Inertia

Inertia (Newton’s First Law of Motion)

- Comes from the Latin word iners, meaning idle or unchanging.

- Definition: An object at rest stays at rest; an object moving keeps moving at the same speed and straight path — unless an unbalanced/outside force acts on it.

- Inertia of rest: Objects don’t start moving on their own.

- Inertia of motion: Moving things don’t slow down or change direction unless pushed or pulled.

- Rule: The more mass something has, the more inertia it has — harder to start, stop, or turn.

Friction

- A force that acts only when two surfaces touch and rub against each other.

- Strength depends on how rough the surfaces are and how hard they press together.

- Always acts opposite to the direction the object wants to move.

- 3 main types (strongest → weakest):

1. Static friction: Holds still objects in place — hardest to overcome.

2. Kinetic / sliding friction: Acts while things slide past each other.

3. Rolling friction: Happens when something rolls — weakest of all.

How they connect

- Inertia keeps motion going or keeps things still.

- Friction is the opposing force that fights inertia — it’s what changes speed or stops movement.

- Example: A rolling ball keeps going because of inertia, but slows and stops because friction pushes back against it.

- Inertia depends directly on mass, not size or speed.

- Friction direction: always opposite motion.

- Clear ranking of friction types: static > sliding > rolling.

- Link: To move/stop, you must overcome inertia and beat friction.