Newton's first law

Imagine floating in space. Someone gives you one small push. Would you ever stop? There is no floor, no air, no friction... so you would keep drifting forever.

For a very long time, people believed a force was needed to keep things moving. In the 17th century, Galileo questioned this. Using thought experiments (careful reasoning in his head), he argued that a ball rolling on a flat surface, with nothing at all to slow it down, would roll forever.

Newton called this tendency inertia: things resist a change in their state. You could call it stubbornness (zid): a thing at rest wants to stay at rest, and a moving thing wants to keep moving. In 1687, Newton published his three laws of motion. The unit of force, the newton (N), is named after him.

Newton's first law: an object at rest stays at rest, and a moving object keeps moving at a constant velocity, unless a net force acts on it. With no net force, there is no acceleration.

You feel inertia every day:

• When a car brakes, a phone charger hanging from the mirror swings forwards: it wants to keep moving.
• Flick a card from under a coin placed on a glass, and the coin drops into the glass: it wanted to stay at rest.

"Can something move if the forces on it are balanced?" Yes, at a steady velocity. Constant velocity means no change in speed and no change in direction: moving in a straight line at a steady speed. A box pushed with a force exactly equal to the friction keeps sliding at a constant velocity, because the net force is zero.

On graphs, zero net force looks like this:

• At rest: the position-time graph is a flat line, and the velocity is zero.
• Moving: the position-time graph is a straight sloping line, and the velocity-time graph is flat.