Evolution of Ideas on Motion and Gravity

Key Insight

Early beliefs, attributed to Aristotle, held that a body's natural state was rest, and movement required a force. This also implied heavier bodies would fall faster due to a greater pull toward Earth, and universal laws could be derived purely through thought, without observation. Galileo challenged these notions through experimentation. For instance, by rolling balls of different weights down a smooth slope, he demonstrated that all bodies increase speed at the same rate, regardless of weight, if air resistance is negligible. An example showed a ball on a slope dropping one meter for every ten meters traveled would increase its speed by approximately one meter per second each second. Astronaut David R. Scott later confirmed this on the moon, where there is no air, by dropping a feather and a lead weight which hit the ground simultaneously.

Building on Galileo's measurements, Newton formulated his laws of motion. His first law, published in 'Principia Mathematica' in 1687, explicitly stated that a body not acted upon by any force will continue moving in a straight line at a constant speed. This redefinition clarified that the true effect of a force is to change a body's speed, or accelerate it, rather than merely setting it in motion. Newton's second law further detailed that a body's acceleration is directly proportional to the applied force and inversely proportional to its mass. For example, a car's acceleration is greater with a more powerful engine but smaller if the car is heavier, for the same engine.

Newton also discovered the law of gravity, stating that every body attracts every other body with a force proportional to the mass of each body, and this force diminishes with increasing distance. For example, the gravitational attraction of a star is one-quarter that of a similar star at half the distance. This explained why all bodies fall at the same rate: a body twice as heavy has twice the gravitational force pulling it down, but also twice the mass, and these two effects precisely cancel each other according to Newton's second law, resulting in identical acceleration. Crucially, Newton's laws implied there is no unique absolute standard of rest, only relative motion (e.g., a tram moving relative to Earth), thereby dismantling Aristotle's belief in a preferred state of rest and making it impossible to assign an absolute position in space to an event.