Friction — Set 3

Correct Answer: A. Streamlined

• **Streamlined** = a shape tapered at both ends so fluid flows smoothly around it with minimal separation, turbulence, and drag. • **Drag ∝ C_d** — a streamlined shape has a very low drag coefficient C_d (≈ 0.05 for a teardrop vs ≈ 1.05 for a flat plate), greatly reducing fluid friction. • 💡 Wrong-option analysis: Circular: a sphere has moderate drag (C_d ≈ 0.47); better than a flat plate but not as effective as a streamlined teardrop form; Square: flat faces cause large pressure drag and flow separation, giving a high drag coefficient; Triangular: sharp edges create turbulence and flow separation; not effective at reducing drag.

Correct Answer: B. It causes noise

• **It causes noise** = friction between moving surfaces converts mechanical energy into vibrations, which radiate as sound — a form of energy waste. • **Unwanted energy conversion** — squeaking hinges, grinding brakes, and chattering gears all demonstrate friction converting kinetic energy into acoustic energy. • 💡 Wrong-option analysis: It stops moving vehicles: braking is a beneficial use of friction, not a disadvantage; It helps in writing: pencil/chalk writing relies on friction — this is an advantage; It helps us walk: walking requires friction between foot and ground — clearly a benefit, not a disadvantage.

Correct Answer: B. Friction decreases

• **Friction decreases** = grease creates a thin lubricating film between the metal links of the chain, separating surfaces and reducing the coefficient of friction. • **Fluid-film lubrication** — the viscous grease layer prevents metal-to-metal contact; the chain now slides through grease rather than on dry metal, sharply reducing friction. • 💡 Wrong-option analysis: The chain stops moving: grease makes the chain move more freely, not stop; Friction remains the same: a lubricating layer physically changes the contact condition and lowers μ; Friction increases: lubricants always reduce friction — the opposite of increasing it.

Correct Answer: D. Greater than

• **Greater than** = static friction (up to its limiting value) is always greater than kinetic friction for the same pair of surfaces and the same normal force. • **μ_s > μ_k** — the static coefficient of friction always exceeds the kinetic coefficient because interlocking irregularities have time to seat deeply when surfaces are at rest. • 💡 Wrong-option analysis: Equal to: μ_s > μ_k for all real surfaces — they are never equal; Less than: if static friction were less than kinetic, objects would accelerate spontaneously once sliding started, which violates observation; Zero compared to: static friction is never zero unless the surface is frictionless (μ_s = 0).

Correct Answer: C. Reduce friction

• **Reduce friction** = talcum powder fills the microscopic pits and irregularities on the carrom board surface, creating a smoother, low-friction interface. • **Solid lubricant** — talc particles act as tiny rollers/ball-bearings between the striker and the board, converting some sliding friction to rolling and lowering overall resistance. • 💡 Wrong-option analysis: Clean the board: talcum powder adds a white coating; it does not clean the board of dirt; Make the striker heavy: powder has negligible mass and cannot make the striker heavier; Increase friction: talcum powder is specifically used to decrease friction for smooth, fast play.

Correct Answer: C. Centripetal friction

• **Centripetal friction** = not a recognised type of friction; centripetal refers to the direction of a force required for circular motion, not a friction category. • **Friction types** — the standard classifications are static friction (no relative motion), kinetic/sliding friction (surfaces sliding), and rolling friction (surfaces rolling). • 💡 Wrong-option analysis: Rolling friction: a well-defined type that acts when one surface rolls over another; Static friction: acts when surfaces are in contact with no relative motion; Sliding friction: also called kinetic friction, acts when surfaces slide against each other.

Correct Answer: D. Water

• **Water** = fluid friction (drag) is much higher in water than in air because water has far greater density and viscosity than air. • **Density ratio** — water is about 800 times denser than air; drag force F = ½ρCdAv² scales directly with density ρ, so drag in water is ~800 times that in air at the same speed. • 💡 Wrong-option analysis: Air: air has low density (~1.2 kg/m³) and low viscosity, so drag is relatively small compared to water; Space: space is essentially a vacuum with no medium, so there is no fluid friction at all; Vacuum: a perfect vacuum has zero fluid, hence zero fluid friction.

Correct Answer: C. To increase friction

• **To increase friction** = spikes penetrate the ground surface, interlocking deeply with the soil or track material to provide a very high-friction grip. • **Grip force** — spikes increase the effective μ between shoe and ground far beyond what a flat sole can achieve, preventing slipping during explosive starts and turns. • 💡 Wrong-option analysis: To look tall: spikes add minimal height and their purpose is functional, not aesthetic; To decrease friction: the entire purpose of spikes is the opposite — to maximise traction; To reduce weight: spikes are small metal/ceramic points that add minimal weight and are not designed to reduce it.

Correct Answer: A. Decreases significantly

• **Decreases significantly** = a thin air layer between surfaces acts as a low-viscosity fluid separator, preventing solid-to-solid contact and dropping friction to near zero. • **Air bearings** — this principle is used in air hockey tables and precision air-bearing spindles where objects float on a thin pressurised air film with negligible friction. • 💡 Wrong-option analysis: Remains the same: introducing an air gap fundamentally changes the contact condition and reduces friction; Becomes infinite: infinite friction would mean the surfaces are locked together — the opposite of the air-layer effect; Increases greatly: an air cushion reduces contact and friction; it cannot increase friction.

Correct Answer: D. Heat energy

• **Heat energy** = friction converts the kinetic energy of sliding motion into thermal energy through molecular-level interactions at the contact surface. • **First Law of Thermodynamics** — work done against friction (W = f_k × d) equals the heat generated (Q = mcΔT), demonstrating energy conservation. • 💡 Wrong-option analysis: Potential energy: potential energy depends on position/height, not on rubbing surfaces; Chemical energy: friction does not cause chemical reactions in ordinary conditions; Nuclear energy: nuclear energy involves changes in atomic nuclei — completely unrelated to frictional surface contact.