Misconceptions About Gravity That Even Adults Carry Around

The ISS orbits at roughly 400 kilometers above Earth's surface. At that altitude, gravitational acceleration is still about 90% of what you experience on the ground. The station doesn't fall straight down because it's also moving horizontally at approximately 28,000 km/h — fast enough that as it falls, Earth's surface curves away beneath it at the same rate. Everything inside the station — crew, equipment, floating water droplets — is falling together, which is why nothing presses against anything else. Physicists call this microgravity, not zero gravity.

Mass is measured in kilograms and stays constant whether you are on Earth, the Moon, or floating between galaxies. Weight is measured in newtons (or pounds-force in everyday US usage) and equals mass multiplied by the local gravitational acceleration. An astronaut with a mass of 80 kg weighs about 784 newtons on Earth but only about 130 newtons on the Moon, where gravity is roughly one-sixth as strong. Their mass doesn't change at all — only the gravitational pull on that mass does. This distinction matters enormously in fields from medicine to aerospace engineering.

This idea dates back to Aristotle and persisted for nearly two thousand years until Galileo challenged it — famously, though possibly apocryphally, by dropping objects from the Leaning Tower of Pisa. The mathematical reason comes directly from Newton's second law: gravitational force is proportional to mass, but so is inertia (resistance to acceleration). The two effects cancel out precisely, leaving the same downward acceleration — about 9.8 m/s² on Earth's surface — for every object. The reason a feather and a hammer don't hit the ground simultaneously when you drop them outdoors is air resistance, not gravity. NASA demonstrated this beautifully on the Moon in 1971, where Apollo 15 astronaut David Scott dropped a feather and a hammer simultaneously in the near-vacuum, and both landed at the same moment.

According to Newton's law of universal gravitation, the gravitational force between two objects decreases with the square of the distance between them — a relationship called an inverse-square law. Double the distance and the force drops to one-quarter; triple it and the force becomes one-ninth. But no matter how large the distance gets, the force approaches zero asymptotically — it never actually reaches it. The gravitational pull of the Milky Way's supermassive black hole, Sagittarius A*, reaches across roughly 26,000 light-years to influence our own solar system in measurable (if extremely small) ways. Gravity truly has no off switch. You can explore how this plays out on inclines in our article on why your car feels heavier going uphill.

In the Newtonian picture, gravity is a force transmitted instantaneously across space — useful for most engineering problems, but ultimately incomplete. Einstein's 1915 general theory of relativity replaced this with a geometric model: massive objects warp the four-dimensional fabric of spacetime around them, and other objects simply follow the straightest possible path through that curved space. What we experience as a gravitational 'pull' is actually the geometry of spacetime directing motion. This framework predicts effects Newton's model cannot, including gravitational time dilation (clocks run slightly slower in stronger gravitational fields — a real correction built into GPS satellites) and gravitational waves, ripples in spacetime detected for the first time by LIGO in 2015. For a complementary look at how false models mislead us in other sciences, see our assumptions that lead new astronomers astray.