A swinging lamp and a pulse
According to a story told by one of his students, the young Galileo Galilei was sitting in the cathedral in Pisa in the 1580s when he noticed a lamp swinging on its chain. Timing the swings against his own pulse, he realised that each swing took the same time whether it was wide or narrow. The Dutch scientist Christiaan Huygens used that idea to build the first PENDULUM clock in 1656.
Galileo is also said to have dropped balls of different weights from the Leaning Tower of Pisa to show that they fall at the same SPEED. He probably never did, but he was right. In 1971 the astronaut David Scott tested it on the Moon, where there is no air to slow things down: he dropped a hammer and a falcon feather together, and they hit the dust at the same moment.
Newton's apple and the unit named after him
Isaac NEWTON told a friend, near the end of his life, that the idea of GRAVITY had come to him while he sat in the garden of his family's farmhouse in Lincolnshire and watched an apple fall. The apple did not hit him on the head; that bit was added later. What struck him was that the same pull that brought the apple down might reach all the way to the Moon and hold it in its orbit.
Scientists measure FORCE in newtons, named after him. By a happy coincidence, a newton is roughly the weight of a small apple. Newton's first law describes INERTIA: things keep doing what they are already doing unless a force changes it. That is the trick behind pulling a tablecloth from under a set of plates, and the reason a seatbelt matters when a car stops suddenly.
Eureka in the bath
A famous story about the Greek thinker Archimedes says a king asked him to find out whether a new golden crown was pure gold or had been mixed with cheaper silver, without damaging it. Stepping into a full bath, Archimedes noticed the water rise and realised he could measure the crown's volume by how much water it pushed aside. Comparing weight and volume would reveal its DENSITY, since gold is much denser than silver. He is supposed to have leapt out and run home shouting eureka, I have found it.
Density explains why a huge steel ship floats: its hollow hull pushes aside a weight of water greater than the ship itself. PRESSURE is force spread over an area: a sharp knife cuts because its force lands on a tiny edge, and snowshoes spread your weight so you do not sink.
Friction, momentum and the very small
FRICTION is the grip between surfaces. Without it you could not walk or keep a knot tied. In curling, players sweep the ice in front of the stone to warm it and make a thin film of water, cutting the friction so the stone slides further. MOMENTUM is MASS times VELOCITY, so a slow lorry is much harder to stop than a fast football.
At the scale of the ATOM and the PARTICLE, physics gets strange. In QUANTUM physics, tiny particles can behave like waves, with a WAVELENGTH, and cannot be pinned down exactly. Lasers and computer chips depend on it. ENERGY, MOTION, a MAGNET and VOLTAGE all have their own pages in this category, and the LEVER and PULLEY are on our simple machines page.