"An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by an external net force."

The Concept: Inertia

Inertia is the natural tendency of objects to resist changes in their state of motion. More mass = more inertia.

$$\vec{F}_{net} = 0 \implies \vec{v} = \text{constant}$$

Real-World Examples

"The net force on an object equals the product of its mass and acceleration."

$$\boxed{\vec{F}_{net} = m\vec{a}}$$

• Larger force → Greater acceleration
• Larger mass → Smaller acceleration (for same force)

Breaking It Down

Real-World Examples

Example 1 — Pushing a Cart:

Push a 10 kg cart with 50 N of force: $$a = \frac{F}{m} = \frac{50}{10} = 5 \ m/s^2$$

Example 2 — Why trucks need bigger engines:

A truck (5000 kg) and a car (1000 kg) need the same acceleration of 3 m/s²

• Car needs: F = 1000 × 3 = 3,000 N
• Truck needs: F = 5000 × 3 = 15,000 N ← much more force!

Example 3 — Cricket ball vs tennis ball:

Same bowling force → lighter tennis ball accelerates far more than a cricket ball

"For every action, there is an equal and opposite reaction."

$$\vec{F}{A \to B} = -\vec{F}{B \to A}$$

⚠️ Critical Points Students Often Miss

• Action and reaction forces act on different objects — they never cancel each other
• Both forces are equal in magnitude, opposite in direction, and simultaneous

Real-World Examples

Why Don't We Fly When We Push a Wall?

You push the wall (action) → wall pushes you back (reaction). The reaction acts on you, but friction from the ground on your feet counters it — so you don't move!

Law 1 → Defines what "no force" means (constant velocity / rest) ↓ Law 2 → Quantifies what "a force" does (F = ma) ↓ Law 3 → Explains where forces come from (always in pairs)

• Law 1 → "Lazy objects don't like change" (Inertia)
• Law 2 → "F = ma — Force equals mass times acceleration"
• Law 3 → "Every push gets a push back" (Action-Reaction)

• codeCopy

  Friction and Newton's Laws

  — How friction modifies Law 1 & 2 problems
• codeCopy

  Free Body Diagrams (FBD)

  — Visual method to solve Law 2 problems
• codeCopy

  Impulse and Momentum

  — Deeper extension of the Second Law (F·t = m·Δv)
• codeCopy

  Normal Force and Weight

  — Classic Law 3 pair (gravity vs. surface reaction)
• codeCopy

  Newton's Laws in circular motion

  — Centripetal force applications
• codeCopy

  Atwood Machine problems

  — Classic exam problems using all 3 laws
• codeCopy

  Newton's Law of Gravitation

  — The fourth law (gravity between masses)