Friction: Factors, Types and Applications

Friction is the opposing force that prevents two solid surfaces, liquid layers, and material components from sliding past one another. The nature of the two surfaces in touch and the force acting on these surfaces are the 2 factors that affect friction There are 2 types of friction forces: Internal friction and External friction. 

• Under External friction, there is dry friction, fluid friction, skin friction, etc. 
• Dry friction is further divided into static, kinetic, and rolling friction. 
• According to microscopic observations, surface imperfections increase friction. 
• The Laws of Static Friction state -The static friction force is independent of the area of contact between two surfaces. 
• The Law of Kinetic Friction states- Kinetic friction is a constant force that opposes relative motion. 
• The Limiting Friction Laws state- The force of limiting friction is independent of the area between two bodies in contact.

Read More: Frictional Force

Key Terms: Friction, Static friction, Kinetic friction, Rolling friction, Force, Surface.

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What Is Friction?

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Friction is defined as the resistance provided by surfaces in contact as they move past each other. Friction is the stopping force that opposes the relative motion or propensity of relative motion of two solid surfaces, fluid layers, and material elements as they slide against each other. 

• It acts indirectly on the contact. 
• Friction is caused by surface irregularities, according to microscopic observations.
• Example: A ball starts to move with some velocity when one throws it on the ground. 
• Newton's first law states that if no force is applied in the direction of motion, the ball should continue to roll, however, this does not happen. 
• Instead, the ball stops after a predetermined distance, suggesting that there is a force at work. 
• Friction is the term used to describe this force.

Read More: Centripetal and Centrifugal Force

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Factors Affecting Friction

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Based on the nature of the two surfaces in touch– 

• Friction is determined by the two surfaces' relative smoothness or roughness. 
• When the surface is smooth, there is less interlocking of flaws, which reduces friction between the two. 
• When the surface is rough, friction increases.

Based on the force acting on these surfaces– 

• Friction rises when force and defects are combined.

Read More: Derivation of Centripetal Acceleration

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What Leads to Friction?

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Friction is created by imperfections on the two surfaces that come into contact. 

• As a result, when one object moves over another, the surface imperfections become interconnected, causing friction. 
• The more defects and roughness, the more friction there is.

Read More: Coefficient of Static Friction

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Different Forms of Friction

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A force called friction works against the movement of two things. Certain frictions simply act on solids, whilst others act on gases and liquids. All of these frictions can be further classified and subdivided. 

1. Internal Friction: It is the force that, at the time of deformation, prevents movement between the components of a solid material.
2. External Friction: It occurs when two bodies in contact with each other tend to move or are in real relative motion. There are different types of external friction- dry friction, fluid friction, skin friction, lubricated friction, etc 
3. Dry-Friction: Dry friction typically occurs between two solid surfaces. 

• This type of friction is always opposed to the movement of two surfaces. 
• This type of friction is also known as coulomb friction. 
• This form of friction is further subdivided into static and dynamic.

4. Static Friction: This friction occurs before the box begins to move. 

• In this situation, the friction is equal to the applied force. 
• As the force increases, so does friction, but there comes a moment when the box begins to move. 
• The impending motion refers to the instant before the box begins to slide.
• The static friction is most significant at this point. 
• The coefficient of static friction determines static friction μs.
• Static friction formula– F = μ_s*N where μ_s is the coefficient of static friction.

Different Forms of Friction

5. Laws of Static Friction: The magnitude of the maximum. The static friction force is independent of the area of contact between two surfaces. The magnitude of maximum force is always relative to the normal force; as the normal force grows, so does the maximum external force that the item can withstand without moving.

6. Kinetic Friction: When the box begins to move, kinetic friction begins to act. It is less than the maximal static friction. 

• In this instance, regardless of the applied force, the value of kinetic friction remains constant. 
• It is determined in this situation by the coefficient of kinetic friction uk.
• Kinetic friction formula– F = μ_k*N where μk is the coefficient of kinetic friction.
• Laws of Kinetic Friction: The four laws of kinetic friction are as follows:

1. Kinetic friction is a constant force that opposes relative motion. Its value is determined by the nature of the two surfaces in contact.
2. Due to the value of kinetic friction fk is independent of the area of contact, the normal reaction is the same throughout.
3. Kinetic friction is not determined by velocity.
4. The kinetic friction value is related to the normal response between the two surfaces in contact.

7. Rolling Friction: This occurs when a disc or ball rolls across a surface. The cause for this appears to be the distribution of energy involved in object twisting.

F_r = μ_rW 

The coefficient used for the Rolling friction is determined as μr and is known as the Dimensionless rolling resistance coefficient.

9. Limiting Friction: It is the maximal static frictional force, i.e. what happens when a body just starts moving over the surface of another body when they come into contact. The following equation determines its value:

F_limiting = μ_sN (μ_s is the coefficient of static friction or limiting friction and N is the normal reaction exerted by the surface)

• Limiting Friction Laws: The following four laws govern limiting friction:

1. The force of limiting friction between two objects is directly proportional to the normal reaction: F ∝ N.
2. As a result, if a body is heavy, the typical reaction, i.e. mg, increases, and more power is required to move the heavy body in contrast to a lighter one.
3. The direction of limiting friction is always the opposite of the direction in which one object is about to pass over another.
4. The force of limiting friction is independent of the area between two bodies in contact.

10. Fluid Friction: This function occurs between opposing fluid layers. This internal flow friction is also known as viscosity. 

• Almost all fluids exhibit shearing resistance and are so viscous. 
• A drag occurs while an airplane is traveling through the air. 
• The viscosity of the air causes this drag. 
• Another example is parachutes slowing down during skydiving.

Read More: Laws of Friction

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Applications of Friction

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Various applications of friction are– 

1. Driving a vehicle over a surface.
2. Using brakes to bring a moving vehicle to a halt.
3. Skating.
4. Walking down the street.
5. Writing in a notebook or on a chalkboard.
6. Plane flying.
7. hammering a nail into a wall.
8. Garden slide sliding.
9. Lighting a matchstick.
10. Dusting a carpet or a foot mat with a stick.

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Things to Remember

• The location and angle of the item have an impact on the friction force.
• The friction coefficient is constant. It is indicated by the load-to-friction ratio (L/F) (F). Friction and load are both expressed in newtons or pounds.
• The force of rolling friction decreases as the surface becomes smoother.
• Rolling friction force is directly proportional to load and inversely proportional to curvature radius.
• The formula for static friction: F = μ_s*N, where s is the static friction coefficient.
• Kinetic friction formula: F = μ_k*N where μ_k is the coefficient of kinetic friction.
• Rolling friction formula: F_r =μ_rW where μ_r is the coefficient of rolling friction.

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Previous Year Questions

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