematician and physicist who lived from 1642-1727.
• The legend is that Newton discovered Gravity when he saw a falling apple while thinking about the forces of nature.
• Whatever really happened, Newton realized that some force must be acting on falling objects like apples because otherwise they would not start moving from rest.
• Newton also realized that the moon would fly off away from Earth in a straight line tangent to its orbit if some force was not causing it to fall toward the Earth. The moon is only a projectile circling around the Earth under the attraction of Gravity.
• Newton called this force "gravity" and determined that gravitational forces exist between all objects.
• Using the idea of Gravity, Newton was able to explain the astronomical observations of Kepler.
• The work of Galileo, Brahe, Kepler, and Newton proved once and for all that the Earth wasn't the center of the solar system. The Earth, along with all other planets,orbits around the sun.
• Two astronomers, J.C. Adams and U.J.J. LeVerrier, later used the concept of Gravity to predict that the planet Neptune would be discovered. They realized that there must be another planet exerting a gravitational force on Uranus because Uranus had odd perturbations in its orbit. (Perturbations are deviations in orbits.)

Albert Einstein -- The General Theory of Relativity

• Einstein developed a whole new idea about Gravity.
• According to Einstein, Gravity arises from the "warping" of space and time.
• Einstein's new theory of Gravity explains a number of phenomena that would violate Newton's theory. For example, light bends when passing near massive objects like the Sun. And a clock raised above the Earth speeds up relative to a clock on the surface.

Basic Facts About Gravity

• Gravity is a force of attraction that exists between any two objects. There is a force of gravity between the sun and the Earth, between the Earth and us, and even between two marbles.
• Projectiles, satellites, planets, galaxies, and clusters of galaxies are all influenced by Gravity.
• Gravity is the weakest of the four known forces of nature, yet the most dominant force. Even though it's the weakest force, Gravity holds together entire solar systems and galaxies!
• The law of universal gravitation says that every object attracts every other object with a force that, for any two objects, is directly proportional to the mass of each object and inversely proportional to the square of the distance between the two objects.
• The gravitational formula is
• This is an example of the "inverse-square law": gravitational force varies as the inverse square of the distance between the two objects. As a result, the effect of Gravity falls rapidly as the distance increases between two objects.
• The best current estimate of G is that it eqauls about 6.67259 10-11 newton-square meter per square kilogram.
• The gravitational field for a planet, g, equals GM/Rsquared, where G is the mass of the planet and R is the distance of the object from the center of the planet (the planet's radius if the object is on the surface of the planet). This means that Gravity is greater where a planet is more massive and where it has a smaller radius. So even though Mars has only about 1/10th of the mass of the Earth, the gravitational force on the surface of Mars is more than 1/10th that on the surface of the Earth -- because Mars' surface is closer to the planet's surface!
• The force of attraction between you and the Earth is your weight.
• Gravity determines the "escape speed" for an object like a rocket. The stronger the gravitational pull of the object, the larger the escape speed. The following chart shows the escape speed for the sun, two planets, and the Earth's moon.

Gravity Fun Facts

• The first reliable measurement of G was made by Henry Cavendish in 1798! He calculated that G was equal to 6.754 10-11 newton-square meter per square kilogram (compared to today's calculation of 6.67259).
• Some theories suggest that G is changing over time and that G varies somewhat in different regions of space!
• No matter how great the distance between an object and the Earth, the Earth's gravitational force does not drop to 0. The gravitational influence of every object is exerted through all space.
• Earth is round because of Gravity. Earth attracted itself together before it became solid.
• There is gravitational attraction between two marbles, but we don't notice it because the force between each of the marbles and the Earth is so much greater.
• A person who weighs 100 pounds on the Earth would weigh only 17 pounds standing on the same scale on the moon.
• According to Albert Einstein, there is no difference between the acceleration of Gravity and any other acceleration. If you were in a rocket accelerating at 32.2 ft/sec2, it would feel just like gravity on the Earth.
• There are no known anti-gravity devices (ah shucks)! But the effects of Gravity can be canceled by free-fall or by placing objects in orbit.

Other Useful Web Sites on Gravity

*A web presentation on Gravity. (5 out of 5)

Exploring Gravity. (4 out of 5)

NASA's Gravity Page (5 out of 5)

A page on gravatation. (3 out of 5)

Institute for gravation research. (3 out of 5)

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