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 N/kg: each kilogram weighs 9.8 N. A 2 kg book weighs W = 2 · 9.8 = 19.6 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.

On the board. In the Forces step, pick a force's label, then drag from the dot on the object the way the force acts. Draw one arrow for each force; a label that is not a force stays unused.