Physics
Newton's Laws of Motion
Newton's three laws of motion are the foundation of classical mechanics. Once you understand them, you can model everything from a falling apple to a rocket launch.
How to Approach Newton's Laws of Motion
First law (inertia)
An object at rest stays at rest, and an object in motion stays in motion in a straight line at constant speed — unless acted on by an external net force.
Second law (F = ma)
The net force on an object equals its mass times its acceleration. F is in newtons (kg·m/s²), m is mass in kg, a is acceleration in m/s².
Third law (action-reaction)
For every action there is an equal and opposite reaction. If A pushes B with force F, B pushes A with force -F.
Frequently Asked Questions
What's the difference between mass and weight?+
Mass (kg) is how much matter an object contains — same everywhere. Weight (N) is the force of gravity on that mass: W = mg, where g ≈ 9.81 m/s² on Earth.
Why don't I feel inertia in a moving car?+
You're moving with the car at the same velocity. You feel inertia when the car accelerates — that's why you get pushed back into the seat.
Do Newton's laws break down anywhere?+
Yes — at speeds close to light (use special relativity) or for very small scales (use quantum mechanics). For everyday physics, Newton's laws are excellent.
Related Topics
More step-by-step guides in Physics and adjacent subjects.
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