The two factors that influence gravitational force are mass and distance. More specifically, the gravitational pull between any two objects depends on the product of their masses and the square of the distance between their centers. These two factors form the foundation of Newton’s law of universal gravitation, which describes how every object in the universe attracts every other object.
Mass: The More Matter, the Stronger the Pull
Gravitational force is directly proportional to the mass of both objects involved. If you double the mass of one object, the gravitational force between the pair doubles. If you double the mass of both objects, the force quadruples. This is because gravity depends on the product of the two masses, not just one of them.
This proportional relationship is why massive objects like planets, stars, and moons dominate gravitational interactions. The Sun holds the entire solar system in orbit because its mass is enormous compared to everything around it. Meanwhile, two tennis balls sitting on a table technically attract each other gravitationally, but their masses are so small that the force between them is undetectable without extremely sensitive instruments.
An important distinction here: mass is not the same thing as weight. Mass is the amount of matter in an object, measured in kilograms, and it stays the same regardless of location. Weight is the gravitational force acting on that mass, measured in newtons, and it changes depending on where you are. You have the same mass on the Moon as on Earth, but your weight on the Moon is only about one-sixth of what it is here.
Distance: The Inverse Square Relationship
The second factor is the distance between the centers of the two objects. Gravitational force is inversely proportional to the square of that distance. This “inverse square” relationship means gravity weakens rapidly as objects move apart. Double the distance between two objects, and the gravitational force drops to one-quarter of what it was. Triple the distance, and it falls to one-ninth.
This square relationship is crucial. It means that small changes in distance have outsized effects on gravitational force. Moving just a little farther away reduces gravity more than you might intuitively expect, while moving closer increases it dramatically. The distance measured is always from center to center, not surface to surface. For a person standing on Earth, that distance is essentially Earth’s radius: about 6,371 kilometers from the surface to the core.
How These Factors Work Together
Newton’s law of universal gravitation combines both factors into a single relationship. The gravitational force between two objects equals a constant (called the gravitational constant, G) multiplied by the product of the two masses, divided by the square of the distance between them. The gravitational constant has a value of 6.67430 × 10⁻¹¹, and it’s the same everywhere in the universe. It’s an extremely small number, which is why gravitational forces between everyday objects are negligible.
The beauty of this law is that it applies universally. The same equation that describes an apple falling from a tree also describes the Moon orbiting Earth and galaxies pulling on each other across millions of light-years. Newton’s key insight was that gravity isn’t something unique to Earth. Every object with mass attracts every other object with mass, in exact proportion to how much matter they contain and how far apart they are.
Earth vs. the Moon: A Clear Example
Comparing Earth and the Moon illustrates how mass and distance shape gravitational force in practice. Earth has a mass of about 5.97 × 10²⁴ kilograms, while the Moon’s mass is only 7.35 × 10²² kilograms, roughly 81 times less. Earth’s equatorial radius is 6,371 kilometers compared to the Moon’s 1,737.5 kilometers.
The result: surface gravity on the Moon is 1.624 m/s², while Earth’s is 9.807 m/s². That’s about one-sixth the pull. Both factors contribute. The Moon has far less mass (weaker pull), but it also has a smaller radius, meaning you’re standing closer to its center (stronger pull). The mass difference wins out, but the smaller radius partially offsets it. If the Moon had the same radius as Earth but kept its current mass, its surface gravity would be even weaker than one-sixth.
Tides: When Distance Beats Mass
Earth’s ocean tides offer a striking example of how distance can outweigh mass. The Sun is 27 million times more massive than the Moon, which should give it an overwhelmingly stronger gravitational influence. But the Sun is also about 400 times farther away from Earth than the Moon is. Because tidal forces weaken with the cube of the distance (even faster than regular gravitational force), the Sun’s tidal effect on Earth is actually about 50% less than the Moon’s. The Moon, despite being tiny compared to the Sun, dominates our tides because it’s so much closer.
Orbits and Satellites
Both factors also determine how fast a satellite needs to travel to stay in orbit. A satellite’s orbital speed reflects the mass of the planet it orbits and its distance from that planet’s center. Closer satellites must travel faster because they experience stronger gravitational pull and need more speed to avoid being pulled down. The International Space Station, orbiting roughly 400 kilometers above Earth’s surface, travels at about 28,000 kilometers per hour. A GPS satellite, orbiting much farther out at around 20,200 kilometers, moves at roughly 14,000 kilometers per hour.
Increase the planet’s mass and a satellite at the same distance must orbit faster. Increase the distance and it can orbit more slowly. This is why the outer planets of our solar system take so much longer to complete an orbit around the Sun, not just because they have farther to travel, but because they move more slowly at that greater distance.
Einstein’s Deeper Explanation
Newton’s framework works extremely well for nearly every situation you’ll encounter, but Einstein’s general theory of relativity offers a deeper explanation of why mass and distance matter. In Einstein’s view, mass and energy curve the fabric of spacetime itself. The more mass an object has, the more it warps the space around it. Other objects then follow the curves in that warped space, which we experience as gravitational attraction. Distance matters because spacetime curvature is strongest close to a massive object and flattens out farther away.
For everyday physics and most calculations, Newton’s two factors, mass and distance, give you everything you need. Einstein’s model becomes essential only in extreme environments: near black holes, at speeds approaching the speed of light, or when precision matters down to fractions of a nanosecond, as it does for GPS satellites correcting for time differences caused by Earth’s gravity.