01 Key Concepts
Newton'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.
Newton'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.
Newton's Third Law
For every action, there is an equal and opposite reaction -- forces always come in pairs acting on two different objects.
Net Force
The vector sum of all forces acting on an object; only the net (unbalanced) force causes acceleration.
Free Body Diagrams
A simplified diagram showing all the forces acting on a single object, useful for applying Newton's second law correctly.
02 Key Formulas
- F = m*a
03 Solved Examples
Example 1 A 5 kg object experiences a net force of 20 N. Find its acceleration.
- Apply F=ma: 20 = 5*a.
- a = 20/5.
Answer: a = 4 m/s^2
Example 2 A 10 kg object accelerates at 3 m/s^2. Find the net force acting on it.
- Apply F=ma: F = 10*3.
Answer: F = 30 N
Example 3 A rocket pushes exhaust gas downward and the rocket moves upward. Which of Newton's laws explains this?
- The rocket exerts a force on the gas (action); the gas exerts an equal, opposite force back on the rocket (reaction).
Answer: Newton's Third Law
04 Practice Questions
1A 2 kg object has net force 10 N. Find acceleration.
5 m/s^2
2A 4 kg object accelerates at 6 m/s^2. Find net force.
24 N
3What does Newton's First Law describe?
Objects resist changes to their state of motion (inertia)
4What does Newton's Third Law state?
For every action force, there is an equal and opposite reaction force
5If net force on an object is 0, what happens to its velocity?
It stays constant (no acceleration)
📄 Newton's Laws — Downloadable Worksheet
10 questions with a full answer key. Grab the PDF to print, or try the interactive version in your browser.