Forces Around Us

A force is a push or a pull on an object. It has a size, measured in newtons ($N$), and a direction, so we draw it as an arrow: the arrow starts on the object, points the way the force acts, and is longer for a bigger force.

A push drawn as an arrow: it points the way the force acts, and a longer arrow means a bigger force.

Weight $W = mg$: gravity pulls every object straight down, toward the Earth. An object of mass $m$ weighs $W = mg$, with $g = 9.8\ \mathrm{N/kg}$: each kilogram weighs $9.8\ \mathrm{N}$. A $2\ \mathrm{kg}$ book weighs $W = 2 \cdot 9.8 = 19.6\ \mathrm{N}$.

Normal force $N$: a surface pushes back on an object resting on it. The push is at right angles to the surface, which is what "normal" means here. A table pushes a book straight up.

A book at rest on a table: the weight mg pulls down, the table pushes up with N.

To show the forces on an object, draw it with one arrow per force, each starting on the object. A book on a table has two: its weight down and the normal force up. At rest the two arrows are the same length: the table pushes up just as hard as gravity pulls down.