Friction — Set 6

Correct Answer: D. Streamlined body

• **Streamlined body** = a bird's body is tapered from beak to tail, allowing air to flow smoothly around it with minimal turbulence and low drag. • **Low drag coefficient** — birds typically have C_d ≈ 0.4 or less; their feathers and body taper reduce flow separation, cutting air resistance for efficient flight. • 💡 Wrong-option analysis: Heavy bones: heavier bones would increase weight and require more lift force, making flight less efficient — not a drag-reduction feature; Feather color: color is an optical property; it has no aerodynamic effect on drag; Large eyes: eye size affects vision, not aerodynamics; large eyes would slightly increase drag, not reduce it.

Correct Answer: B. Apparent area of contact

• **Apparent area of contact** = friction force is independent of the geometric contact area for dry solid surfaces — this is Amontons' First Law of Friction. • **Amontons' First Law** — f = μN holds whether a brick is laid flat (large area) or on its edge (small area); as area increases, contact pressure decreases proportionally, keeping friction constant. • 💡 Wrong-option analysis: Normal force: f = μN — friction is directly proportional to N, so normal force absolutely affects friction; Roughness: a rougher surface has a higher μ, directly increasing the friction force; Nature of surfaces: different material pairs have different μ values, directly determining the magnitude of friction.

Correct Answer: D. To increase friction

• **To increase friction** = sand particles add roughness to the icy surface, giving tyres and shoes more points of mechanical interlocking and raising the effective μ. • **Traction improvement** — ice has μ ≈ 0.03; adding sand can raise effective μ to 0.3–0.5, which is a 10× improvement in available traction. • 💡 Wrong-option analysis: To decorate the track: safety, not aesthetics, is the reason sand is spread on icy tracks; To melt the snow: sand does not generate significant heat and cannot melt snow; To make the track soft: sand does not soften ice — it adds surface roughness while the ice remains hard.

Correct Answer: D. It may increase due to adhesion

• **It may increase due to adhesion** = over-polished surfaces can have increased friction because molecules come close enough for intermolecular attractive forces (adhesion) to activate. • **Molecular adhesion** — on atomic scales, perfectly flat surfaces allow more contact between surface atoms; van der Waals and covalent forces create a 'stickiness' that raises friction beyond the normal-polishing reduction. • 💡 Wrong-option analysis: It always decreases: mild polishing reduces friction, but excessive polishing triggers adhesion effects that can increase it; It becomes infinite: adhesion effects are strong but finite; 'infinite' friction would require the surfaces to be inseparable; It becomes zero: perfect smoothness would theoretically give zero irregularity-based friction but increases adhesion-based friction — net result is not zero.

Correct Answer: C. A child on a playground slide

• **A child on a playground slide** = the child's clothing slides across the slide surface — two solid surfaces moving relative to each other — which is the definition of sliding (kinetic) friction. • **Kinetic friction** — sliding friction f_k = μ_k × N acts between the child's clothes and the slide material, determining the speed of descent. • 💡 Wrong-option analysis: A boat floating in water: the boat experiences fluid friction (drag) from water, not sliding friction between two solid surfaces; A car parked on a hill: the car is stationary, so static friction (not sliding friction) acts between tyres and road; A ball rolling on the ground: rolling involves rolling friction, not sliding friction.

Correct Answer: A. Mu

• **Mu** = the Greek letter μ (mu) is the universal symbol for the coefficient of friction in physics, representing the ratio of frictional force to normal force. • **μ = f / N** — μ_s (static) and μ_k (kinetic) are dimensionless values specific to each pair of surfaces; typical values range from 0.01 (ice on ice) to 1.0 (rubber on concrete). • 💡 Wrong-option analysis: Beta (β): used in optics (velocity as fraction of c) and statistics, not for friction coefficient; Delta (δ or Δ): used for small changes, deflection, or uncertainty — not for friction; Alpha (α): used for angular acceleration, thermal expansion coefficient, or significance level — not the friction coefficient symbol.

Correct Answer: B. To reduce friction for the coins

• **To reduce friction for the coins** = talcum powder fills microscopic depressions in the wooden board, creating a smoother, lower-friction surface for the coins and striker. • **Solid lubricant** — talc particles roll between the coin and board like tiny ball-bearings, converting some sliding friction to rolling friction and reducing overall resistance. • 💡 Wrong-option analysis: To make the board smell good: talcum powder is used for its friction-reducing properties, not its scent; To protect the wood from moisture: talc does not seal the wood or offer meaningful moisture protection; To increase the weight of the striker: talc powder is too light to add any significant mass to the striker.

Correct Answer: C. Treading of tires

• **Treading of tires** = grooves cut into the tyre surface increase the tyre's contact-edge length with the road and channel water away, improving grip and increasing friction. • **Wet grip** — on a wet road, tread channels disperse up to 30 litres of water per second per tyre; without treads, a water film lifts the tyre off the road (aquaplaning). • 💡 Wrong-option analysis: Using ball bearings: ball bearings convert sliding to rolling friction, reducing resistance — used to decrease friction, not increase it; Applying grease: grease is a lubricant that reduces friction — the opposite of what is needed when grip is required; Polishing: a polished smooth surface has fewer irregularities and lower friction — reduces grip rather than increasing it.

Correct Answer: A. Viscosity

• **Viscosity** = the internal resistance of a fluid to flow or to the motion of objects through it; it is the fluid analogue of solid-surface friction. • **η (eta)** — dynamic viscosity η quantifies how strongly a fluid resists shear; water η ≈ 0.001 Pa·s vs honey η ≈ 10 Pa·s, explaining why honey flows much more slowly. • 💡 Wrong-option analysis: Dry friction: friction between unlubricated solid surfaces — not the resistance offered by gases or liquids; Rolling friction: resistance when a round object rolls on a surface — does not apply to fluid flow; Static friction: the force preventing stationary objects from starting to slide — irrelevant to fluid resistance.

Correct Answer: A. Friction

• **Friction** = rubbing two surfaces together converts the mechanical work done into thermal energy (heat) at the contact interface, warming both surfaces. • **W = Q** — every joule of work done against friction becomes heat; rubbing hands together at moderate force produces ~0.5–1 W of heat, noticeably warming them in seconds. • 💡 Wrong-option analysis: Radiation: thermal radiation heats objects from a distance — rubbing is a contact process, not radiation; Chemical reaction: no chemical change occurs when skin rubs against skin; the heat is purely mechanical; Conduction: conduction transfers heat between objects already at different temperatures — it does not generate new heat from mechanical motion.