Gravity is the force that attracts two bodies toward each other, the force that causes apples to fall toward the ground and the planets to orbit the sun. The more massive an object is, the stronger its gravitational pull.

Gravity is one of the four fundamental forces, along with the electromagnetic, strong and weak forces.

It is what causes objects to have weight. When you weigh yourself, the scale tells you how much gravity is acting on your body. The formula for determining weight is: weight equals mass times gravity. On Earth, gravity is a constant 9.8 meters per second squared, or 9.8 m/s2.

Historically, philosophers such as Aristotle thought that heavier objects accelerate toward the ground faster. But later experiments showed that wasn't the case. The reason that a feather will fall more slowly than a bowling ball is because of the drag from air resistance, which acts in the opposite direction as the acceleration due to gravity.

Sir Isaac Newton developed his Theory of Universal Gravitation in the 1680s. He found that gravity acts on all matter and is a function of both mass and distance. Every object attracts every other object with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them. The equation is often expressed as:

Fg = G (m1 ∙ m2) / r2

• Fg is the gravitational force
• m1 and m2 are the masses of the two objects
• r is the distance between the two objects
• G is the universal gravitational constant

Newton's equations work extremely well to predict how objects such as planets in the solar system behave.

Newton published his work on gravitation in 1687, which reigned as the best explanation until Einstein came up with his theory of general relativity in 1915.  In Einstein's theory, gravity isn't a force, but rather, the consequence of the fact that matter warps space-time. One prediction of general relativity is that light will bend around massive objects.

Fun facts

• The gravity on the moon is about 16 percent of that on Earth, Mars has about 38 percent of Earth's pull, while the biggest planet in the solar system, Jupiter, has 2.5 times the gravity of Earth.
• Though nobody "discovered" gravity, legend has it that famous astronomer Galileo Galilei did some of the earliest experiments with gravity, dropping balls off the Tower of Pisa to see how fast they fell.
• Isaac Newton was just 23 years old and back from university when he noticed an apple falling in his garden and began unraveling the mysteries of gravity. (It's probably a myth that the apple bonked him on the head though.)
• An early measure of Einstein's theory of relativity was the bending of starlight near the sun during a solar eclipse on May 29, 1919.
• Black holes are massive collapsed stars with such strong gravity that even light cannot escape from it.
• Einstein's theory of general relativity is incompatible with quantum mechanics, the bizarre laws that govern the behavior of the tiny particles — such as photons and electrons — that make up the universe.