Inertia and the first law

Aristotle thought a moving body needs a force to keep it moving. Galileo saw that a body only slows down because of friction and air; without them, a moving body would keep moving for ever.

Newton made this his first law: every body stays at rest, or keeps moving at a constant velocity in a straight line, unless an external force makes it change. An external force is one applied from outside the body.

This unwillingness to change is called inertia, a kind of laziness every body has. Mass measures it: the heavier the body, the harder it is to start, stop or turn. You can see three kinds:

• Inertia of rest: a standing passenger falls backwards when a train starts suddenly. The feet move with the floor, but the upper body stays where it was.
• Inertia of motion: the same passenger leans forward when the train brakes. The feet stop, the upper body keeps going.
• Inertia of direction: when a car turns sharply, you seem to be thrown towards the outside of the turn. Nothing pushes you out: your body simply wants to go straight on while the car turns.

The law also tells us what a force is: whatever changes a body's state of rest or motion.

Read it the other way round too: if a body is at rest, or moves at a constant velocity, the net force on it is zero. Several forces may act, but they cancel. A lift rising at a steady 2 m/s carries you with the floor pushing up exactly as hard as gravity pulls down.

Zero net force does not mean the body is not moving. A body at rest is in static equilibrium; a body moving at a constant velocity is in dynamic equilibrium. Both have zero acceleration.

When the force stops, the body keeps the velocity it had at that instant. A stone whirled on a string is always moving along the tangent, so when the string breaks it flies off along the tangent, not outwards from the centre.