Q.When the object is moving at constant velocity on the rough surface:

Newton's First Law: The Law of Inertia

Imagine you're sitting in a bus that's stopped at a signal. The bus suddenly lurches forward. What happens to you? You jerk backwards against the seat. Now imagine the bus is moving at a steady speed and the driver slams the brakes. You lurch forwards toward the front.

Why? Your body was trying to keep doing what it was already doing — staying still when the bus was still, and moving forward when the bus was moving. That instinct is the heart of Newton's First Law.

The Intuition: Objects Are Lazy

Things don't change their motion on their own. A ball sitting on the ground will stay sitting forever unless something pushes or pulls it. A rolling ball will keep rolling forever in a straight line — unless friction, air resistance, or a wall stops it.

This "laziness" of objects is called inertia. The more massive an object, the more inertia it has — a truck is much harder to start moving or stop than a bicycle.

The Precise Statement

Let's break that down:

• "At rest stays at rest" — A book on a table won't slide off unless you push it or the table shakes.
• "In motion stays in motion" — A hockey puck on ice keeps gliding because friction is very low. On rough ground, it stops quickly — but that's because friction (a force) is acting on it.
• "Unbalanced external force" — If you push a box and someone else pushes it equally from the opposite side, the forces cancel (balanced). The box doesn't move. Only when the net force is non-zero does motion change.

Why This Law Matters

Newton's First Law defines what a force is: anything that changes an object's state of motion. It also introduces the idea of inertial reference frames — if you're in a smoothly moving train with no windows, you can't tell you're moving at all. All physics works the same as if you were at rest.

Real-Life Examples

| Situation | What happens | Why |

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