Q.Launching of a rocket is based upon which principle? (A) Nuclear fusion
(B) Nuclear fission
(C) Newton's law of motion
(D) Work-energy theorem

Newton's Laws of Motion

Imagine you're sitting in a bus that suddenly lurches forward. Your body jerks backward, pressed into the seat. Then the bus brakes hard — you fly forward. That feeling in your body, the way things seem to "want" to keep doing what they were doing, is the raw experience Newton captured in his first law.

The Core Intuition

Motion is not natural or unnatural. Objects don't "prefer" to be at rest. They don't "prefer" to move either. They simply keep doing whatever they're already doing — unless something forces them to change.

A ball on a table stays put. A hockey puck sliding on ice keeps sliding. The difference? Friction. On ice, there's almost no force to stop it, so it keeps going. On a rough table, friction acts as a force that slows it down. Newton realised: if you could remove all forces, an object would keep moving forever in a straight line at constant speed.

This was revolutionary. Before Newton, people thought you needed a continuous push to keep something moving. Newton said: no, you only need a push to change its motion — to start it, stop it, speed it up, slow it down, or turn it.

The Three Laws — Precisely

The key word is unbalanced. If multiple forces act on an object but cancel each other out (like a book on a table — gravity pulls down, the table pushes up equally), the net force is zero. The object behaves as if no force acts on it.

This is the workhorse. It tells you exactly how much motion changes when you apply a force.

• Force ($\vec{F}$) is what causes acceleration — measured in newtons (N).
• Mass ($m$) is the measure of inertia — how much an object resists acceleration. A heavier object needs more force to achieve the same acceleration.
• Acceleration ($\vec{a}$) is the rate of change of velocity — how quickly speed or direction changes.

Example: A 2 kg block is pushed with a net force of 10 N.

If object A exerts a force on object B, then object B exerts a force of the same magnitude but opposite direction on object A. These forces always come in pairs, act on different objects, and are simultaneous.

Common confusion: If action and reaction are equal and opposite, why does anything move? Because they act on different bodies. When you push a wall, the wall pushes you back with equal force. Your hand doesn't move through the wall — but you can move backward (your feet push the floor, the floor pushes you forward). The pair of forces never cancel because they're on different objects.

Putting It All Together — A Simple Problem

A 5 kg box is pulled horizontally with a force of 20 N. The friction opposing motion is 5 N.

Step 1: Find the net force.

Step 2: Apply the second law.

The box accelerates at 3 m/s² in the direction of the pull. …