Physics
Newton's Law of Universal Gravitation
Quick fact
Newton formulated this law in the 1680s, but it took another century for the gravitational constant G to be measured. The force is incredibly weak — even between you and this screen, a tiny attraction exists, though far too small to feel.
Why this is interesting
Why does an apple fall straight down, yet the Moon seems to defy gravity by staying in the sky? The answer lies in one simple, universal rule that connects them both.
Read the full explanation
Understanding Newton's Law of Universal Gravitation
Imagine holding two magnets. If you bring them close, they pull together. Now picture that every object with mass — you, a planet, a star — behaves like a tiny magnet for gravity. Newton's law says that the gravitational force between any two objects depends on two things: how much mass they have (more mass = more pull) and how far apart they are (closer = stronger pull). Actually, the pull weakens with distance in a squared way: if you double the distance, the force becomes one-quarter as strong. This is called an inverse-square law. So the apple falls toward Earth because Earth has huge mass and the apple is near. The Moon, though massive, is far away and also moving sideways — its path curves around Earth, making it orbit rather than fall straight down.
A deeper explanation
The underlying mechanism is that gravity is a fundamental interaction between all matter. Newton's law is expressed as F = G (m1 m2) / r^2, where G is the gravitational constant, m1 and m2 are masses, and r is the distance between their centers. The inverse-square relation arises from the geometry of a sphere: as you move away from a mass, its influence spreads over an ever-larger spherical surface area. This law unified Kepler's empirical laws of planetary motion by showing that the same force governing falling apples also governs the orbits of planets. It allowed precise predictions of tides (due to Moon's pull) and later enabled the discovery of Neptune from deviations in Uranus's orbit. Newton's law remained the complete description of gravity until Einstein's general relativity refined it for extreme conditions, but for most everyday and astronomical purposes, it is highly accurate and profound.