On This Page
Overview
Universal gravitation is the principle that every mass attracts every other mass. The strength of the gravitational attraction depends on the masses involved and the distance between them. In classical physics, Newton's law of universal gravitation provides the framework for understanding falling objects, planetary motion, moons, satellites, tides, and many large-scale astronomical structures.
Core Idea
Gravity is not limited to Earth. Every object with mass attracts every other object with mass. The force may be tiny for ordinary objects, but it becomes extremely important when one or both objects have large mass, such as planets, moons, stars, and galaxies.
Mass and Distance
Gravitational force increases when mass increases and decreases as distance increases. If two objects are more massive, their attraction is stronger. If they are farther apart, their attraction is weaker. Distance is especially important because gravitational force follows an inverse-square relationship.
Inverse-Square Relationship
An inverse-square relationship means the force weakens with the square of the distance. Doubling the distance between two masses makes the gravitational force one fourth as strong. Tripling the distance makes it one ninth as strong.
Gravity Near Earth
Near Earth's surface, gravity gives objects a nearly constant downward acceleration in simple introductory models. This is why falling objects are often analyzed using approximately constant gravitational acceleration when height changes are small compared with Earth's radius.
Connection to Orbital Motion
Orbital motion occurs when gravity continuously pulls an object inward while the object also has sideways motion. The object keeps falling toward the central body, but its sideways motion causes it to keep missing the surface.
Weight and Mass
Mass is the amount of matter and a measure of inertia. Weight is the gravitational force on a mass. An object's mass is the same in different locations, but its weight can change if the local gravitational field changes.
Tides and Gravitational Differences
Tides occur because gravity can pull differently on different parts of an extended body. The Moon's gravitational influence on Earth is a major cause of ocean tides, with the Sun also contributing.
Common Mistakes
A common mistake is thinking gravity only exists near Earth. Another is assuming astronauts in orbit have no gravity acting on them. In orbit, gravity is still acting, but the spacecraft and astronauts are in continuous free fall around Earth.
Why This Matters in Physics
Universal gravitation connects mechanics to astronomy. It explains falling motion, planetary systems, satellite paths, tides, orbital energy, escape velocity, and the structure of many cosmic systems.