Friction — Set 4

Correct Answer: A. Applying oil

• **Applying oil** = oil fills the microscopic gaps between the hinge surfaces, creating a fluid-film that prevents metal-on-metal contact and reduces squeaking friction. • **Viscous lubrication** — the oil layer sustains a pressure gradient that separates the surfaces; the shear stress within the oil is far lower than solid-surface friction. • 💡 Wrong-option analysis: Tightening screws: tightening increases the normal force between hinge surfaces, which would increase friction, not reduce it; Adding sand: abrasive material increases surface roughness and friction — the opposite of reducing it; Painting it: paint adds a thin coating but does not lubricate the metal surfaces against each other.

Correct Answer: A. Lower

• **Lower** = rolling friction is much smaller than sliding friction because only a tiny contact patch deforms rather than two full surfaces rubbing against each other. • **μ_rolling << μ_sliding** — for steel on steel, μ_rolling ≈ 0.001–0.005 vs μ_sliding ≈ 0.15–0.5; the wheel was invented specifically to exploit this difference. • 💡 Wrong-option analysis: Identical: rolling and sliding friction are governed by different mechanisms and have very different coefficients; Higher: if rolling friction were higher than sliding, wheels would be harder to use than dragging — the opposite of reality; Much higher: rolling friction is much lower than sliding friction, not higher.

Correct Answer: D. It stops the car

• **It stops the car** = when brakes are applied, friction between brake pads and disc (or drum) converts the car's kinetic energy into heat, decelerating it to a stop. • **Kinetic energy dissipation** — all of the car's ½mv² must be absorbed by frictional braking; this is why brakes get hot after repeated heavy braking. • 💡 Wrong-option analysis: It has no role: without friction between brake pads and wheels (and between tyres and road), braking would be impossible; It speeds up the car: friction is a resistive force — it decelerates, never accelerates; It turns the car: turning involves steering and lateral tyre friction — not braking friction.

Correct Answer: C. Teflon

• **Teflon** = polytetrafluoroethylene (PTFE) has one of the lowest coefficients of friction of any known solid material (μ ≈ 0.04 against steel). • **μ_PTFE ≈ 0.04** — compared to rubber on steel (μ ≈ 0.8) or concrete on steel (μ ≈ 0.45), Teflon's coefficient is remarkably low due to its non-polar fluorine bonds. • 💡 Wrong-option analysis: Rubber: rubber has a very high friction coefficient against most surfaces, which is why it is used for tyres and grips; Wood: wood on steel has moderate friction (μ ≈ 0.2–0.5), much higher than Teflon; Concrete: concrete is an abrasive material with high friction coefficient against steel.

Correct Answer: B. Nature of the materials

• **Nature of the materials** = friction depends on the microscopic surface properties and atomic interactions of both materials in contact, defining their coefficient μ. • **Material-pair μ** — rubber on concrete (μ ≈ 0.8) vs ice on ice (μ ≈ 0.03) illustrates how material nature alone can change friction by a factor of 25. • 💡 Wrong-option analysis: The magnetic field: friction is a mechanical phenomenon; magnetic fields do not influence surface friction between non-magnetic materials; The color of the surfaces: color is an optical property related to light reflection, not to surface roughness or intermolecular forces; Atmospheric pressure: while extreme pressures affect friction, normal atmospheric pressure variations have negligible effect.

Correct Answer: C. Rolling friction is less than sliding friction

• **Rolling friction is less than sliding friction** = wheels replace sliding contact with rolling contact, reducing the resistive force and requiring much less effort to move the suitcase. • **Work saved** — dragging a 20 kg suitcase on a rough floor might require ~40 N of force, while rolling it requires only ~1–2 N; wheels reduce effort by ~20–40 times. • 💡 Wrong-option analysis: Wheels increase weight: adding small wheels adds negligible weight compared to the suitcase contents; Wheels increase sliding friction: wheels eliminate sliding contact — they cannot increase sliding friction; Wheels change the center of gravity: the center of gravity shifts slightly but this is not why wheeled suitcases are easier to pull.

Correct Answer: B. It increases

• **It increases** = rubbing stones together does work against friction; this work is converted to heat (kinetic energy of molecules), raising the temperature of both stones. • **W = Q** — the mechanical work done by rubbing (W = F × d) equals the heat generated (Q = mcΔT); early humans used this to make fire. • 💡 Wrong-option analysis: It drops to zero: temperature cannot fall when work is done against friction; heat is always generated; It decreases: friction always produces heat and increases temperature — it cannot decrease temperature; It stays the same: if temperature stayed the same, energy would not be conserved; friction always converts work to heat.

Correct Answer: D. Static electricity

• **Static electricity** = friction between ice crystals and air molecules within clouds causes charge separation, leading to large static electrical build-up. • **Triboelectric effect** — when two different materials rub together, electrons transfer from one to the other; in clouds this charge accumulates until lightning discharges it. • 💡 Wrong-option analysis: Magnetism: cloud friction does not generate magnetic fields; static charge builds up, not magnetic poles; Gravity: gravity is the fundamental attractive force between masses and is not generated by cloud friction; Pressure: wind and convection create pressure differences, but friction between clouds and air specifically produces static charge.

Correct Answer: C. Maximum static friction

• **Maximum static friction** = limiting friction is the peak value of static friction, reached at the precise moment just before an object begins to slide. • **f_lim = μ_s × N** — once the applied force exceeds this value, kinetic friction (μ_k × N, which is smaller) takes over and the object slides. • 💡 Wrong-option analysis: Minimum static friction: static friction can be as small as zero (no applied force); 'limiting' refers to the maximum, not minimum, value; Average kinetic friction: kinetic friction is relatively constant during sliding and is not the same as limiting friction; Minimum rolling friction: rolling friction is a separate category unrelated to the static friction limit.

Correct Answer: D. Graphite powder

• **Graphite powder** = a solid lubricant with a layered hexagonal structure; the layers slide easily over each other, reducing friction without attracting dust. • **Layered structure** — in graphite, carbon layers (graphene sheets) are held by weak van der Waals forces and slide apart readily, giving μ ≈ 0.1 as a dry lubricant. • 💡 Wrong-option analysis: Sand: an abrasive that increases surface roughness and friction — the opposite of a lubricant; Iron filings: metal particles that can scratch surfaces and increase friction; Water: can cause corrosion of metal parts and is not a suitable lubricant for most machine components.