Friction – JSS2 Intermediate Science Lesson Note (Third Term)

Friction is a force that opposes the motion of an object when two surfaces rub against each other.

Behavioural objectives

1. Define friction and state the direction in which it acts.
2. Explain why friction happens between two surfaces.
3. List the types of friction with one example of each.
4. State the advantages and disadvantages of friction.
5. Describe ways of reducing and increasing friction.

What is friction?

Friction is a force that acts between two surfaces in contact and opposes the movement of one over the other.

• Direction: friction always acts opposite to the direction in which an object is moving or trying to move.
• Effect: it slows moving objects down or stops them from sliding.
• Unit: friction is a force, so it is measured in newtons (N).
• Everyday case 1: you can walk on a rug without slipping, but you may slip on a wet, soapy tiled floor.
• Everyday case 2: a rolling ball slows down and stops even if nobody touches it.

Why does friction happen?

Friction happens because no surface is perfectly smooth, so the rough parts of two touching surfaces catch on each other.

• Tiny bumps: even a smooth glass table has tiny “hills and valleys” when seen under a powerful microscope.
• Hooking: when two surfaces touch, the hills of one fit into the valleys of the other and catch.
• Resistance: this catching resists sliding and produces friction.
• Rougher surfaces: they have bigger hills and valleys, so they give more friction.

What are the types of friction?

There are four common types of friction: static, sliding, rolling and fluid friction.

Static friction

• Meaning: the friction that holds an object still before it starts to move.
• Example: a heavy table that does not move when you first push it gently.

Sliding friction

• Meaning: the friction between two surfaces that slide over each other.
• Example: a box dragged across a floor.

Rolling friction

• Meaning: the friction when an object rolls over a surface.
• Example: a ball or a bicycle wheel rolling on the road.
• Note: it is usually less than sliding friction.

Fluid friction

• Meaning: the friction (resistance) when an object moves through a liquid or gas.
• Example: a swimmer moving through water or a plane moving through air.

What affects the size of friction?

The size of friction depends mainly on the kind of surfaces and on how hard they press together.

• Roughness: rougher surfaces give more friction than smooth ones.
• Weight pressing down: a heavier object presses harder on the surface and gives more friction.

What are the advantages of friction?

Friction is useful because many everyday actions depend on it.

• Walking: friction between the soles of your shoes and the ground stops you from sliding. Shoes have treads for this reason.
• Writing: friction lets the pencil lead rub off onto paper. Without it, the pencil would slide and leave no mark.
• Braking: friction between the brake pads and the wheels stops a car or bicycle.
• Lighting a match: rubbing the match head on the side of the box produces the heat that starts the fire.
• Holding things: friction helps you grip a pen, a spoon or a heavy bag.

What are the disadvantages of friction?

Friction is harmful when it wastes energy and damages objects.

• Wear and tear: surfaces wear away. Shoe soles become thin and smooth, and car tyres must be replaced.
• Unwanted heat: friction produces heat. Too much friction in an engine can make parts very hot or make them seize up.
• Loss of energy: more energy or fuel is needed to push a heavy box over a rough floor than over a smooth one.
• Noise: a squeaky door hinge makes noise because of friction between the metal parts.

How can friction be reduced?

Friction can be reduced by making surfaces smoother, separating them or changing sliding into rolling.

1. Lubrication: put oil, grease or wax between the surfaces. It fills the “valleys” so the surfaces slide easily. Examples: oiling a door hinge, greasing a bicycle chain.
2. Ball bearings: place small metal balls between moving parts, because rolling is easier than sliding.
3. Streamlining: give objects moving through air or water a smooth, pointed shape. Planes, boats and fish are examples.
4. Polishing: sanding wood or polishing a floor makes the “hills” smaller.
5. Using wheels or rollers: a suitcase with wheels is easier to pull than one without.

How can friction be increased?

Friction can be increased by making surfaces rougher or by pressing them together harder.

• Treads on shoes and tyres: the patterns grip the ground and prevent slipping.
• Rough surfaces: rough steps and floor mats give a better grip, especially when wet.
• Sand on slippery ground: sprinkling sand on a slippery path gives a better grip.
• Pressing harder: pressing a brake pad harder on a wheel stops it faster.

Evaluation

1. In which direction does friction act compared to the movement of an object?
2. Mention two things that would be impossible to do if there was zero friction on Earth.
3. Why do mechanics put grease on the moving parts of a car?
4. Explain why a smooth floor is more dangerous to walk on than a rough one.
5. Name the four types of friction.

Assignment

List three places in your home or school where friction is useful and three where it is harmful. For each, say how friction could be increased or reduced.