Friction — Set 2

Correct Answer: D. Limiting friction

• **Limiting friction** = the maximum value of static friction, acting at the instant just before an object begins to slide on a surface. • **f_lim = μ_s × N** — limiting friction equals the static coefficient times the normal force; once the applied force exceeds this, motion begins. • 💡 Wrong-option analysis: Rolling friction: the resistance to rolling motion — unrelated to the maximum static condition; Dynamic friction: another name for kinetic friction, which acts after motion has started; Fluid friction: drag in liquids or gases — not relevant to the static-to-kinetic transition of solid surfaces.

Correct Answer: B. Making surfaces rough

• **Making surfaces rough** = increases the number of microscopic interlocking irregularities, requiring more force to overcome and thus raising friction. • **Higher μ** — greater surface roughness corresponds to a higher coefficient of friction μ, increasing f = μN for any given load. • 💡 Wrong-option analysis: Polishing surfaces: smoothing reduces interlocking of irregularities, lowering μ and decreasing friction; Using ball bearings: converts sliding to rolling friction, which reduces rather than increases resistance; Using lubricants: lubricants create a film that separates surfaces and lowers the effective friction.

Correct Answer: A. Friction produces heat

• **Friction produces heat** = kinetic energy lost to friction is converted into thermal energy at the contact surfaces, raising their temperature. • **Energy dissipation** — the work done against friction (W = f × d) becomes internal energy of the surface molecules, measurable as a temperature rise. • 💡 Wrong-option analysis: Friction is always a nuisance: friction is essential for walking, writing, braking — it has many beneficial uses; Friction only occurs in solids: fluid friction (drag) exists in liquids and gases as well; Friction does not depend on weight: friction depends directly on the normal force, which equals weight for horizontal surfaces.

Correct Answer: B. Normal force

• **Normal force** = the coefficient of friction μ = f / N, where f is the frictional force and N is the normal (perpendicular) contact force. • **μ is dimensionless** — since both f and N are forces, their ratio has no units; μ characterises the roughness of a specific pair of surfaces. • 💡 Wrong-option analysis: Applied force: friction is not defined relative to applied force — it equals μN only when motion is about to start or during sliding; Tension: tension acts along strings or ropes and is unrelated to the definition of the friction coefficient; Gravitational force: on a horizontal surface N equals weight (gravity), but on inclined surfaces N < mg, so 'gravitational force' is not the general definition.

Correct Answer: B. To increase friction on wet roads

• **To increase friction on wet roads** = tyre treads channel water away from the contact patch, maintaining rubber-to-road contact and preventing aquaplaning. • **Aquaplaning** — at high speed, water builds up under a smooth tyre faster than it can escape, lifting the tyre off the road; treads prevent this by providing drainage channels. • 💡 Wrong-option analysis: To decrease the cost: tread patterns add manufacturing complexity and cost; they exist for safety, not cost reduction; To reduce weight: grooves remove rubber, marginally reducing weight, but that is not the design purpose; To look good: aesthetic appeal is not the engineering reason for tread patterns.

Correct Answer: B. Area of contact

• **Area of contact** = for dry solid surfaces, friction does NOT depend on the apparent (geometric) area of contact — this is Amontons' First Law. • **Amontons' First Law** — f = μN holds regardless of the contact area, because the real microscopic contact area is proportional to N, not to geometric area. • 💡 Wrong-option analysis: Roughness: affects μ and therefore directly changes friction; Nature of surfaces: different materials have different μ values, directly affecting friction; Normal force: friction = μN, so it is directly proportional to normal force.

Correct Answer: A. Speed of the object

• **Speed of the object** = fluid (drag) friction increases with speed; at low speeds it is proportional to v, and at high speeds it is proportional to v². • **Stokes' Law / Drag equation** — F_drag = 6πηrv (Stokes) at low Re, and F_drag = ½ρCdAv² at high Re; both show direct speed dependence. • 💡 Wrong-option analysis: Color of the object: color is an optical property with no effect on drag or fluid friction; Material of the container: the fluid's viscosity matters, not the container's material; Mass of the object: mass affects inertia and terminal velocity but is not a direct factor in the formula for drag force.

Correct Answer: B. Writing with a pencil

• **Writing with a pencil** = graphite rubs off onto paper due to friction between the pencil tip and paper surface — friction is essential for this action. • **Useful friction** — without sufficient friction between lead and paper, no mark would transfer; the same principle applies to chalk on blackboards. • 💡 Wrong-option analysis: Energy loss in machines: heat generated by friction in engines and gearboxes is a harmful waste of energy; Wear of tires: tread erosion from road friction shortens tire life — a disadvantage; Overheating of engines: excess friction in engine parts converts fuel energy into unwanted heat, damaging components.

Correct Answer: D. Adhesion

• **Adhesion** = when surfaces are polished to extreme flatness, molecules come so close that intermolecular attractive forces (adhesion) significantly increase friction. • **Cold welding** — very clean, flat metal surfaces pressed together in a vacuum can actually cold-weld due to this molecular adhesion, illustrating the extreme case. • 💡 Wrong-option analysis: Magnetism: magnetic attraction only affects ferromagnetic materials and is not the reason polished surfaces stick; Evaporation: evaporation is a phase transition unrelated to surface friction or adhesion; Gravity: gravity provides the normal force but does not directly cause the adhesion effect between ultra-smooth surfaces.

Correct Answer: B. Heavier objects have more friction

• **Heavier objects have more friction** = a heavier object exerts a greater normal force on the surface, and since f = μN, friction increases proportionally. • **f = μN** — doubling the weight doubles the normal force and therefore doubles the frictional force for the same μ. • 💡 Wrong-option analysis: Weight has no effect: weight determines the normal force which directly scales friction — weight is never irrelevant; Lighter objects have more friction: the opposite is true — less weight means less normal force and less friction; Friction becomes zero for heavy objects: friction increases with weight; it would only be zero if μ = 0 regardless of weight.