Drop a stone and it falls. The Earth pulls it down. Gravity is the force of attraction between objects with mass. It is the weakest of the four fundamental forces, but it has infinite range and it is always attractive. It holds you to the ground, keeps the moon in orbit, and shapes the large-scale structure of the universe. Newton described it as a force. Einstein described it as the curvature of spacetime. Both descriptions work, depending on the scale and precision needed.

Gravity follows an inverse square law. Double the distance between two objects and the force falls to one quarter. That is why the moon orbits the Earth, and the Earth orbits the sun, at stable distances. Gravity is also the reason objects fall at the same rate regardless of mass. Drop a feather and a hammer on the moon, as Apollo 15 astronaut David Scott did, and they hit the ground at the same time. There is no air resistance to slow the feather. Gravity is responsible for tides, for the formation of stars and planets, and for the fact that you can walk without floating away.

Gravity facts

• Definition. Attraction between objects with mass.
• Weakest force. But infinite in range.
• Inverse square. Force falls with the square of distance.
• Acceleration. 9.8 m/s² near Earth's surface.
• Einstein. Curvature of spacetime.

Gravity is not the same as magnetism. Magnetism acts on charged particles. Gravity acts on all mass and energy.