Physics: Friction Types and Definition

Friction

Definition

• Friction is a force that opposes motion between two surfaces in contact.
• It is a contact force that arises from the interaction between two surfaces.

Types of Friction

• Static Friction: The force that prevents an object from moving when a force is applied to it.
• Kinetic Friction: The force that opposes the motion of an object that is already moving.
• Rolling Friction: The force that opposes the motion of an object that is rolling on a surface.
• Fluid Friction: The force that opposes the motion of an object through a fluid (liquid or gas).

Factors Affecting Friction

• Surface Roughness: Rougher surfaces increase friction, while smoother surfaces decrease friction.
• Normal Force: Increased normal force (force perpendicular to the surface) increases friction.
• Surface Area: Increased surface area in contact increases friction.
• Material: Different materials have different coefficients of friction.

Coefficient of Friction

• Static Coefficient of Friction (μs): The ratio of static friction to normal force.
• Kinetic Coefficient of Friction (μk): The ratio of kinetic friction to normal force.
• μs > μk: Static friction is always greater than kinetic friction.

Reducing Friction

• Lubrication: Applying a substance (e.g. oil, grease) to reduce friction between surfaces.
• Polishing: Smoothening surfaces to reduce friction.
• Ball Bearings: Using ball bearings to reduce rolling friction.

Friction

Definition and Types

• Friction is a contact force that opposes motion between two surfaces in contact, arising from their interaction.
• There are four main types of friction: static, kinetic, rolling, and fluid friction.

Static Friction

• Prevents an object from moving when a force is applied to it.
• Occurs when an object is stationary and a force is applied to try to make it move.

Kinetic Friction

• Opposes the motion of an object that is already moving.
• Always less than static friction.

Rolling Friction

• Opposes the motion of an object that is rolling on a surface.
• Typically less than kinetic friction.

Fluid Friction

• Opposes the motion of an object through a fluid (liquid or gas).
• Examples include air resistance and water resistance.

Factors Affecting Friction

Surface Roughness

• Rougher surfaces increase friction, while smoother surfaces decrease friction.
• This is because rough surfaces have more points of contact, increasing the frictional force.

Normal Force

• Increased normal force (force perpendicular to the surface) increases friction.
• This is because a greater normal force results in a greater frictional force.

Surface Area

• Increased surface area in contact increases friction.
• This is because a greater surface area provides more points of contact, increasing the frictional force.

Material

• Different materials have different coefficients of friction.
• This is because materials have different properties that affect the frictional force.

Coefficient of Friction

Static Coefficient of Friction (μs)

• The ratio of static friction to normal force.
• A measure of the maximum frictional force before an object starts moving.

Kinetic Coefficient of Friction (μk)

• The ratio of kinetic friction to normal force.
• A measure of the frictional force once an object is moving.

Relationship Between μs and μk

• μs is always greater than μk.
• This means that it takes more force to start moving an object than to keep it moving.

Reducing Friction

Lubrication

• Applying a substance (e.g. oil, grease) to reduce friction between surfaces.
• Works by reducing the contact between surfaces, decreasing the frictional force.

Polishing

• Smoothening surfaces to reduce friction.
• Works by reducing the surface roughness, decreasing the frictional force.

Ball Bearings

• Using ball bearings to reduce rolling friction.
• Works by replacing the surface contact with rolling contacts, decreasing the frictional force.

Learn about the different types of friction, including static, kinetic, and rolling friction, and understand the definition of friction as a force that opposes motion.