Class 9 Science Chapter 6 How Forces Affect Motion – Revision Notes

Quick revision notes for Class 9 Science Chapter 6 (How Forces Affect Motion). These Class 9 Science Chapter 6 notes are ideal for quick revision just before exams.

Balanced vs Unbalanced Forces

• Balanced: cancel out, no change in motion.
• Unbalanced: don’t cancel, change speed/direction.

Friction

• Opposes relative motion between surfaces.
• Reduce: lubricants, smoother surfaces, rollers/wheels.
• Increase: rough surfaces (e.g. tyre treads).

Newton’s Laws

• 1st law (Inertia): object stays at rest/uniform motion unless acted on by unbalanced force.
• 2nd law: F = ma; rate of change of momentum ∝ force.
• 3rd law: every action has an equal and opposite reaction, on two different bodies.

One-Line Summary

Motion changes only under unbalanced forces, friction opposes relative motion between surfaces, and Newton’s three laws together explain why objects resist changes in motion (inertia), how force relates to acceleration (F=ma), and why forces always come in equal-and-opposite pairs acting on different objects.

Written by Satish

NCERTBooks.org is an independent educational resource run by a small team focused on making official NCERT textbooks easy to find, read, and download for students, parents, and teachers across India. We are not affiliated with NCERT or the Ministry of Education -- we organise publicly available NCERT content by class and subject, verify links against official sources, and build tools (like our in-browser reader) that make studying more convenient. Every guide we publish is written and reviewed by our team based on the actual NCERT curriculum and syllabus.