What is gravity?
Gravity is a fundamental phenomenon associated with the attraction between objects with mass. It explains why Earth holds us and its atmosphere, why the Moon orbits Earth, and why planets move around the Sun.
Mass, weight and distance
The attraction between two bodies increases with their masses and decreases with the distance between them. Mass is a property associated with the amount of matter in an object; weight is the gravitational force on it at a particular location. A person’s mass therefore stays the same on the Moon while their weight changes. Near Earth’s surface, freely falling objects accelerate at about 9.8 m/s²; air resistance can alter actual motion. Gravity should not be thought of only as a universal downward direction: local down points approximately toward Earth’s center.
Orbits and general relativity
An orbiting satellite is continually falling toward Earth, but its sideways speed keeps it from hitting the surface. Astronauts appear weightless because they and their spacecraft fall together, not because gravity disappears. Newton’s law works very well for many everyday and spaceflight calculations. Einstein’s general relativity says mass and energy affect the geometry of spacetime and provides a broader account for strong gravity and precise measurements. Lunar and solar gravity, together with Earth’s rotation, contribute to tides.