Physics
Newton's Law of Universal Gravitation
Quick fact
Newton formulated this law in 1687, but the gravitational constant G was not measured accurately until over a century later by Henry Cavendish in 1798.
Why this is interesting
Ever wondered what connects a falling apple to the Moon's orbit around Earth? The same invisible force does both, and it reaches across the entire universe.
Read the full explanation
Understanding Newton's Law of Universal Gravitation
Imagine you and a friend standing a few meters apart. According to Newton's law, you are attracted to each other gravitationally—but the force is so tiny you never notice. Now replace your friend with a planet. Mass matters: the larger the masses, the stronger the pull. Distance matters too: double the separation, and the force becomes four times weaker. This is the 'inverse square' idea. Think of a light bulb: doubling your distance from it makes the light spread out over four times the area, so it appears dimmer. Gravity works the same way. Newton realized that the same force that pulls an apple to the ground also keeps the Moon in orbit around Earth. He didn't just describe gravity on Earth; he made it universal.
A deeper explanation
The law is expressed mathematically as F = G (m1 m2) / r^2, where F is the gravitational force, G is the gravitational constant, m1 and m2 are the masses, and r is the distance between their centers. The constant G ensures the units work out and its tiny value (about 6.674×10⁻¹¹ N·m²/kg²) explains why everyday objects don't stick together. Why does gravity obey an inverse square? One way to see it: imagine gravity spreading out from a mass in a sphere; the surface area of a sphere increases with the square of the radius, so the force per unit area—the intensity—drops as 1/r². This law matters because it predicts planetary orbits with astonishing precision, explains tides (due to the Moon's differential pull), and even allows us to calculate the mass of distant stars. It was the first true universal law of physics, showing that the same rules govern Earth and the heavens. Newton's law remains accurate for most everyday and astronomical situations, though Einstein's general relativity refines it for extreme gravity.