Friction — Types & Factors

• Friction — force that opposes relative motion between surfaces in contact.
• Three types: static, sliding (kinetic), rolling friction.
• Friction depends on: nature of surfaces and normal force (weight pressing surfaces together).
• Friction is both useful and harmful — examples of each.
• Ways to increase or reduce friction.

Types of friction

Rolling friction < Sliding friction < Static friction (in general order).

Factors affecting friction

1. Nature of surfaces — rougher surfaces → more friction; polished/smooth → less friction.
2. Normal force — heavier object → more friction (more pressing force between surfaces).

Friction does NOT significantly depend on:

• Area of contact (a broad flat box vs a narrow one of same weight → same friction).
• Speed of sliding (approximately, for most school-level problems).

Friction is useful

• Walking, running (feet grip ground).
• Writing (pen/chalk grips paper/board).
• Brakes on vehicles.
• Matches lighting (friction produces heat).
• Conveyor belts, tyres gripping roads.

Friction is harmful

• Wears out machine parts, tyres, soles of shoes.
• Produces unwanted heat in engines → energy loss.
• Slows moving vehicles, reduces efficiency.

Increasing friction

• Using rough surfaces (treads on tyres, grooves on brake pads).
• Sprinkling sand on icy roads.
• Using rubber soles on sports shoes.

Reducing friction

• Polishing surfaces (ball bearings in wheels).
• Lubricants (oil, grease, graphite) between moving parts.
• Streamlining — smooth shape reduces air resistance (fluid friction).
• Using wheels instead of sliding (rolling < sliding friction).

Fluid friction (drag)

• Friction between an object and a fluid (liquid or gas) = drag or fluid friction.
• Streamlined shapes (fish, aircraft, bullet) reduce drag.
• Reduced with: oil, air-filled tyres, boat hulls.

Why do kabaddi players rub their hands with soil before the game?

Rubbing soil on hands → increases roughness of skin.
Rough skin + rough opponent's skin → more static and sliding friction.
This gives better grip when holding opponents → harder to escape.
Physics: higher friction force needed to prevent sliding motion (static friction).

• Name three types of friction in increasing order.
• Why do tyres have treads?
• Name two uses of friction and two harmful effects.
• How does a lubricant reduce friction?
• Why are wheels used on heavy luggage trolleys?

Open the Practice tab for graded questions on Friction.