Friction is the force that opposes motion between two surfaces in contact. Slide a book across a table and it slows down and stops — that's friction acting against its movement. Friction always acts in the direction opposite to the motion.
What causes friction?
No surface is perfectly smooth. Even surfaces that look flat have tiny bumps and grooves. When two surfaces touch, these irregularities lock into each other, and that interlocking resists movement. Rougher surfaces have more friction; smoother surfaces have less.
What affects how much friction there is?
- ✓ How rough or smooth the surfaces are.
- ✓ How hard the surfaces press together — a heavier object presses harder, so there's more friction.
Types of friction
- ✓ Static friction: acts when an object is still and you're trying to move it.
- ✓ Sliding friction: acts when one surface slides over another. It is smaller than static friction — that's why it's harder to start moving a box than to keep it moving.
- ✓ Rolling friction: acts when something rolls. It is the smallest, which is why wheels and ball bearings are so useful.
Friend and foe
Friction is helpful: it lets us walk without slipping, write with a pen, and stop a cycle with brakes. It's also harmful: it wears out machine parts and wastes energy as heat. We increase friction with treads on tyres and shoes, and reduce it using lubricants like oil, polished surfaces, and ball bearings. Fluid friction (called drag) is the resistance air or water adds — which is why fast vehicles, fish and birds have streamlined shapes.
Want this taught aloud in your language?
Concepts like static vs. sliding friction stick better when explained with examples your child can picture. Upload this NCERT chapter to Tutorfic and it will teach it aloud in your child's language, then quiz them on the types of friction with the reasoning behind each answer. It's one of all the Class 8 subjects Tutorfic teaches.