What are Newton's laws of motion?
Newton's laws of motion are three laws that Isaac Newton laid out in 1687 in Philosophiæ Naturalis Principia Mathematica. They describe how forces affect motion and form the foundation of classical mechanics. Building on Galileo's work on inertia, and paired with the law of universal gravitation in the same book, they explained motion on Earth and in the heavens with one set of principles.
The three laws at a glance
| Law | Statement | Everyday example |
|---|---|---|
| First law (inertia) | With zero net force, an object at rest stays at rest and a moving object keeps moving in a straight line at constant speed | You lurch forward when a bus brakes hard |
| Second law | Acceleration is proportional to net force and inversely proportional to mass: F = ma | An empty shopping cart speeds up more easily than a full one |
| Third law (action-reaction) | If A pushes on B, B pushes back on A with an equal and opposite force | A rocket pushes exhaust backward and moves forward |
How should you think about F = ma?
The more general form of the second law is force equals the rate of change of momentum (F = dp/dt); with constant mass it becomes F = ma. One newton (N) is the force that accelerates 1 kg at 1 m/s². A 1 kg object weighs about 9.8 N at Earth's surface (roughly 2.2 lb).
In the third law, action and reaction act on different objects, so they don't cancel. A classic exam trap: a book's weight and the table's normal force balancing out is a first-law (equilibrium) situation, not a third-law pair.
Where do the laws apply, and where don't they?
Newton's laws hold in non-accelerating reference frames, called inertial frames. Motion near the speed of light needs Einstein's relativity, and atomic-scale behavior needs quantum mechanics. Still, for cars, buildings and satellite orbits they're extremely accurate. Extending them to rotation leads to conservation of angular momentum; for related ideas see the principle of the lever and the laws of thermodynamics.
FAQ
- If action and reaction are equal, why does anything move?
- Because the two forces act on different objects. Each object's motion depends only on the net force acting on it.
- What's the difference between mass and weight?
- Mass is how much matter an object has (kg); weight is the gravitational force on it (N). On the Moon your mass is the same but your weight is about one-sixth.
Sources reviewed: 2026-10-25
References
- Newton, I. (1687). Philosophiæ Naturalis Principia Mathematica. London: Royal Society.
- Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol. I. Reading, MA: Addison-Wesley.
- Halliday, D., Resnick, R., & Walker, J. (2013). Fundamentals of Physics (10th ed.). Hoboken, NJ: Wiley.