Revision Notes: Class 8 Science Chapter 5 Exploring Forces

Class 8 Science Chapter 5: Exploring Forces — Quick Revision Notes

• Force: a push or pull arising from an interaction between two objects; SI unit is the newton (N).
• Effects of force: can start motion, stop motion, change speed, change direction, or change shape — often a combination.
• Contact forces (require touching): muscular force (from muscle contraction) and friction (opposes relative motion between surfaces; greater on rough surfaces).
• Non-contact forces (act without touching): magnetic force (attract/repel between magnets), electrostatic force (from charge, like charges repel/unlike attract), and gravitational force (Earth’s pull, always attractive).
• Weight vs mass: mass is the fixed amount of matter (g/kg), constant everywhere; weight is the gravitational force pulling an object down (N), which varies by location — e.g. Moon’s gravity is about 1/6th of Earth’s.
• Spring balance: measures weight — the stretch of a calibrated spring is proportional to the weight hung on it.
• Buoyant force (upthrust): the upward force a liquid exerts on an object placed in it; per Archimedes’ Principle, it equals the weight of liquid displaced.
• Floating vs sinking: an object floats if buoyant force ≥ its weight, and sinks if its weight exceeds the buoyant force.

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.