A force is a push or pull that can speed a thing up, slow it down or turn it. Bigger pushes make bigger changes, and a given push moves a heavy thing less than a light one. Long ago, the Greek thinker Aristotle explained motion by a thing's nature. Earth and water, he taught, are heavy and move down, while air and fire are light and move up. That was his explanation for why things fall or rise.

People believed for nearly 2,000 years before Newton that all objects naturally tend to come to rest. Galileo Galilei was born in Pisa on February 15, 1564. His father was a court musician. Galileo left the University of Pisa with no degree, yet he became a central figure of the Scientific Revolution. He lived until 1642. He reasoned that moving things stop because of a force, friction, and that stopping is not natural for them. That reasoning helped lead to the idea of inertia.

Galileo used ramps. A ball rolled down one slope and up a facing slope rose to about the same height. Galileo reasoned that friction caused any small difference. Friction is a force that slows things sliding or rubbing against one another. Rough surfaces create more of it than smooth ones, so two pieces of coarse sandpaper are harder to slide together than two pieces of fine sandpaper. Friction also affects things at rest. Starting a heavy crate sliding takes more force than keeping it sliding, because friction on something sitting still is bigger than friction on something moving.

Inertia means a thing resists changes in its motion. No force is needed to keep a moving thing going. Think of a book sliding across a table. Friction is what stops it, and without friction it would continue at the same speed and in the same direction. A thing sitting still stays still unless a push or pull moves it. Equal forces that point in opposite directions are balanced, and balanced forces cancel, leaving the thing's motion unchanged. Only an unbalanced force changes how a thing moves.

Galileo argued against the idea that heavy things fall faster than light things. Astronauts later demonstrated his point on the Moon, where there is almost no air. A hammer and a feather released side by side there fell at the same rate. Take away the air, and heavy and light things fall alike. The reason things fall is gravity, a pull. Earth's gravity draws things down toward the ground, and a dropped thing speeds up as it falls. Gravity pulls harder when more mass is involved and weaker when things are farther apart, and all of Earth's mass is the source of its gravity.

Isaac Newton was born on 25 December 1642 by the calendar then in use, in Woolsthorpe, a small village in Lincolnshire, England. That was a few days less than a year after Galileo's death. Newton stated three laws of motion, which underlie the science of how forces change motion. With Kepler's laws, they explain why planets follow elliptical orbits instead of circles. The amount a thing speeds up depends on the push and the mass of the thing: more force gives more change, and more mass gives less.

Newton's third law is about pairs. If one thing pushes a second thing, the second pushes back on the first with equal force in the opposite direction. Pushing on anything brings back an equal push, and a fish moves ahead by driving water backward. Swimmers use the same rule against a pool wall, because the wall pushes back. In a jet engine, hot exhaust gas is pushed out the back, and the gas pushes the engine forward in reaction. A wing deflects air downward, and the air in turn pushes the wing up.

Weight is the pull of gravity on a thing, but mass is the amount of stuff in it. On Earth, weight equals mass times g, and g is 9.8 newtons per kilogram. Weight is a force that depends on where you are, while mass does not. A person on the Moon weighs much less than on Earth, because lunar gravity is only about one-sixth of Earth's. Scientists measure mass in kilograms, and they measure force, weight included, in newtons.

Drag is a contact force. It arises when a solid object moves through a fluid, which can be air or water. If there is no fluid, or no motion between the object and the fluid, drag does not exist. Air and water slow down things moving through them, and that is the force called drag. A skydiver falling through air speeds up until the air's push equals gravity's pull, and from then on the speed stays the same. That speed is called terminal velocity.

Forces that need touching include pushes and friction. Gravity and magnets act across a gap. Two magnets can push or pull on each other when they are not touching. Each magnet has a north pole and a south pole. Poles that are alike repel, and poles that are different attract. A compass needle's north end points toward Earth's geographic North Pole, yet the magnetic pole nearby is in fact a south magnetic pole. Gravity also reaches across a gap, pulling things toward the center of Earth.

Words to know

• inertia: resisting change
• elliptical: oval
• fluid: air or water
• poles: magnet ends
• deflects: pushes aside