Basic Information

On this page you will find things that you may or may not know about gravity.

(Put cursor on blue vocabulary words to view their meanings, or on the pictures if you're not sure what they are.)

Gravity is a that acts between any two of in the universe to pull them together.

When something happens again and again in the same way, the event takes place according to a law.

The first scientist to develop the law of gravity was Isaac Newton. Isaac Newton was an English physicist and mathematician who lived from 1642 – 1727.

Newton was curious about the motion of the moon. In 1665 he realized that the force which pulled an apple toward the earth and the force that kept the moon in its were the same. This led him to develop the law of gravity.

Everything on earth is pulled down toward the earth’s surface. Gravity holds us on earth. The pull of gravity also keeps the moon in orbit around the earth and the earth in orbit around the sun.

All bodies of matter exert a force of gravity. The strength of gravity depends on the of the two bodies and the distance between them.

The earth’s mass is greater than the moon’s. So the earth’s pull on the moon is greater than the moon’s pull on the earth. Gravity is stronger between two bodies that are closer together and weaker between bodies that are farther apart.

Newton worked out an equation for gravity. The force of gravity equals the mass of one body multiplied by the mass of the other body divided by the distance multiplied by itself. OR

Our bodies are pulled down to the earth’s surface by gravity. This pull gives us our weight.

Gravity exerts 9.81 on every 2.2 pounds or 1 kilogram of matter. A scientist would say that an astronaut’s weight is 154 pounds or 70 kilograms and his mass is 9.81 times his weight in kilograms.

Mass stays the same no matter what the force of gravity is. Out in space, in the absence of gravity, an astronaut will float freely with no weight but his mass will still be the same 154 pounds. On the moon, the mass will still remain the same but the astronaut will weigh 26 pounds or 12 kilograms because the gravitational pull is 1/6 as strong.

The astronaut’s weight changes from place to place, but the mass remains the same. The weight of an object on the earth changes from place to place also even though the mass stays the same. An object at the equator weighs less than the same object would weigh at the North Pole.

In over 300 years of scientific study, scientists have learned a lot about gravity. But gravity can also still be quite a mystery.