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Mechanics · Quick Reference
Newton's Laws Cheat Sheet
In one line: newton's three laws of motion form the foundation of classical mechanics, explaining exactly how and why objects move the way they do.
Key Ideas
1Newton's First Law (Inertia). An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force.
2Newton's Second Law. The net force on an object equals its mass times its acceleration: F = m*a. A larger force or smaller mass produces greater acceleration.
3Newton's Third Law. For every action, there is an equal and opposite reaction -- forces always come in pairs acting on two different objects.
4Net Force. The vector sum of all forces acting on an object; only the net (unbalanced) force causes acceleration.
5Free Body Diagrams. A simplified diagram showing all the forces acting on a single object, useful for applying Newton's second law correctly.
Worked Examples
A 5 kg object experiences a net force of 20 N. Find its acceleration.
a = 4 m/s^2
A 10 kg object accelerates at 3 m/s^2. Find the net force acting on it.
F = 30 N
A rocket pushes exhaust gas downward and the rocket moves upward. Which of Newton's laws explains this?
Newton's Third Law
Formulas
F = m*a
Practice Yourself
A 2 kg object has net force 10 N. Find acceleration.
5 m/s^2
A 4 kg object accelerates at 6 m/s^2. Find net force.
24 N
What does Newton's First Law describe?
Objects resist changes to their state of motion (inertia)