Class 11 Physics Chapter 4 Laws of Motion – Revision Notes

Chapter 4 of Class 11 Physics rests on Newton’s three laws of motion, from inertia through F=ma to action-reaction pairs. It also covers how momentum behaves in isolated systems and how static and kinetic friction resist motion differently.

Last Updated: September 23, 2026

Newton’s Three Laws

• 1st Law: inertia; no motion change without net force.
• 2nd Law: F=dp/dt=ma.
• 3rd Law: action-reaction pairs, equal and opposite, on different bodies.

Momentum

• p=mv; conserved when no external force acts.

Friction

• Static: f_s(max)=μ_sN.
• Kinetic: f_k=μ_kN.

Circular Motion

• F=mv²/r; banked road ideal speed v=√(rg tanθ).

One-Line Summary

Newton’s three laws govern motion and force; momentum is conserved in isolated systems, and friction is a self-adjusting resistive force between contacting surfaces.

Quick visual: a worked diagram from the full Solutions page, for reference.

Frequently Asked Questions

How does the First Law of Motion given by Newton define the concept of inertia?
The First Law of Motion states that an object remains at rest or in uniform motion unless acted upon by an external force. This defines inertia as the natural tendency of any body to resist a change in its state of motion.

What is the difference between mass and weight according to the Laws of Motion?
Mass is the measure of inertia of a body and remains constant everywhere, while weight is the force of gravity acting on that mass, so weight varies depending on the local value of gravitational acceleration.

Written by Satish

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