Friction is essential for locomotion and numerous practical functions. Five main advantages: (1) WALKING: static friction between foot and ground allows a person to walk. The foot pushes backward (action), and friction from the ground pushes the person forward (reaction). Without friction, feet would slip. This is also why we cannot walk on extremely smooth ice — there is insufficient friction. (2) STICKING/GRIPPING: two surfaces can be held together by friction. Nuts and bolts stay tight due to friction between threads. You hold a glass without it slipping due to friction between hand and glass. (3) BRAKING: vehicle brakes work by pressing brake pads onto rotating discs/drums — kinetic friction decelerates the wheel, stopping the vehicle. Without friction, no braking is possible. (4) WRITING: the transfer of ink from pen to paper or graphite from pencil to paper requires friction between the writing instrument and the paper surface. Smooth glass paper is difficult to write on. (5) BELT DRIVE: friction between drive belt and pulleys transmits mechanical power from one shaft to another in machines (engines, fans, industrial machinery).

• Walking mechanism in detail: when you push your foot backward against the ground (action force, backward), the ground exerts friction forward on your foot (reaction force, forward). This forward friction is what propels you forward — it is static friction because your foot does not slide while in contact with the ground. The 'contact point' of your foot has zero velocity relative to the ground during each step. The same mechanism applies to vehicle acceleration: tyre pushes backward on road (action); road pushes tyre forward via friction (reaction) → vehicle accelerates forward.
• Braking mechanism: when brakes are applied, brake pads press against the wheel disc. Kinetic friction between pad and disc generates a torque opposing wheel rotation, slowing the wheel. Simultaneously, static friction between tyre and road decelerates the vehicle body (as long as the wheel rolls without skidding). If the wheel locks (skids), kinetic friction between tyre and road decelerates the vehicle — but steering becomes impossible since rolling contact is lost. Anti-lock braking systems (ABS) prevent wheel lock to maintain rolling friction and steering.
• Belt drive energy transfer: a belt wrapped around two pulleys (driving and driven) transmits rotational motion via friction between belt and pulley surfaces. The maximum torque that can be transmitted is limited by the limiting friction at the contact between belt and pulley. If the driving torque is too large, the belt slips — kinetic friction replaces static friction and power transmission becomes less efficient. Increasing belt tension or using higher friction materials increases the maximum transmittable torque.

Three main disadvantages of friction: (1) ENERGY DISSIPATION: friction always opposes relative motion. Any machine with sliding parts must continuously supply energy to overcome friction, reducing mechanical efficiency. Energy consumed by friction appears as heat (W_friction = f × d). A machine with high friction has low efficiency = (useful output work) / (total input work). (2) WEAR AND TEAR: repeated sliding breaks off material at the contact asperities, causing the surfaces to gradually wear down. Machine parts must be replaced periodically due to wear. Wear is proportional to sliding distance, normal force, and inversely proportional to material hardness (Archard's wear law). (3) HEAT GENERATION: frictional work converts entirely to thermal energy. This heat can damage temperature-sensitive machine components (bearings, seals, electronics), requiring cooling systems. Five methods to REDUCE friction: (1) Polishing; (2) Lubrication; (3) Proper material selection; (4) Streamlining; (5) Ball bearings.

• Lubrication mechanism: a thin lubricant film (oil, grease) is interposed between the two sliding surfaces. The lubricant fills micro-asperities, preventing direct metal-to-metal contact. Friction now occurs within the fluid film rather than at the solid surfaces — fluid friction is much less than solid contact friction. Viscosity of the lubricant determines the fluid friction. Lower viscosity → less fluid friction but less protection. Higher viscosity → more fluid friction but better protection against contact. Engine oil, bearing grease, and cutting fluids are all lubricants. Coefficient of friction can drop from μ ≈ 0.3–0.5 (dry metal) to μ < 0.01 (well-lubricated).
• Ball bearings convert sliding friction to rolling friction: at a rotating axle, without bearings, the shaft slides inside the housing — continuous kinetic (sliding) friction generates heat and resists rotation. With ball bearings: small hardened steel balls roll between the inner race (on shaft) and outer race (on housing). Friction becomes rolling friction (magnitude F_rolling = μ_r R/r << f_k). Wear is minimal because rolling involves no sliding at contact points. Result: drastically reduced friction, less heat, longer part life.
• In manufacturing of tyres, synthetic rubber is preferred because its coefficient of friction with the road is larger. This is an example of INCREASING friction intentionally using material selection — the opposite of friction reduction. Synthetic rubber has higher μ_s and μ_k with road surfaces than natural rubber, improving braking and cornering. Sand thrown on slippery ground also increases friction by increasing the effective surface roughness (more asperities between foot/tyre and ground). NEET may ask: which method INCREASES friction? → coarser material or throwing sand; which method decreases friction? → polishing, lubrication, ball bearings.