Gravity is one of the four fundamental forces in nature. It's the force that attracts all objects with mass toward each other. We experience gravity every day when objects fall to the ground, but gravity also acts between all masses in the universe. The Earth pulls on the apple, causing it to fall, and the Earth and Moon pull on each other, keeping the Moon in orbit. This universal attraction is what we call gravitational force.
Newton's universal law of gravitation 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 equation F equals G times m1 times m2 divided by r squared describes this relationship mathematically. G is the gravitational constant, a fundamental constant of nature. When masses increase, the gravitational force increases proportionally. However, when distance increases, the force decreases rapidly according to the inverse square law - double the distance means one-fourth the force.
A gravitational field is the region of space around a massive object where other masses experience gravitational force. We can visualize this field using field lines that point toward the center of mass. The density of these field lines indicates the strength of the gravitational field - closer lines mean stronger field. The field strength equation g equals GM over r squared shows that field strength decreases with the square of distance from the mass. Different celestial bodies create fields of different strengths. The Sun has the strongest field, followed by Earth, then the Moon. Test masses placed in these fields will experience forces proportional to the local field strength.
Orbital mechanics describes how objects move in space under gravitational influence. For circular orbits, gravitational force provides exactly the centripetal force needed to keep objects in orbit. The orbital velocity equation shows that velocity equals the square root of GM over r, where closer orbits require higher speeds. Earth orbits faster than Mars because it's closer to the Sun. Satellites orbit Earth using the same principles - the closer the satellite, the faster it must travel. Elliptical orbits are also possible, like those of comets, where objects speed up when closer to the central mass. Escape velocity is the minimum speed needed to break free from gravitational pull completely.
Gravity acts on all scales throughout the universe, but its relative importance varies dramatically. At the microscopic scale, gravitational forces between atoms are negligible compared to electromagnetic forces. At human scales, the gravitational attraction between two people is incredibly weak, about one billionth of a Newton, while Earth's gravity exerts hundreds of Newtons on each person. At planetary scales, gravity becomes the dominant force, controlling orbital motion and planetary structure. At cosmic scales, gravity shapes the large-scale structure of the universe, forming galaxies, galaxy clusters, and the cosmic web. Tidal effects demonstrate how gravity varies with distance - the Moon's gravity pulls more strongly on the near side of Earth than the far side, creating ocean tides. This universal force connects the smallest interactions to the grandest structures in the cosmos.