What is Friction?
Friction is a force that opposes the relative motion or tendency of motion between two surfaces in contact.
However, friction can also cause wear and tear, generate unwanted heat, and reduce the efficiency of machines.
When two surfaces touch, friction can resist their movement against each other. For example, when you push a heavy box across the floor, friction acts against its movement, making it harder to push.
Friction is an important part of everyday life. It helps us walk without slipping, allows vehicles to brake, and enables us to hold objects securely.
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Why Does Friction Occur?
Even surfaces that appear perfectly smooth have tiny irregularities when examined closely.
When two surfaces come into contact, these irregularities interact. Adhesion between the surfaces can also contribute to friction.
As one surface moves, or attempts to move, against the other, these interactions create a force that resists relative motion.
The amount of friction depends on several factors, including the nature of the surfaces, the force pressing them together, and whether the surfaces are stationary or moving relative to each other.
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Types of Friction
Friction can be classified into four main types:
- Static Friction
- Sliding Friction
- Rolling Friction
- Fluid Friction
1. Static Friction
Static friction acts between two surfaces that are in contact but are not sliding against each other.
For example, when you try to push a heavy cupboard, it may remain stationary because static friction prevents it from moving.
Static friction adjusts to oppose the applied force, up to a maximum value. Once the applied force exceeds this maximum, the cupboard begins to slide.
2. Sliding Friction
Sliding friction acts when one surface slides over another.
For example, when you slide a book across a table, friction acts opposite to the book’s movement relative to the table.
Sliding friction is generally lower than the maximum static friction between the same surfaces.
3. Rolling Friction
Rolling friction, often called rolling resistance, opposes the motion of an object rolling over a surface.
For example, the wheels of a bicycle experience rolling resistance as they move along a road.
Rolling resistance is usually much smaller than sliding friction under comparable conditions. This is why wheels make it easier to move heavy objects.
4. Fluid Friction
Fluid friction is the resistance experienced by an object moving through a fluid, such as a liquid or gas.
For example, a swimmer experiences resistance while moving through water, and a moving car experiences air resistance.
Fluid friction depends on factors such as the object’s speed, shape, size, and the properties of the fluid.
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Factors Affecting Friction
Several factors influence the frictional force:
- Nature of the Surfaces
- Normal Force
- Surface Conditions
Nature of the Surfaces
The materials and surface conditions affect friction.
For example, rubber tyres generally provide better grip on a dry road than smooth, hard surfaces.
Normal Force
The normal force is the contact force acting perpendicular to a surface.
In the simple model of dry friction, increasing the normal force increases the maximum static friction and the sliding friction.
For example, pushing a heavily loaded box across the floor generally requires more force than pushing an empty box.
Surface Conditions
Water, oil, grease, and other substances between surfaces can change the frictional force.
Lubricants are commonly used in machines to reduce direct contact between moving parts.
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Formula for Friction
For sliding friction, a commonly used approximate formula is:
where:
- Fk = kinetic frictional force
= coefficient of kinetic friction- N = normal force
For static friction, the formula is:
where:
- Fs = static frictional force
= coefficient of static friction
The maximum static friction is:
The coefficient of friction is dimensionless, meaning it has no unit.
These equations describe a simplified model of dry friction and do not apply universally to all surfaces or conditions.
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Advantages and Disadvantages of Friction
Advantages
Friction is essential for many everyday activities.
- Walking: Friction between our feet and the ground allows us to walk without slipping.
- Writing: Friction helps a pencil leave graphite marks on paper.
- Braking: Friction in braking systems slows down vehicles.
- Gripping: Friction allows us to hold objects without dropping them.
- Driving: Friction between tyres and roads enables vehicles to accelerate, brake, and turn.
Disadvantages
Although friction is useful, it can also have unwanted effects.
- Wear and Tear: Friction causes surfaces in contact to wear away over time.
- Heat Generation: When two surfaces slide against each other, friction converts some of their mechanical energy into thermal energy. This causes the surfaces to become warmer and can lead to unwanted heating in machines.
- Energy Loss: Friction can reduce the amount of mechanical energy available for useful work.
- Reduced Efficiency: Machines may require additional energy to overcome friction.
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Controlling Friction
How Can Friction Be Reduced?
Friction can be reduced in several ways.
- Using Lubricants: Oil and grease reduce direct contact between moving surfaces and can lower friction.
- Using Wheels and Rollers: Wheels and rollers replace much of the sliding motion with rolling motion, often reducing resistance.
- Using Ball Bearings: Ball bearings contain small metal balls that roll between moving components. They reduce sliding contact and allow rotating parts to move with less resistance.
- Streamlining: Streamlined shapes reduce fluid resistance, particularly air resistance, when objects move through fluids.
How Can Friction Be Increased?
Sometimes, increasing friction is necessary for safety and better performance.
- Tyre Treads: Treads help tyres maintain grip, especially on wet roads.
- Rough Surfaces: Textured flooring improves grip and reduces slipping.
- Rubber Soles: Footwear with suitable soles provides better traction.
- Grip Materials: Rubber coverings on handles make tools easier to hold.
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Friction in Everyday Life
- Friction plays an important role in many everyday activities. It helps us walk, run, hold objects, write, and control vehicles. However, friction can also resist movement and produce unwanted heat.
Here are some familiar examples.
Walking and Running
When we walk or run, our feet push backwards against the ground. Static friction between our footwear and the ground provides the forward force that helps us move without slipping.
Without sufficient friction, walking or running becomes difficult, especially on wet or slippery surfaces.
Holding and Gripping Objects
Friction between our fingers and an object helps us hold it securely.
For example, when we hold a glass, friction prevents it from slipping through our fingers. Rubber coverings on tool handles provide additional grip.
Writing on Paper
When we write with a pencil, friction between the pencil tip and paper helps transfer graphite onto the paper, leaving visible marks.
Driving and Braking
Friction between vehicle tyres and the road provides the grip needed to accelerate, turn, and brake.
In vehicles such as scooters and cars, friction between the brake pads and rotating components helps slow the wheels. Tyre-road friction allows the braking force to slow the vehicle.
Moving Furniture
When we push a heavy table or cupboard across the floor, friction resists its movement.
Using wheels or rollers makes moving heavy furniture easier because rolling resistance is generally lower than sliding friction.
Rubbing Our Hands
When we rub our hands together, friction converts some of the mechanical energy into thermal energy, making our hands feel warmer.
These examples show that friction can be both useful and unwanted, depending on the situation.
Key Points
- Friction is a fundamental force that influences the motion of objects.
- It helps us walk, grip objects, control vehicles, and perform everyday activities.
- At the same time, it can cause wear, generate heat, and reduce the efficiency of machines.
- Understanding friction helps us explain how objects move and how we can control their movement.
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Glossary of Key Terms
Recap of the Key Terms in Friction
- Adhesion: The attraction between molecules of two surfaces in contact, which can contribute to friction.
- Air Resistance: A type of fluid friction that opposes the movement of an object through air.
- Applied Force: A push or pull exerted on an object by a person or another object.
- Ball Bearings: Small spherical components used in machines to reduce friction between moving parts.
- Coefficient of Friction: A dimensionless quantity that describes the relationship between frictional force and normal force in a simplified friction model.
- Contact Force: A force that acts when two objects or surfaces are in physical contact.
- Drag: A resistive force experienced by an object moving relative to a fluid, such as air or water.
- Fluid Friction: The resistance experienced by an object moving through a liquid or gas, or between layers of fluid moving relative to each other.
- Friction: A force that opposes relative motion or the tendency of relative motion between surfaces in contact.
- Frictional Force: The force acting parallel to contacting surfaces that resists their relative sliding motion or tendency to slide.
- Kinetic Friction: Friction acting between two solid surfaces sliding against each other. Also called sliding friction.
- Lubricant: A substance, such as oil or grease, used to reduce friction and wear between moving surfaces.
- Maximum Static Friction: The greatest static frictional force that can act between two surfaces before sliding begins.
- Normal Force: The contact force exerted by a surface on an object, acting perpendicular to the surface.
- Rolling Friction: Resistance that opposes the rolling motion of an object over a surface. Also called rolling resistance.
- Sliding Friction: Friction that acts when one solid surface slides over another.
- Static Friction: Friction that prevents or resists the tendency of two surfaces in contact to slide against each other.
- Streamlining: Designing an object’s shape to reduce resistance as it moves through a fluid.
- Surface Irregularities: Microscopic bumps and uneven features on surfaces that can contribute to friction.
- Traction: The grip between contacting surfaces, such as tyres and a road, that allows forces to be transmitted without unwanted slipping.
- Wear and Tear: The gradual deterioration of surfaces caused by repeated contact, friction, and movement.
- Weight: The gravitational force acting on an object, usually directed towards the centre of the Earth.
Questions and Answers
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is friction?
Friction is a force that opposes the relative motion or tendency of motion between two surfaces in contact.
Why does friction occur?
Friction occurs because of interactions between surfaces in contact, including microscopic surface irregularities and adhesion between the surfaces.
In which direction does friction act?
Friction acts parallel to the surfaces in contact, opposing their relative sliding motion or tendency to slide.
Is friction a contact force?
Yes. Friction is a contact force because it arises from interactions between surfaces or materials in contact.
What are the main types of friction?
The four main types discussed in this lesson are static friction, sliding friction, rolling friction, and fluid friction.
What is static friction?
Static friction acts between two surfaces that are not sliding relative to each other. It prevents or resists the tendency of one surface to slide over another.
What is sliding friction?
Sliding friction acts when one solid surface slides over another. For example, a book sliding across a table experiences sliding friction.
What is rolling friction?
Rolling friction, or rolling resistance, opposes the rolling motion of an object over a surface. It is usually much smaller than sliding friction under comparable conditions.
What is fluid friction?
Fluid friction is the resistance associated with motion through a fluid, such as air or water, or relative motion between layers of fluid.
What is air resistance?
Air resistance is a type of fluid resistance that opposes the motion of an object relative to the surrounding air.
What is the difference between static and sliding friction?
Static friction acts when surfaces are not sliding relative to each other, while sliding friction acts when they are sliding.
What is maximum static friction?
Maximum static friction is the greatest static frictional force that can act between two surfaces before sliding begins.
Is static friction always constant?
No. Static friction adjusts to oppose the tendency to slide, up to its maximum value.
What happens when the applied force exceeds maximum static friction?
If the applied force tending to cause sliding exceeds the maximum static friction, the object begins to slide.
Is sliding friction generally less than maximum static friction?
Yes. For many pairs of dry surfaces, sliding friction is lower than maximum static friction, although this is not true in every situation.
What factors affect friction?
Friction depends on factors such as the materials in contact, surface conditions, normal force, and whether the surfaces are stationary or moving relative to each other.
Do rough surfaces always produce greater friction?
No. Although surface roughness can affect friction, the relationship depends on the materials and contact conditions. Rougher surfaces do not always produce greater friction.
What is the normal force?
The normal force is the contact force exerted by a surface on an object, acting perpendicular to the surface.
How does normal force affect friction?
In the simplified model of dry friction, increasing the normal force increases the maximum static friction and the sliding friction.
What is the coefficient of friction?
The coefficient of friction is a dimensionless quantity that describes the relationship between frictional force and normal force in a simplified friction model.
Does the coefficient of friction have a unit?
No. The coefficient of friction is a ratio of forces, so it has no unit.
What is the formula for sliding friction?
The approximate formula for sliding friction is:
where Fk is the sliding frictional force, 
is the coefficient of kinetic friction, and N is the normal force.
What is the formula for static friction?
Static friction satisfies the relationship:
The maximum static friction is:
where 
is the coefficient of static friction and N
is the normal force.
Why is it difficult to push a heavy box across the floor?
A heavy box generally produces a greater normal force on a horizontal floor, increasing the frictional resistance. More force may therefore be required to start or maintain sliding.
Why is rolling usually easier than sliding?
Rolling resistance is generally much smaller than sliding friction under comparable conditions. This is why wheels and rollers make it easier to move heavy objects.
Why do we need friction to walk?
Static friction between our feet and the ground provides the grip needed to push against the ground and move forward without slipping.
How does friction help vehicles move?
Friction between tyres and the road allows vehicles to accelerate, brake, and turn without unwanted slipping.
How does friction help in braking?
Friction in the braking system converts mechanical energy into thermal energy. Friction between the tyres and road also helps slow the vehicle.
Why do vehicle tyres have treads?
Tyre treads help maintain grip, particularly on wet roads, by allowing water to escape from the contact region and reducing the risk of hydroplaning.
Why do we use rubber soles in footwear?
Rubber soles can provide good traction on many walking surfaces, helping reduce the risk of slipping.
How does friction help us write?
Friction between a pencil tip and paper helps transfer graphite onto the paper, producing visible marks.
What are the advantages of friction?
Friction enables walking, writing, gripping objects, braking, and controlling the motion of vehicles and machines.
What are the disadvantages of friction?
Friction can cause wear and tear, produce unwanted heat, waste useful mechanical energy, and reduce the efficiency of machines.
How does friction produce heat?
When surfaces slide against each other, friction converts some of their mechanical energy into thermal energy, increasing their temperature.
How can friction be reduced?
Friction can be reduced by using lubricants, wheels, rollers, ball bearings, and suitable surface treatments. Streamlining helps reduce fluid resistance.
How do lubricants reduce friction?
Lubricants, such as oil and grease, form a layer between moving surfaces, reducing direct contact and often lowering friction and wear.
Why are ball bearings used in machines?
Ball bearings allow components to rotate with less resistance by replacing much of the sliding contact with rolling contact.
What is streamlining?
Streamlining is the process of designing an object’s shape to reduce resistance as it moves through a fluid, such as air or water.
How can friction be increased?
Friction can be increased by choosing materials and surface textures that provide better grip, such as suitable rubber soles, textured flooring, and grip coverings.
Is friction always harmful?
No. Friction is essential for many everyday activities, including walking, writing, gripping objects, and controlling vehicles. Although it can cause wear and energy losses, it is also extremely useful.