Newton's First Law
Newton's first law states that an object remains at rest, or continues moving in a straight line at constant velocity, unless acted upon by a net external force.
Curious
Newton's first law is often called the law of inertia. Inertia is an object's built-in resistance to having its motion changed. A hockey puck sliding on frictionless ice would slide forever in a straight line at the same speed if nothing touched it — not because it has some ongoing "force of motion," but because nothing is acting to change what it's already doing.
The reason objects on Earth don't behave this way — a rolling ball eventually stops, a thrown paper airplane eventually falls — is that forces are always acting: friction, air resistance, gravity. Remove those forces, and the object just keeps doing whatever it was already doing.
This flips the ancient, intuitive assumption (associated with Aristotle) that motion requires a continuous push to sustain it. Newton's insight was the opposite: it's changing motion, not motion itself, that requires a cause.
Exploring
Formally, Newton's first law establishes the existence of inertial reference frames — frames of reference in which an object with zero net force on it moves at constant velocity (including the special case of remaining at rest). This is not automatically true in every frame: inside an accelerating car, a stationary cup will appear to slide backward with no obvious force acting on it in that frame. The first law is a statement about a special class of frames, and every subsequent law of Newtonian mechanics is stated relative to inertial frames.
| Condition | Resulting motion |
|---|---|
| Net force = 0, object at rest | Remains at rest |
| Net force = 0, object moving | Continues at constant velocity (same speed and direction) |
| Net force ≠ 0 | Accelerates — governed by Newton's Second Law |
Worked example: A puck of mass 0.3 kg slides across a nearly frictionless air-hockey table at 2 m/s. If the net horizontal force is measured to be 0 N, the first law predicts it continues at exactly 2 m/s in a straight line — no additional force is needed to "keep it going."
Deep Dive
The first law's deepest function is definitional rather than predictive: it defines an inertial frame as any reference frame in which a body subject to zero net force moves with constant velocity. Once inertial frames are defined this way, the second and third laws can be stated as holding specifically within such frames. In non-inertial (accelerating or rotating) frames, apparent violations of the first law occur — objects appear to accelerate with no applied force — and are accounted for by introducing fictitious forces (e.g., centrifugal force, Coriolis force) that have no physical source but are required to make Newton's laws hold in that frame's coordinates.
Some historians of science credit Galileo Galilei's earlier work on inertia (in his 1632 Dialogue Concerning the Two Chief World Systems and later writings) as a direct precursor; Newton's contribution was to state it as a precise, general law applicable to all bodies and integrate it into a complete axiomatic system alongside the second and third laws.
In special relativity, the first law survives essentially unchanged — inertial frames remain the frames in which force-free motion is uniform — but the transformation between inertial frames changes from the Galilean transformation to the Lorentz transformation, altering how velocities and time intervals appear between frames in relative motion.
Concept Relationship Map
Requires: inertial frames, force
Enables: Newton's Second Law, relative motion
Commonly confused with: Newton's Second Law under zero net force (mathematically equivalent, but historically and conceptually distinct as a definitional statement)
Sources
- Newton, I. (1687). Philosophiæ Naturalis Principia Mathematica, Book I, Axioms, Law I. Public domain.
- Goldstein, H., Poole, C., Safko, J. (2002). Classical Mechanics, 3rd ed. Addison Wesley, Ch. 1.
- Feynman, R.P. et al. The Feynman Lectures on Physics, Vol. I, Ch. 9. feynmanlectures.caltech.edu