Physics · Lesson 8

Friction, viscosity & surface tension

JAMBWAECNECO

What the syllabus asks

JAMB: Friction: static and dynamic friction, coefficient of limiting friction, advantages and disadvantages, methods of reducing friction; viscosity and terminal velocity; surface tension and capillarity (simple treatment)

WAEC: Friction: static and dynamic (kinetic) friction, laws of solid friction, coefficient of friction, angle of friction (inclined plane), advantages and disadvantages of friction, methods of reducing friction; viscosity: fluid friction, terminal velocity, effect of temperature; surface tension: molecular explanation, cohesion and adhesion, capillarity, effects of detergents and temperature,…

NECO: Friction: static and dynamic (kinetic) friction, laws of solid friction, coefficient of friction, angle of friction (inclined plane), advantages and disadvantages of friction, methods of reducing friction; viscosity: fluid friction, terminal velocity, effect of temperature; surface tension: molecular explanation, cohesion and adhesion, capillarity, effects of detergents and temperature,…

What you will learn

Explain the nature of friction and distinguish static, limiting and dynamic friction.
State the laws of solid friction.
Define and calculate the coefficient of friction on horizontal surfaces and slopes.
Apply friction to problems involving motion, work and stopping distances.
State advantages and disadvantages of friction and methods of reducing it.
Explain viscosity and terminal velocity, and describe an experiment to show terminal velocity.
Explain surface tension and capillarity in terms of molecular forces, and describe their effects and applications.

Key points

Friction opposes relative motion between surfaces; it acts along the surface.
Static friction self-adjusts up to limiting friction; dynamic friction is slightly smaller.
Laws: depends on the surfaces and the normal reaction, not on the area of contact.
μ = F/R (no unit); on a level surface R = mg.
On a slope R = mg cos θ; a block just slides when μ = tan θ.
Work against friction becomes heat; skidding deceleration = μg.
Reduce friction with lubricants, bearings, polishing and streamlining; increase it with treads and rough surfaces.
Viscosity is fluid friction; it falls as a liquid is heated.
Terminal velocity: weight = upthrust + drag, so acceleration is zero.
Surface tension makes the surface act like a stretched skin; detergents and heat reduce it; adhesion > cohesion gives capillary rise (water), cohesion > adhesion gives depression (mercury).

Start here: What is friction?

Try pushing a heavy wardrobe across a tiled floor. At first, nothing happens: something is pushing back. Push harder and it suddenly starts to slide. That “something” is friction.

Friction is a force that opposes the relative motion, or the attempted motion, between two surfaces in contact.

Where does it come from? Even surfaces that feel smooth, like a polished table, are rough under a microscope, with tiny hills and valleys. Where the high points touch, they press and partly lock together, and the molecules at these points attract each other. To slide, these contact points must be broken or pushed past one another. That resistance is friction.

Friction always acts along the surface and in the direction opposite to the motion (or to the way the object would move). It does not appear until something tries to move the object.

Worked example 8.1

A box rests on level ground with no one pushing it. What is the friction force on it?

Nothing is trying to move the box sideways, so there is no motion for friction to oppose.
Friction is zero. It only appears in response to an applied force.

Zero

Worked example 8.2

A boy pushes a 20 kg box with a horizontal force of 50 N, but it does not move. What is the friction force?

The box is at rest, so the resultant force is zero (Newton’s first law).
Friction must exactly balance the push: 50 N, acting opposite to it.

50 N opposite to the push

⚠️ Common mistake: thinking friction acts in the direction of motion. It always opposes the relative motion or the tendency to move.

Try it yourself. In example 15.2, the boy increases his push to 70 N and the box still does not move. What is the friction now?

Show solution

70 N. Static friction adjusts to match the push, up to its limit.

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Practice questions

1. A 6 kg block rests on a floor (μ = 0.25). What force just moves it? (g = 10 m/s²)

Show worked answer

F = 0.25 × 60 = 15 N.

2. A force of 20 N moves a 5 kg box at constant speed. Find μ_k.

Show worked answer

20/50 = 0.4.

3. A body just begins to slide on a plane tilted at 30°. Find μ.

Show worked answer

tan 30° = 0.577.

4. A 40 N pull acts on an 8 kg box on a floor with μ_k = 0.3. Find the acceleration.

Show worked answer

Friction 24 N; resultant 16 N; a = 2 m/s².

5. Explain why engine oil flows more easily when the engine is hot.

Show worked answer

Heating lowers the viscosity: faster molecules have weaker attraction between layers.

Key words

Friction: A force that opposes relative motion between surfaces in contact.
Static friction: Friction acting when surfaces are not sliding over each other.
Limiting friction: The maximum value of static friction, reached just before sliding begins.
Dynamic (kinetic) friction: Friction acting while surfaces slide over each other.
Normal reaction (R): The force a surface exerts perpendicular to itself on an object resting on it.
Coefficient of friction (μ): The ratio of frictional force to normal reaction.
Angle of friction: The angle of a slope at which an object just begins to slide; tan θ = μ.
Lubricant: A substance (oil, grease) used to reduce friction between surfaces.
Viscosity: The internal friction of a fluid, which resists flow.
Terminal velocity: The constant maximum velocity reached when the resultant force on a falling body is zero.
Surface tension: The force per unit length acting along a liquid surface, making it behave like a stretched skin.
Cohesion: Attraction between molecules of the same substance.

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