Physics · StudentHub Lesson
Newton's Laws of Motion
The three laws that describe how forces affect the motion of objects.
What you will learn
- State Newton's three laws of motion
- Explain inertia
- Apply F=ma to solve problems
- Explain action-reaction force pairs
- Relate the laws to everyday examples
Watch the lesson
Newton's first law | Physics | Khan Academy · Khan Academy
Watch on YouTubeTopic notes
Main Idea
Newton's three laws of motion describe the relationship between a body and the forces acting on it.
Key Concepts
- First law: an object stays at rest or in motion unless acted on by a net force (inertia)
- Second law: force equals mass times acceleration (F=ma)
- Third law: for every action, there is an equal and opposite reaction
Definitions
- Inertia: tendency of an object to resist changes in motion
- Force: a push or pull measured in Newtons
- Acceleration: rate of change of velocity
Processes
When a net force acts on a mass, it accelerates according to F=ma; forces always occur in pairs per the third law.
Examples
A seatbelt keeps you from continuing forward (inertia) when a car suddenly stops.
Common Mistakes
- Confusing mass with weight
- Thinking heavier objects always accelerate slower regardless of force
- Forgetting action-reaction pairs act on different objects
Key concepts
Important terms
- Inertia
- The tendency of an object to resist a change in its state of motion.
- Net force
- The overall force acting on an object after combining all forces.
- Newton
- The SI unit of force.
Worked examples
Problem
A 10 kg object experiences a net force of 20 N. Find its acceleration.
- 1. Use F = ma
- 2. Rearrange: a = F/m
- 3. a = 20 N / 10 kg
Answer: 2 m/s^2
Quick revision
- First law: inertia
- Second law: F = ma
- Third law: action-reaction pairs
- Net force causes acceleration
- Mass resists acceleration
- Forces are pushes or pulls
- Force pairs act on different objects
Check your understanding
Question 1 · Multiple choice
Which law states F=ma?
Question 2 · Multiple choice
What does Newton's first law describe?
Question 3 · True or false
Action-reaction force pairs act on the same object.
Question 4 · Short answer
A 5 kg object accelerates at 4 m/s^2. What force is applied?
Question 5 · Short answer
What quantity measures resistance to a change in motion?
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