Friction
Friction is the contact force that always opposes the relative motion between two surfaces touching each other, making it both indispensable and wasteful in everyday life.
Friction is core NCERT physics whose simple, application-rich facts are favourite Prelims fodder — the ordering of static/sliding/rolling friction, factors affecting it, lubricants, ball bearings and fluid drag. For GS-III it underpins science-and-technology basics like energy efficiency, biomimicry (streamlining) and engineering design choices. Conceptual clarity here also helps decode everyday-science MCQs.
Understand the chapter
Force of Friction: The Universal Opposer
Any object moving over another surface slows and finally stops even without an external push, because a resisting contact force — friction — acts opposite to its motion. Push a book left and friction acts right; push it right and friction acts left, so friction always opposes the applied force and the relative motion. The force can be measured with a spring balance by noting the reading when an object just begins to move.
- Friction acts between the two surfaces in contact and on both of them.
- It always opposes relative motion — it has no fixed direction of its own.
- Spring balance: measures the applied force / force of friction.
What Friction Depends On
Friction arises because even surfaces that look smooth carry countless minute irregularities that interlock; moving a surface needs force to overcome this interlocking. Rougher surfaces have more irregularities and so generate greater friction, while pressing the surfaces harder increases friction further. Hence friction depends on the nature and smoothness of the surfaces and on how hard they are pressed together.
- Cause: interlocking of irregularities on the two surfaces.
- More irregularities on rough surfaces → greater friction.
- Greater pressing force (e.g., a heavier load) → greater friction.
Static, Sliding and Rolling Friction
Static friction is the force needed to set a resting object into motion, while sliding friction keeps an already-moving object sliding at the same speed. Sliding friction is slightly smaller than static friction because, once sliding, the contact points get too little time to lock into the opposite surface — that is why it is easier to keep a box moving than to start it. When a body rolls instead of slides, rolling friction comes into play and is the smallest of the three.
- Static friction: opposes the start of motion (largest).
- Sliding (kinetic) friction: opposes ongoing sliding (smaller than static).
- Rolling friction: opposes rolling (smallest).
- Decreasing order: static > sliding > rolling.
Friction: A Necessary Evil
Friction is indispensable — we could not walk, grip objects, write with chalk or pencil, fix a nail, tie a knot or construct buildings without it, and brakes and tyres rely on it entirely. Yet it is also harmful: it wears out shoe soles, screws and ball bearings, and produces heat that wastes energy in running machines. So friction is simultaneously a friend and a foe — a true 'necessary evil'.
- Friend: walking, gripping, writing, braking, nails/knots, construction.
- Foe: wear and tear of materials; heat and energy loss in machines.
- Heat from friction: rubbing palms, striking a matchstick, a mixer jar warming up.
Increasing and Reducing Friction
Where grip matters, friction is deliberately increased — shoe soles and tyres are treaded/grooved, brake pads press the rim, and kabaddi players or gymnasts roughen their hands. Where it wastes energy, friction is reduced using lubricants like oil, grease or graphite, which form a thin layer that prevents direct rubbing and avoids interlocking of irregularities. Where oil is unsuitable, an air cushion is used; sprinkling powder on a carrom board works on the same principle.
- Increase: treading of soles/tyres, brake pads, soil/coarse powder for grip.
- Reduce: lubricants (oil, grease, graphite); air cushion.
- Lubricant logic: a thin layer keeps surfaces from directly rubbing.
Wheels and Ball Bearings
Because rolling friction is smaller than sliding friction, replacing sliding with rolling greatly eases motion — this is why rollered luggage is easy to pull and the wheel is called one of mankind's greatest inventions. In most machines, sliding is replaced by rolling using ball bearings, for example between the hub and the axle of ceiling fans and bicycles.
- Rolling friction < sliding friction → the 'wheel' advantage.
- Ball bearings convert sliding contact into rolling contact.
- Examples: hubs and axles of ceiling fans and bicycles.
Fluid Friction and Streamlining
Fluids — the common name for gases and liquids — also exert friction, called drag, on objects moving through them. The drag depends on the object's speed relative to the fluid, on its shape, and on the nature of the fluid. To lose less energy, objects are given special streamlined shapes copied from nature, which is why aeroplanes resemble birds and fast-moving bodies mimic fishes.
- Fluids = gases + liquids; their friction is called drag.
- Drag depends on speed, shape of the object, and nature of the fluid.
- Streamlined shapes (birds, fish, aeroplanes) minimise drag.
Key terms
- Friction
- Contact force that opposes the relative motion between two surfaces touching each other.
- Static friction
- Force that must be overcome to start a stationary object moving; the largest of the three types.
- Sliding friction
- Force opposing an object already sliding at steady speed; slightly smaller than static friction.
- Rolling friction
- Resistance when one body rolls over another; the smallest of the three types.
- Fluid friction (drag)
- Frictional force exerted by gases and liquids on objects moving through them.
- Lubricants
- Substances like oil, grease and graphite that form a thin layer to reduce friction between moving parts.
- Ball bearing
- Device that replaces sliding with rolling friction between machine parts such as hubs and axles.
- Interlocking
- Locking together of minute surface irregularities that gives rise to friction.
- Spring balance
- Device with a coiled spring and graduated scale used to measure the force acting on an object.
- Streamlining
- Giving bodies special shapes (like birds or fish) to minimise fluid friction or drag.
Must-know facts exam-ready
- Friction always opposes relative motion and acts on both surfaces in contact.
- Friction is caused by the interlocking of minute irregularities on the surfaces.
- Rougher surfaces and harder pressing both increase friction.
- Decreasing order of friction: static > sliding > rolling.
- Sliding friction is slightly less than static friction because contact points get no time to lock.
- Rolling friction is the smallest, which is why the wheel is a landmark invention.
- Ball bearings reduce friction by converting sliding into rolling.
- Lubricants — oil, grease, graphite — reduce friction; an air cushion is used where oil is unsuitable.
- Fluids = gases + liquids; the friction they exert is called drag.
- Drag depends on speed relative to the fluid, shape of the object, and nature of the fluid.
- Streamlined shapes (birds, fish, aeroplanes) minimise fluid friction.
- Friction can never be reduced to zero — no surface is perfectly smooth.
Memory tricks remember it for good
Traps to avoid
- Sliding friction is LESS than static friction, not equal or greater — it is easier to keep a box moving than to start it.
- The correct decreasing order is static > sliding > rolling; rolling is the smallest, not the largest.
- Spilling soapy water makes walking HARDER, not easier — the soap acts as a lubricant and reduces grip.
- A 'fluid' means both liquids AND gases; air too exerts friction (drag).
- Friction can never be made zero, however much you polish or lubricate — some irregularities always remain.
- A heavier box needs MORE force because pressing harder increases friction (Seema applies more force than Iqbal).
Exam focus
🧠 Prelims angles
- Arranging rolling, static and sliding friction in decreasing order (static > sliding > rolling).
- Ordering surfaces by friction: wet marble < dry marble < newspaper < towel (increasing).
- Factors affecting friction: nature/smoothness of surfaces and how hard they press together.
- Methods to reduce friction: lubricants, ball bearings, air cushion and streamlining.
- Fluid friction is called drag and depends on speed, shape and nature of the fluid.
- Spring balance is the device used to measure force.
✍️ Mains angles GS-III
- 'Friction is a necessary evil.' Examine with examples from daily life and technology.Balance friend-roles (walking, gripping, braking, construction) against foe-roles (wear, heat, energy loss), then close with lubrication and ball bearings as management strategies.
- How does biomimicry — streamlining inspired by birds and fish — inform modern vehicle and aircraft design?Link drag-reduction to energy efficiency; cite the aeroplane–bird shape similarity noted in the chapter.
- Discuss how reducing friction improves energy efficiency in machines.Use lubricants, ball bearings and air cushions; connect frictional heat as energy wastage to the sustainability angle.
Last-minute revision tick as you recall
- Friction = contact force opposing relative motion; acts on both surfaces.
- Cause: interlocking of surface irregularities.
- Rougher surfaces + harder pressing → more friction.
- Order: static > sliding > rolling (rolling smallest).
- Sliding < static: contact points cannot lock in time.
- Reduce friction: lubricants (oil/grease/graphite), ball bearings, air cushion.
- Fluid friction = drag; depends on speed, shape and nature of fluid.
- Streamlining (birds/fish/aeroplane) cuts drag.
- Friction can never be reduced to zero.
Distilled from NCERT Class 8 · Science (Class 8) for UPSC. Always cross-check facts with the original NCERT.