Definition · Plain-language
Newton’s first law
Newton’s first law, the law of inertia, states that an object remains at rest or moves at constant velocity unless acted upon by a net external force.
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The law of inertia
Newton’s first law overturns the intuitive idea that motion needs a constant push to continue. Instead it states that an object will keep doing whatever it is already doing — staying still, or moving in a straight line at a steady speed — until a net external force changes that. The tendency of an object to resist changes in its motion is called inertia, and an object’s mass is the measure of how much inertia it has. A heavy lorry is harder to start moving, and harder to stop, than a bicycle precisely because it has more inertia.
Net force is what counts
The key phrase is net force — the single combined force left after all the pushes and pulls on an object are added together as vectors. An object can have many forces acting on it yet still move at constant velocity, provided those forces balance to zero. A car cruising at a steady motorway speed has its engine’s forward force exactly cancelled by friction and air resistance, so the net force is zero and its velocity stays constant. Only when the forces do not balance — an unbalanced, net force — does the object speed up, slow down or change direction.
Inertial frames and everyday examples
Newton’s first law also defines what physicists call an inertial frame of reference — one in which the law holds, meaning the frame is not accelerating. The law explains countless daily experiences: when a bus brakes suddenly, passengers lurch forward because their bodies tend to keep moving while the bus decelerates; a tablecloth can be whipped from under crockery because the crockery’s inertia resists the brief sideways force. Seatbelts, headrests and airbags are all engineering responses to inertia — they apply the force needed to change a passenger’s motion safely.
Key facts
At a glance
- Definition: an object keeps its velocity unless a net external force acts
- Also called: the law of inertia
- Inertia: an object’s resistance to a change in its motion
- Measured by: mass — more mass means more inertia
- Key condition: applies when the net (combined) force is zero
- Example: a seatbelt supplies the force to stop you continuing forward in a crash
Common misconceptions
What people often get wrong
Often heard: Inertia is a force that keeps objects moving.
Actually: Inertia is not a force; it is the property of resisting changes in motion. No force is needed to keep an object moving at constant velocity — only to change that motion.
Often heard: Newton’s first law only applies to objects that are standing still.
Actually: It applies equally to moving objects. A body already moving keeps its straight-line, constant-speed motion unless a net force acts, just as a still body stays still.
Often heard: If forces are acting on an object, it must be accelerating.
Actually: Only an unbalanced net force causes acceleration. Many balanced forces can act at once and cancel, leaving the object at constant velocity.
Going deeper
