In science, a force means a push or a pull. It can make a thing speed up, slow down or turn. Each one has a size, and each one points in a direction. Push harder, and the change is bigger. Gravity is a pull too, and Earth's gravity pulls things down toward the ground. When something is dropped, it speeds up as it falls.

Things resist changes in their motion. Scientists call this tendency inertia. Something sitting still stays still, and something moving keeps moving. It is why a thing needs a push or a pull to get going. Drag is one more way a motion changes. It is the name for air or water slowing a thing that moves through it.

Now picture two pushes or pulls on one thing. If they match in size and point opposite ways, they are balanced. Balanced ones cancel each other out, so how the thing moves stays the same. An unbalanced force does change how the thing moves. With no unbalanced force, there is no change, and a moving thing carries on as before until a force changes its motion.

The amount of speeding up depends on two things. One is the push, and the other is the mass of the thing. A bigger force makes the thing speed up more, and a bigger mass makes it speed up less. So one push affects heavy things and light things differently. The heavy one moves less, and it needs a bigger push to move as much.

Newton's third law describes pairs of pushes and pulls. Say one thing pushes on another. The other pushes back on it, with an equal force pointing the opposite way. When a swimmer pushes against the wall of a pool, the swimmer moves away, since the wall pushes back. A fish moves forward by pushing water backward.

Mass and weight are easy to mix up. Mass is how much stuff makes up a thing. Mass stays the same whether a thing is on Earth, in orbit or on the Moon. Weight is gravity's pull on a thing, and gravity draws a thing toward the center of Earth. The Moon pulls with about one-sixth of Earth's gravity. A person on the Moon would weigh much less.

Pushes and pulls reach things in different ways. Pushes and friction need touching, and friction slows things when they slide or rub together. Gravity and magnets can reach across a gap, so a pair of magnets can push or pull without touching. Rough surfaces create more friction than smooth ones. Rough sandpaper is harder to slide against itself than fine sandpaper is.

Words to know

• inertia: resisting change
• balanced: equal and opposite
• mass: amount of stuff
• weight: gravity pull
• friction: slowing rub

The short version

Pushes and pulls are forces. A force can change how a thing moves by speeding it up, slowing it down or turning it. Forces that are equal and opposite are balanced and cancel, while an unbalanced force changes the motion. When one thing pushes on another, the second pushes back with an equal force.

Quick facts

• A force means a push or a pull, with a size and a direction.
• Inertia is how a thing resists any change in its motion.
• Balanced forces match in size and point opposite ways, so they cancel.
• More force gives more change in motion, and more mass gives less.
• Mass stays the same whether a thing is on Earth, in orbit or on the Moon.