Q.A swimmer pushes the water backward and is propelled forward through the pool. This is a direct application of Newton's Law of:

Inertia: The Reluctance to Change

Imagine you're sitting in a bus that's stopped at a stand. The driver suddenly floors the accelerator. What happens to you? You get thrown backwards against your seat. Now imagine the bus is moving fast and the driver slams the brakes. You lurch forward. Why?

Your body was at rest when the bus was at rest. It wanted to stay at rest. When the bus accelerated forward, your body tried to keep doing what it was doing — nothing. That's why you felt pushed back. Similarly, when the bus braked, your body was moving forward and wanted to keep moving forward. That's why you lunged ahead.

This property — the tendency of an object to resist any change in its state of motion — is called inertia.

The Precise Statement

Newton's First Law of Motion gives the formal definition:

Every object continues in its state of rest or of uniform motion in a straight line, unless compelled to change that state by an external force.

That "continuing in its state" part is inertia. An object at rest stays at rest. An object moving at constant speed in a straight line keeps moving exactly like that — unless something pushes or pulls it.

The more mass an object has, the more inertia it has. A cricket ball has more inertia than a table-tennis ball. It's harder to start a heavy box sliding than a light one, and harder to stop it once it's moving. That's mass doing its job.

Two Kinds of Inertia (for exams)

| Type | What it resists | Example |

|------|----------------|---------| …