Friction | Science Grade 8 CBSE

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The six faces of friction

Friction comes in kinds and can be turned up or down. Tap each term to see what it means and where you meet it.

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The three big ideas

Friction is the force that opposes the relative motion between two surfaces in contact. It always acts along the surfaces, against the direction of motion.
It is caused by the interlocking of tiny irregularities on the surfaces. Even a surface that looks and feels smooth is full of microscopic bumps.
Friction is greater on rougher surfaces and when the two surfaces are pressed together harder (a heavier object).
Without friction we could not walk, hold a pencil or stop a moving cycle — yet the same force also slows things we want to keep moving.

Common mistake: believing a "smooth" surface has no friction. No real surface is perfectly smooth — friction is only smaller, never truly zero, on polished surfaces.

Static friction acts when an object is at rest but you are trying to move it. It is the largest of the three — you must overcome it to start motion.
Sliding friction acts once the object is sliding. It is smaller than static friction, which is why a box is easier to keep moving than to start.
Rolling friction acts when something rolls. It is the smallest — rolling a load is far easier than dragging it.
The order is: rolling < sliding < static. This single fact explains wheels, ball bearings and why we roll heavy drums instead of pushing them.

Worked example. Workers must shift a heavy water drum across a yard. Why do they tip it on its side and roll it rather than slide it upright?

Sliding the drum means fighting sliding friction; rolling it brings the much smaller rolling friction into play. Rolling needs far less effort to move the same load.

Friction as a friend: it lets us walk without slipping, grip objects, write with a pencil, light a matchstick, and stop vehicles with brakes.
Friction as a foe: it wears out shoe soles, tyres and machine parts, and converts useful energy into heat, wasting fuel.
To reduce friction: polish surfaces, use lubricants (oil, grease), use ball bearings to swap sliding for rolling, and streamline shapes to cut fluid drag.
To increase friction: add treads and grooves to tyres and soles, make handles rough, and spread sand on slippery tracks.

Common mistake: assuming friction is always bad. Without it the world would be unusable — you could not even stand up. The skill is knowing when to reduce it and when to increase it.

Where you'll meet it

Friction at work

Treads and grips

The deep grooves on cycle and truck tyres, the patterns on sports shoes, and the rough surface of a cricket bat handle all exist to increase friction so they grip and do not slip — crucial on a wet monsoon road.

Oil, grease and bearings

Bicycle chains are oiled, fan and wheel axles ride on ball bearings, and machine joints are greased — all to reduce friction, save energy and stop parts from wearing out and overheating.

Streamlined shapes

Fish, birds and aeroplanes share a smooth, tapered shape so air or water slips past with the least drag. Engineers copy these shapes for fast trains and vehicles to cut fluid friction and fuel use.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and case studies, testing whether you can use the ideas, not just recall them.

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