Gravity is one of the four fundamental forces in nature. It's the force that attracts any two objects with mass toward each other. This invisible force is what keeps our feet on the ground, causes objects to fall when dropped, and holds the planets in orbit around the sun. Every object with mass, no matter how small, exerts a gravitational pull on every other object with mass.
Sir Isaac Newton formulated the universal law of gravitation, which states that every particle attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. The formula F equals G times m1 times m2 divided by r squared shows this relationship mathematically. G is the gravitational constant, a fundamental constant of nature. As masses increase, the force increases. As distance increases, the force decreases rapidly due to the inverse square relationship.
On Earth, gravity creates an acceleration of 9.8 meters per second squared for all objects. This means that weight equals mass times gravity. It's important to understand that mass and weight are different - mass is the amount of matter in an object, while weight is the gravitational force acting on that mass. A famous discovery is that all objects fall at the same rate in a vacuum, regardless of their mass. This was demonstrated by Galileo and later confirmed by Apollo astronauts who dropped a hammer and feather on the Moon, where there's no air resistance.
In space, gravity governs the motion of all celestial bodies. Planets orbit the sun, moons orbit planets, and artificial satellites orbit Earth. The orbital velocity depends on the mass of the central body and the distance from its center. Objects in orbit are actually in continuous free fall - they're falling toward the central body but moving fast enough sideways that they keep missing it. This is why astronauts experience weightlessness in space stations. They're not beyond Earth's gravity, they're simply in free fall along with their spacecraft. Escape velocity is the minimum speed needed to break free from a gravitational field completely.
Einstein revolutionized our understanding of gravity with his theory of general relativity. Instead of thinking of gravity as a force, Einstein showed that massive objects actually bend the fabric of spacetime itself. Imagine a heavy ball placed on a stretched rubber sheet - it creates a depression that causes other objects to roll toward it. This is analogous to how mass curves spacetime. Objects then follow the straightest possible paths through this curved spacetime, which we perceive as gravitational attraction. This theory explains phenomena that Newton's law couldn't fully account for, such as the precise orbit of Mercury and the bending of light around massive objects. It also predicts the existence of gravitational waves and time dilation effects near massive objects.