Friction – JSS2 Basic Technology Lesson Note (Second Term)

Friction is the force that opposes the relative motion of two surfaces that touch each other.

Behavioural objectives

1. Define friction and name the types of friction.
2. Explain the difference between static friction and kinetic friction.
3. State the three laws of dry friction.
4. Explain the normal force and the coefficient of friction, and use F = μN in a simple calculation.
5. Describe the effects, advantages and disadvantages of friction.
6. Give ways of increasing and reducing friction.

What causes friction?

Friction is caused by the way the surfaces of two objects interact when they touch.

• Surface roughness: even smooth-looking surfaces have tiny bumps, called asperities, that catch on each other.
• Adhesion: the surfaces stick slightly to each other where they touch.
• Surface deformation: the surfaces change shape a little under pressure.
• Surface contamination: dust, water or oil on the surfaces changes how strongly they grip.

What are the types of friction?

There are five types of friction: dry, fluid, lubricated, skin and internal friction.

• Dry friction: opposes the sliding of two solid surfaces in contact, such as a box pushed across a floor. It is either static or kinetic.
• Fluid friction: acts between layers of a thick (viscous) fluid moving past each other, as when honey or palm oil is stirred.
• Lubricated friction: a case of fluid friction where a lubricant, such as engine oil, separates two solid surfaces.
• Skin friction: a part of drag, the force that resists a fluid (air or water) moving across the surface of a body.
• Internal friction: resists motion between the parts of a solid material while it is being bent or stretched.

What is the difference between static and kinetic friction?

Static friction acts between surfaces at rest, while kinetic friction acts between surfaces that are sliding.

• Static friction: the force that stops a resting object from starting to move. It is sometimes called “stiction”.
• Kinetic friction: the force that slows down an object already sliding. It is also called sliding or dynamic friction.

What are the laws of dry friction?

The laws of dry friction are three rules found by experiment, and they describe how sliding friction behaves.

• Amontons’ First Law: the force of friction is directly proportional to the applied load.
• Amontons’ Second Law: the force of friction is independent of the apparent area of contact.
• Coulomb’s Law of Friction: kinetic friction is independent of the sliding velocity.

What is Coulomb friction?

Coulomb friction is an approximate model used to calculate the force of dry friction, named after Charles-Augustin de Coulomb.

F_f ≤ μF_n

• F_f: the force of friction. It is parallel to the surface and opposite to the applied force.
• μ: the coefficient of friction, which depends on the two materials in contact.
• F_n: the normal force, which acts at right angles (perpendicular) to the surface.

How does friction behave in each case?

• Static case: friction is exactly as large as the push trying to move the object, up to a maximum of μF_n.
• Threshold: once the push is larger than this maximum, the object starts to slide.
• Direction: friction always acts against the movement (kinetic) or the possible movement (static).
• Kinetic example: a box sliding across a floor is slowed down by kinetic friction.
• Potential movement example: the drive wheels of an accelerating car feel a frictional force pointing forward.
• Why the car moves: without that forward force, the wheels would spin and the rubber would slide backward along the road.

What is the normal force?

The normal force is the force that presses two surfaces together, acting at right angles to the surfaces.

• Level surface: the normal force equals the weight of the object, so N = mg, where m is mass and g is the acceleration due to gravity.
• Tilted surface: on an inclined plane the normal force is smaller, because less of the weight presses straight into the surface.
• Other forces: a push or pull with an upward or downward part also changes the normal force.
• Friction on a level surface: F_f = μ × m × g.

What is the coefficient of friction?

The coefficient of friction (COF), symbol μ, is the ratio of the force of friction between two bodies to the force pressing them together.

• Formula: μ = F_f ÷ F_n.
• Unit: none, because it is a ratio of two forces.
• Depends on the materials: ice on steel has a low value, while rubber on a road has a high value.
• Not affected by size: a large aluminium block and a small aluminium block have the same coefficient.
• Friction force does change: the large block has a bigger normal force, so it has a bigger friction force.
• Rough surfaces: rougher surfaces tend to have higher values.
• Similar metals: two surfaces of the same metal usually grip more than two different metals, so brass on brass has a higher value than brass on steel.
• Found by experiment: the value must be measured and cannot be worked out by calculation alone.

What are typical values of the coefficient of friction?

• Most dry materials together: between 0.3 and 0.6.
• Teflon: can be as low as 0.04.
• Rubber on other surfaces: about 1 to 2.
• Zero: would mean no friction at all, which does not happen in practice.
• Values above 1: are possible. They mean the force needed to slide the object is greater than the normal force.

Is the coefficient of friction a material property?

The coefficient of friction is better described as a system property, because it depends on more than the two materials.

• Temperature: heating the surfaces can change the value.
• Speed: fast sliding can heat the surfaces and change the value.
• Surroundings: air, water or dust on the surfaces affect it.
• Surface structure: the shape and roughness of the contact surface affect it.

How do we calculate friction?

Friction on a level surface is found with F_f = μN, where N is the normal force.

Worked example: a 10 kg block rests on a level floor. The coefficient of kinetic friction is 0.4. Take g = 10 m/s².

1. Find the normal force: N = mg = 10 × 10 = 100 N.
2. Write the formula: F_f = μN.
3. Substitute the values: F_f = 0.4 × 100.
4. Answer: F_f = 40 N.

What are the effects of friction?

The main effects of friction are heat, wear and traction.

Heat

• Energy change: friction changes kinetic energy (movement energy) into heat energy.
• Fire making: rubbing two pieces of dry wood together can start a fire.
• Stirring: stirring a thick liquid also produces a little heat.
• Machines: moving parts of an engine become hot from friction.

Wear

• Meaning: wear is the slow loss of material from surfaces that rub together.
• Result: worn parts perform poorly and may break.
• Examples: worn tyres, worn shoe soles and worn brake pads.
• Tribology: the study of friction, wear and lubrication.

Traction

• Meaning: traction is the grip between a tyre or shoe and the ground.
• Vehicles: most land vehicles rely on friction to speed up, slow down and turn.
• Danger: a sudden loss of traction, for example on a wet road, can cause loss of control and accidents.

What are the advantages and disadvantages of friction?

Advantages of friction

• Walking: our shoes grip the ground so we do not slip.
• Braking: brake pads grip the wheel and stop the vehicle.
• Holding things: nails, screws and knots stay in place.
• Making fire: rubbing a matchstick on the box lights it.

Disadvantages of friction

• Wear: machine parts and tyres wear out.
• Waste of energy: some energy is lost as heat, so machines are less efficient.
• Overheating: moving parts can become too hot and fail.
• Noise: rubbing parts can squeak or grind.

How can friction be increased or reduced?

Ways of reducing friction

• Lubrication: put oil or grease between moving parts.
• Polishing: make the surfaces smoother.
• Ball bearings and rollers: replace sliding with rolling.
• Streamlining: give vehicles a smooth shape so air and water flow past easily.

Ways of increasing friction

• Treads: grooves on tyres and shoe soles improve grip.
• Rough surfaces: make surfaces rougher, as on the handles of tools.
• Sand: spread sand on a slippery path.
• Brake pads: use rubbery material that grips well.

What kind of force is friction?

Friction is not a fundamental force, and it is a non-conservative force.

• Not fundamental: it comes from the combined effect of adhesion, roughness, deformation and contamination.
• Empirical methods: because it is so complex, friction is studied by experiment.
• Non-conservative: the work done against friction depends on the path taken.
• Energy loss: some energy always becomes heat, so mechanical energy is not conserved.

Evaluation

1. Define friction.
2. Name the five types of friction described in this lesson.
3. State Amontons’ First Law, Amontons’ Second Law and Coulomb’s Law of Friction.
4. What is the difference between static friction and kinetic friction?
5. A 20 kg box rests on a level floor with μ = 0.3. Take g = 10 m/s². Calculate the friction force.

Assignment

1. Write a short note on three effects of friction, with one everyday example of each. Then give two ways of reducing friction and two ways of increasing it.