Friction — Set 5

Correct Answer: A. Contact force

• **Contact force** = friction is classified as a contact force because it only exists when two surfaces (or a surface and fluid) are physically touching. • **Contact vs non-contact** — contact forces (friction, normal, tension) require physical touch; non-contact forces (gravity, magnetism, electrostatics) act at a distance. • 💡 Wrong-option analysis: Fundamental force: the four fundamental forces are gravity, electromagnetism, strong nuclear, weak nuclear — friction is not one of them; Nuclear force: nuclear forces act inside atomic nuclei at femtometre scales, unrelated to macroscopic surface contact; Non-contact force: friction requires direct surface contact — it cannot act without physical touching.

Correct Answer: B. Surface irregularities

• **Surface irregularities** = even apparently smooth surfaces have microscopic peaks (asperities) and valleys that interlock when surfaces press together, creating friction. • **Real contact area** — actual contact occurs only at the tips of asperities; the real contact area is much smaller than geometric area, but the interlocking creates measurable resistance. • 💡 Wrong-option analysis: Air pressure: atmospheric pressure acts uniformly on all surfaces and does not cause the directional resistance characteristic of friction; Magnetic attraction: only relevant for ferromagnetic materials; it is not the primary cause of friction between ordinary surfaces; Gravity: provides the normal force that presses surfaces together, but the interlocking irregularities are the actual cause of friction.

Correct Answer: D. It is opposite to the relative motion

• **It is opposite to the relative motion** = friction always acts as a retarding force, opposing whichever direction the surfaces move (or tend to move) relative to each other. • **Vector direction** — if object A moves right relative to object B, friction on A points left; friction on B points right (Newton's Third Law pair). • 💡 Wrong-option analysis: It is in the direction of the applied force: friction opposes applied force, never assists it in the same direction; It is always horizontal: on an inclined surface, friction acts along the incline — not necessarily horizontal; It is always vertical: the normal force is perpendicular (often vertical), but friction acts along the contact surface, not perpendicular to it.

Correct Answer: C. Rolling friction

• **Rolling friction** = generally the smallest type of friction because the contact region is a tiny point or line, and deformation energy is much less than in sliding contact. • **Magnitude order** — static (limiting) ≥ sliding (kinetic) >> rolling; μ_rolling ≈ 0.001–0.01 compared to μ_sliding ≈ 0.1–0.8 for typical surfaces. • 💡 Wrong-option analysis: Sliding friction: also called kinetic friction, it is larger than rolling friction and acts when surfaces slide against each other; Limiting friction: the maximum static friction — larger than kinetic friction and much larger than rolling friction; Static friction: ranges from zero up to its limiting value, which exceeds kinetic friction.

Correct Answer: B. Contact between surfaces

• **Contact between surfaces** = friction requires physical interaction between surfaces or between a surface and a fluid; without contact, there is no friction. • **No contact = no friction** — an object floating freely in space experiences zero friction because nothing touches it; friction appears the instant contact is made. • 💡 Wrong-option analysis: High temperature: friction can exist at any temperature — it is contact, not temperature, that is the necessary condition; Motion: static friction exists even without motion (surfaces at rest can have static friction resisting an applied force); Vacuum: friction between solid surfaces can exist in vacuum (e.g., spacecraft mechanisms) — vacuum eliminates fluid friction but not solid contact friction.

Correct Answer: A. Lubricants

• **Lubricants** = substances like oil, grease, or graphite powder used to create a film between surfaces, minimising direct contact and reducing friction. • **Film separation** — lubricants work by keeping the two solid surfaces apart; the shear stress needed to slide within the lubricant film is far less than solid-on-solid friction. • 💡 Wrong-option analysis: Conductors: materials (metals) that allow electric current to flow — unrelated to surface friction reduction; Abrasives: rough materials (sandpaper, grinding wheels) that increase friction and remove material from surfaces; Adhesives: glue-like substances that bond surfaces together, increasing resistance to relative motion — the opposite of lubricants.

Correct Answer: A. Due to air friction

• **Due to air friction** = meteors travelling at 11–72 km/s compress and heat the surrounding air through friction and compression, generating temperatures up to 1,650°C. • **Atmospheric drag** — the enormous drag force (F ∝ v²) at hypersonic speeds converts the meteor's kinetic energy into heat fast enough to vaporise rock. • 💡 Wrong-option analysis: Due to solar heat: solar radiation heats objects in space gently; it cannot cause the rapid ablation seen during atmospheric entry; Due to gravity: gravity accelerates the meteor but does not directly generate the heat — that comes from air friction; Due to lack of oxygen: oxygen in the atmosphere can support combustion but the primary heat source is compressive air friction, not simply lack of oxygen.

Correct Answer: D. Production of heat

• **Production of heat** = friction always dissipates mechanical energy as thermal energy; every surface contact in motion generates heat as its direct by-product. • **Q = f_k × d** — the heat produced equals friction force times sliding distance; this is why brakes, bearings, and gearboxes require cooling in continuous operation. • 💡 Wrong-option analysis: Creation of gravity: gravity is a fundamental force determined by mass — friction cannot create gravity; Decrease in time: friction slows machines but does not decrease time itself; Increase in mass: mass is an intrinsic property; friction cannot change the mass of any component.

Correct Answer: B. It is useful but causes energy loss

• **It is useful but causes energy loss** = friction enables walking, gripping, writing, and braking (necessary), yet wastes energy as heat and wears out parts (evil). • **Dual role** — engineers design systems to maximise beneficial friction (tyres, brakes, clutches) while minimising harmful friction (bearings, pistons, gears) simultaneously. • 💡 Wrong-option analysis: It is always good: friction causes heat generation, wear, and energy waste in machines — it is not always beneficial; It is very expensive to create: friction occurs naturally at all contact surfaces and needs no manufacturing cost to create; It is always bad: without friction we could not walk, drive, write, or hold anything — it is clearly not always bad.

Correct Answer: B. It increases friction

• **It increases friction** = friction force f = μN; increasing N directly and proportionally increases f, since μ (dependent on material) stays constant. • **Linear relationship** — doubling the normal force (e.g., stacking another weight on a box) doubles the friction; this is Amontons' Second Law. • 💡 Wrong-option analysis: It makes friction zero: f = μN; only if μ = 0 would friction be zero regardless of N — increasing N cannot make friction zero; It has no effect: N directly multiplies μ to give f; any change in N changes f proportionally; It decreases friction: increasing N increases surface interlocking, raising friction — it cannot decrease it.