What makes things move? Forces do. Pushing and pulling are both forces. Either one can change a thing's motion. It can make the thing go faster, go slower or change direction. A larger push brings a larger change. Mass matters as well. Give a light object and a heavy object equal pushes, and the heavy one moves less. To get the heavy one to move just as much, you need a bigger push.

Gravity is one more pull. Earth's gravity tugs everything downward, toward the planet's center. That is why a ball you let go of falls. Something that falls goes faster and faster. The pull of gravity gets stronger with more mass. It gets weaker as things move farther apart. All of Earth's mass is the source of Earth's gravity.

Weight and mass sound alike, yet they differ. Mass tells how much stuff makes up a thing. Weight is the pull of gravity on that thing. Mass stays constant whether a thing is on Earth, in orbit or on the Moon. The pull of gravity on the Moon is only about one-sixth of the pull on Earth. For that reason, a person would weigh much less on the Moon than at home.

Friction is a force that works against sliding. When two things slide or rub together, friction slows them down. Rough surfaces make more friction than smooth ones. Sliding two sheets of coarse sandpaper over each other is harder than sliding two sheets of fine sandpaper. Friction acts on things sitting still, too. A heavy crate takes extra force to get started and less to keep sliding. The reason is that friction on a crate that sits still is bigger than friction on a crate that moves.

When something moves through air or water, the air or water slows it down. That force is called drag. A falling skydiver shows drag and gravity at work. The skydiver speeds up more and more. Then the air's push grows as big as gravity's pull. After that, the speed stops changing. The speed at that moment has a name: terminal velocity.

Pushing goes both ways. A push on something is met by an equal push back on you. A fish shoves water toward its tail, and the water shoves the fish ahead. A swimmer leaving a pool wall does the same. The swimmer pushes the wall, the wall pushes the swimmer, and the swimmer moves away. Scientists call this Newton's third law. A push from one thing on a second thing meets an equal push from the second back on the first, aimed the opposite way.

Forces fall into two groups. Some need contact, such as a push or friction. Others reach across a gap, such as gravity or magnets. Two magnets can attract or repel each other with no touching. Simple machines, a lever for example, let a small push or pull get a job done that would otherwise need a big force.

A thing sitting still will not start moving until some push or pull acts on it. Moving things keep their motion until a force changes it. Scientists use the word inertia for the way things resist a change in their motion. In other words, things at rest stay at rest, and things in motion stay in motion.

Galileo lived from 1564 to 1642, and he was a scientist. He did not accept that stopping is natural for moving things. He reasoned that a force, friction, makes them stop. That thinking helped lead to inertia as an idea. Galileo also argued that heavy things fall no faster than light things. Astronauts later tested this on the Moon, where there is almost no air. They dropped a hammer along with a feather, and the two fell at the same rate.

Isaac Newton lived from 1642 to 1727. He stated three laws of motion. Those laws are the base of the science that explains how forces change motion.

Words to know

• force: push or pull
• friction: rubbing slowdown
• drag: slowing force
• inertia: resisting change