Friction: Reducing Friction – JSS2 Basic Technology Lesson Note (Second Term)

Friction is reduced by changing sliding friction into rolling friction, by placing a lubricant between two surfaces, or by adding small vibrations to one surface.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Name devices that change sliding friction into rolling friction.
2. Give examples of lubricants and state what tribology is.
3. Explain what happens to energy lost through friction.
4. State the harmful effects of the work of friction on surfaces.
5. Describe how friction is used in disc brakes and drum brakes.

What is friction?

Friction is the force that opposes the motion of one surface over another surface in contact with it.

• Static friction: acts between surfaces that are not moving, such as a heavy box at rest on the floor.
• Sliding friction: acts when one surface slides over another, such as a box dragged along the floor.
• Rolling friction: acts when a round object rolls over a surface, such as a wheel on a road. It is much smaller than sliding friction.

How can friction be reduced?

There are three ways to reduce friction: use devices that roll, use lubricants, and use vibration.

1. Using devices that roll

Rolling devices change sliding friction into a much smaller rolling friction.

• Wheels: a loaded cart on wheels is easier to push than a box dragged along the ground.
• Ball bearings: small steel balls between moving parts, as in bicycle hubs and electric fans.
• Roller bearings: small steel cylinders between moving parts, used where loads are heavy.
• Air cushions: a layer of air that lifts an object so that it does not touch the surface.
• Fluid bearings: a thin layer of liquid or gas that separates the moving parts.

Some plastics are used to make low-friction bearings.

• Examples: nylon, HDPE and PTFE.
• Advantage: their coefficient of friction falls as the load on them increases.
• Heavy-duty bearings: grades with very high molecular weight are chosen for better wear resistance.

2. Using lubricants

A lubricant is a substance placed between two surfaces to reduce friction.

• Liquid lubricants: oil and water.
• Semi-solid lubricants: grease.
• Solid lubricants: powders such as graphite and talc.
• Lubrication: the use of a lubricant to reduce friction between moving surfaces.
• Tribology: the science of friction and lubrication.
• Lubricant technology: the application of science to lubricants, especially for industry and commerce.

Other points about lubrication:

• Superlubricity: a recently observed effect in graphite, where friction between two sliding objects falls close to zero.
• Acoustic lubrication: uses sound as a lubricant, so a lubricant need not be a fluid or a powder.

3. Using vibration

Friction between two parts can be reduced by adding very small vibrations to one of the parts.

• Sinusoidal vibration: a smooth, regular vibration, as used in ultrasound-assisted cutting.
• Dither: a vibration noise added to one part.

What happens to the energy lost through friction?

Energy lost through friction is not destroyed but changed into heat.

• The law of conservation of energy says energy cannot be created or destroyed.
• Friction changes other forms of energy into thermal (heat) energy.
• The energy is lost to the system, because heat spreads out quickly into the surroundings.

Example: a sliding hockey puck

1. The puck slides over the ice with kinetic energy.
2. Friction acts against its motion.
3. The kinetic energy changes into heat.
4. The heat raises the thermal energy of the puck and the ice.
5. The puck comes to rest.

What is the work of friction?

The work of friction is the work done by the friction force as surfaces rub against each other.

• Static friction: does no work, because the surfaces do not move over each other.
• Kinetic (sliding) friction: acts opposite to the motion, so it does negative work.

Useful effects of the work of friction

• Polishing: rubbing wears a surface smooth.
• Friction welding: the work of friction mixes and joins materials.

Harmful effects of the work of friction

• Heat: moving parts become hot.
• Wear: sliding surfaces are worn away when the work of friction becomes too high.
• Fretting: hard particles trapped between moving surfaces make the wear worse.
• Bearing seizure or failure: excessive wear can make a bearing jam or break.
• Poor fit and finish: worn parts may no longer fit well or work properly.

What are the applications of friction in transportation?

Friction is needed in transport for braking, for grip and for road safety.

Brakes

Automobile brakes work by friction, slowing a vehicle by changing its kinetic energy into heat.

• Disc brakes: brake pads are squeezed against a rotating disc to create friction.
• Drum brakes: brake shoes or pads are pressed outwards against a rotating cylinder called the brake drum.
• Heat: getting rid of this large amount of heat safely is one challenge in designing brakes.

Grip and road safety

• Rail adhesion: the grip that the wheels of a train have on the rails.
• Road slipperiness: an important design and safety factor for vehicles.
• Split friction: a dangerous condition where the friction on one side of a car differs from the other side.
• Road texture: affects how well tyres grip the driving surface.

Evaluation

1. Name four devices that change sliding friction into rolling friction.
2. What is tribology?
3. What happens to the kinetic energy of a sliding hockey puck as it comes to rest?
4. State two harmful effects of excessive work done by friction on sliding surfaces.
5. Why do disc brakes have better stopping performance than drum brakes?

Assignment

Define lubrication, then name three lubricants and state whether each is a liquid, a semi-solid or a solid.