Newton's First Law
The statement
A body at rest stays at rest. A body moving in a straight line at constant speed continues to do so. Neither state changes unless a net external force acts on the body.
This is Newton's First Law of Motion.
In symbols: if , then .
What this really says
Before Newton, the prevailing view (following Aristotle) was that a force was needed to keep a body moving — that motion was the result of a continuously applied push. Rest was the "natural" state.
Newton overturned this completely. The First Law says:
- Rest is not special. Uniform motion is just as natural as rest.
- Force does not cause motion. Force causes change in motion.
- A moving body keeps moving. It does not slow down on its own — something must act on it to slow it down.
The reason things slow down in everyday experience is friction and air resistance — forces that we often forget to account for. In the absence of all forces, a moving body would continue forever.
Inertia
The tendency of a body to resist any change in its state of motion is called inertia.
- A body at rest resists being set in motion → inertia of rest
- A body in motion resists being stopped or deflected → inertia of motion
- A body moving in a direction resists change of direction → inertia of direction
Mass is the measure of inertia. A heavier body has more inertia — it is harder to start, stop, or change direction.
A truck and a bicycle moving at the same speed — the truck is far harder to stop. That is because it has far greater inertia.
Why "net external force"
Two words matter here: net and external.
Net — it is the vector sum of all forces. A book sitting on a table has gravity pulling it down and the normal force pushing it up. The net force is zero, so the book stays at rest. Both forces are present, but they cancel.
External — forces within the system do not count. The internal forces between the atoms of a body do not change its overall motion. Only forces from outside the system matter.
The First Law defines inertial frames
The First Law also serves a deeper purpose: it defines what we mean by a valid reference frame for doing physics.
An inertial frame is a reference frame in which the First Law holds — where a body with no net force moves in a straight line at constant speed (or stays at rest).
A non-inertial frame (an accelerating frame — like a car braking hard) is one where objects appear to accelerate even with no force acting on them. In such frames, Newton's laws as usually stated do not hold without correction.
All of classical mechanics is built on inertial frames.
Everyday examples
| Situation | First Law at work |
|---|---|
| Passenger lurches forward when bus brakes suddenly | Passenger's body continues forward — inertia of motion |
| Dust falls off a carpet when beaten | Carpet moves, dust stays — inertia of rest |
| A spinning top continues spinning | No net torque — continues rotating |
| Satellite orbits without engine | Tangential inertia balanced by gravitational centripetal force |