Every force, whether it is a push or a pull, has a size and a direction. A force can make something go faster, go slower or turn. The bigger the push, the bigger the change in the way a thing moves.

In science, mass is counted in kilograms. Force, weight included, is counted in newtons. People in the United States also use the pound as a unit of weight. Weight is how hard gravity pulls on a thing, and mass is how much stuff the thing holds. A person would weigh much less on the Moon, where gravity is roughly one-sixth as strong as on Earth. A trip to orbit or to the Moon leaves a thing's mass unchanged.

Forces sort into those that touch and those that reach. Pushing and friction need touching. Gravity and magnets work across a gap. Earth's gravity draws things down to the ground, toward the center of Earth, so a dropped ball goes down. Two magnets can push or pull on one another while a space still separates them.

Net force is the sum of every force acting on a thing. Equal forces that point opposite ways cancel out. Forces like that are balanced, and balanced forces leave the way a thing moves unchanged. An unbalanced force does change the way a thing moves, and with none, nothing changes. A moving thing goes on in the same way until a force alters its motion. Inertia is a thing's tendency to resist changes in its motion, whether it is at rest or moving.

How much a thing speeds up depends on two things: the push and the mass. More force gives more change, and more mass gives less. So one push moves a heavy thing less than a light one, and a bigger push is needed to move the heavy thing the same amount.

Friction slows things that slide or rub against each other. A heavy crate is harder to start than to keep going, because friction on a resting object is bigger than friction on a moving one. Air and water slow moving things too, and drag is the name of that force.

Isaac Newton, who lived from 1642 to 1727, stated three laws of motion. His third law covers pairs of forces: when one thing pushes a second thing, the second pushes back on the first equally, in the opposite direction. Pushing off a pool wall sends a swimmer away because the wall pushes back. A fish swims ahead by shoving water backward.

A thing that moves in a circle needs a force that pulls it toward the center. The name of that force is centripetal, meaning center-seeking. Picture a ball whirling on a string. Let the string snap and the ball shoots off in a straight line, since nothing pulls it around once the string is gone.

Momentum is mass in motion, found by multiplying a thing's mass by its velocity. A heavy thing moving fast has more momentum than a light thing moving slowly. Momentum depends on both mass and velocity.

Words to know

• newtons: units of force
• centripetal: center-seeking
• momentum: mass in motion
• balanced: equal and opposite