Friction — Set 1

Correct Answer: D. Static friction

• **Static friction** = the resistive force that prevents two surfaces from sliding relative to each other when no relative motion exists. • **f_s ≤ μ_s × N** — static friction adjusts from zero up to the limiting value; it does not have a fixed magnitude. • 💡 Wrong-option analysis: Rolling friction: occurs when one surface rolls over another, not when surfaces are stationary; Fluid friction: the drag force in liquids or gases, not between solids at rest; Sliding friction: acts only when the surfaces are already moving relative to each other.

Correct Answer: A. It increases

• **Friction increases** = rougher surfaces have more microscopic peaks and valleys that interlock, requiring greater force to overcome. • **f = μN** — a higher coefficient of friction μ (due to roughness) directly increases the frictional force f for the same normal force N. • 💡 Wrong-option analysis: It remains constant: friction is not fixed; it depends on the roughness (μ) of surfaces; It becomes zero: zero friction requires a perfectly smooth or lubricated surface; It decreases: decreasing friction requires smoothing or lubrication, not increased roughness.

Correct Answer: A. Ball bearings

• **Ball bearings** = convert sliding friction to rolling friction, drastically reducing the resistance in rotating machine parts. • **Rolling friction << sliding friction** — rolling contact has a much smaller deformation zone, so energy loss is far lower than sliding contact. • 💡 Wrong-option analysis: Glue: adhesives increase bonding between surfaces, making motion harder, not easier; Rough paper: rough surfaces increase interlocking and friction; Sand: abrasive materials increase surface roughness and friction.

Correct Answer: D. Opposite direction

• **Opposite direction** = friction always opposes the relative motion (or tendency of motion) between two surfaces, acting as a retarding force. • **Newton's Third Law context** — friction is a reaction force at the contact surface; it cannot act in the same direction as motion since that would accelerate rather than resist. • 💡 Wrong-option analysis: Random direction: friction always has a definite direction opposing motion — it is never random; Same direction: if friction acted in the direction of motion it would be a driving force, not friction; Perpendicular direction: the normal force is perpendicular to the surface; friction acts along the surface.

Correct Answer: C. Oil

• **Oil** = a liquid lubricant that forms a thin film between moving surfaces, preventing direct contact and drastically reducing friction. • **Viscosity** — the oil film's viscosity allows it to maintain a continuous layer under pressure, keeping μ (coefficient of friction) very low. • 💡 Wrong-option analysis: Alcohol: volatile and not viscous enough to form a sustained lubricating film; Mercury: toxic heavy metal liquid, not used as a practical lubricant; Water: too thin a film and promotes corrosion — not an effective lubricant for metals.

Correct Answer: D. Drag

• **Drag** = the resistive force exerted by a fluid (liquid or gas) on an object moving through it, also called fluid friction. • **F_drag ∝ v²** — drag force increases with the square of speed, which is why streamlined shapes are critical at high velocities. • 💡 Wrong-option analysis: Contact friction: a general term for friction between touching solids, not specifically for fluids; Static friction: opposes the start of motion between two solid surfaces at rest; Dry friction: friction between solid surfaces without lubrication — not applicable to fluid media.

Correct Answer: B. Friction decreases

• **Friction decreases** = water acts as a lubricant on marble, filling surface irregularities and greatly reducing the coefficient of friction. • **μ_wet << μ_dry** — the friction coefficient of wet marble is much lower than dry marble, so less grip is available for walking. • 💡 Wrong-option analysis: Surface becomes hard: water does not alter the hardness of the marble; Friction increases: water reduces, not increases, friction between shoe sole and marble; Gravity increases: gravity is constant — it does not change with the presence of water on a floor.

Correct Answer: D. Sole

• **Sole** = the outer bottom layer of a shoe, textured or grooved to maximise friction between the foot and the walking surface. • **Tread pattern** — the grooves in a shoe sole channel away water and increase the contact pressure at ridges, boosting grip on slippery surfaces. • 💡 Wrong-option analysis: Insole: the inner cushioning layer inside the shoe — it contacts the foot, not the ground; Tongue: the padded flap under the laces — has no contact with the ground at all; Laces: secure the shoe to the foot but play no role in ground friction.

Correct Answer: B. Kinetic friction

• **Kinetic friction** = also called sliding friction, acts between two surfaces that are already in relative motion, like a book sliding on a table. • **f_k = μ_k × N** — kinetic friction is constant at a given normal force and is slightly less than the maximum static friction for the same surfaces. • 💡 Wrong-option analysis: Limiting friction: the maximum value of static friction just before motion begins — the book is already moving, so this does not apply; Static friction: acts when there is no relative motion between surfaces; Rolling friction: occurs when a round object rolls — a sliding book does not roll.

Correct Answer: D. Frictional force

• **Frictional force** = causes surfaces of machine parts to rub against each other, removing material and causing gradual wear and tear. • **Heat + abrasion** — friction converts kinetic energy into heat and microscopic abrasion, both of which degrade the surface finish and dimensions over time. • 💡 Wrong-option analysis: Gravitational force: pulls parts downward but does not cause rubbing or surface wear directly; Electrostatic force: significant only at nano/micro scale and does not cause macroscopic machine part wear; Magnetic force: can attract ferromagnetic parts but does not cause the sliding abrasion that wears surfaces.