Knowing the answer to the question, “What is friction in physics?” can help you in many ways in everyday life.

Introduction

To make smarter choices and save resources, it is essential to know: “What is friction in physics?” It is a fundamental concept in physics that affects countless aspects of our daily lives.

From walking on a road to the functioning of machines, friction plays an important role. Here, we shall find out about its nature, basic principles, and associated concepts.

What is Friction in Physics?

Before processing this question, let us first familiarise ourselves with some important terminologies.

Applied Force

It is the external force exerted on an object to move it. It works against friction to start the motion.

Frictional Force

It is the force that resists the motion of an object. It acts parallel to the surfaces in contact and the opposite direction of the applied force.

Normal Force

It is the perpendicular force that is exerted by a surface in opposition to the weight of an object resting on it (surface). It is also referred to as normal reaction force.

Relationship between the Terms

When an applied force acts on a body, the frictional force initially resists it. This resistance increases until it reaches a certain maximum value (force of limiting friction).

At this point, the applied force is equal to the frictional force, and the object remains stationary. Once the applied force exceeds this limit, the frictional resistance is overcome, and the body starts to move.

The frictional force is also directly proportional to the normal force. If the weight of the object increases, the normal force also increases. This increased normal force leads to a higher frictional force. Conversely, lighter objects experience less normal force and, therefore, less frictional force.

Additionally, both the frictional and normal forces are influenced by the direction and magnitude of the applied force. Their relationship can be effectively analysed and understood using vector diagrams.

Friction

It is defined as,

“the resistive force that arises when two surfaces come into contact and attempt to move relative to each other.”

Nature of Friction

Friction is a contact force, meaning it only occurs when two surfaces are in physical contact. It acts parallel to the contact surface and opposes the motion.

Factors behind Friction in Solids

Several factors are responsible for determining the amount of friction between two surfaces. These factors include:

Surface Roughness

Rough surfaces have higher friction than smoother surfaces due to more prominent irregularities.

Nature of Materials in Contact

The type of material in contact affects friction significantly. For example, rubber on asphalt generates more friction than ice on steel.

Normal Force

As stated earlier, friction and the normal force (the force pressing the two surfaces together) are directly proportional to each other. In other words, the heavier the object, the greater the normal force, and hence more will be the frictional force.

How Does Friction Work?

Friction is the result of complex interactions that exist in both smoother and rough surfaces. Two prominent factors responsible for the friction are:

1. Microscopic Interactions

In the smoothest surfaces, there are asperities (microscopic peaks and valleys) that intertwine when they come into contact and thus offer resistance.

2. Surface Irregularities

In rough surfaces, the irregularities on surfaces hinder the motion by creating intertwining zones.

Cold-Welds

The microscopic interactions and/or surface irregularities that resist the movement of surfaces on each other are referred to as cold welds.

Example

When a block placed on the floor is pushed with significant force, it experiences a hindrance due to the interactions and/or irregularities of surfaces.

Why Does Friction Oppose Motion?

Friction arises to counter any relative motion or its tendency, maintaining equilibrium until the applied force exceeds its limit.

Energy Dissipation

When motion occurs despite friction, energy is dissipated. This dissipation of energy is primarily in the form of heat.

For instance, while rubbing our palms together, we generate warmth due to friction.

Importance of Friction in Daily Life

Friction is both a boon and a bane.

Positive Uses of Friction

It enables us to walk on the ground, grip objects, write on paper, and apply vehicle brakes.

Challenges

It causes wear and tear, reduces efficiency, and generates unwanted heat that is bad economically.

Force of Limiting Friction

It is defined as,

“the maximum frictional force that must be overcome to initiate motion.”

Significance of Force of Limiting Friction

It determines when an object switches from the state of rest to the state of motion. Once the applied force exceeds the force of limiting friction, the object begins to move, and kinetic friction takes over.

It is important to note that the force of limiting friction and normal reaction force are directly proportional.

Mathematical Formulation

Mathematically, it is represented as given below.

Here,

• = force of limiting friction
• = normal reaction force
• = weight of the object
• = mass of the object
• = gravitational acceleration
• = coefficient of limiting friction

This equation highlights how limiting friction depends on the material properties and the normal force.

Coefficient of Limiting Friction

It is defined as,

“the ratio between the force of limiting friction and normal reaction force.”

Unit

It () is a dimensionless quantity because it is a ratio between two same type of physical quantities.

Mathematical Formulation

Mathematically, it is represented as given below.

Factors Affecting the Coefficient of Limiting Friction

It is affected by different factors. These factors include:

• Material properties (e.g., rubber has a higher compared to steel)
• Presence of lubricants
• Surface smoothness

Conclusion

Friction, a fundamental force in physics, plays a dual role in our lives. It facilitates essential tasks while also posing challenges like energy loss and wear.

Understanding its principles allows us to harness its benefits effectively while mitigating its drawbacks. From designing efficient machinery to improving road safety, friction is at the heart of modern innovation.

Frequently Asked Questions (FAQs)

What is friction in physics?

Friction is the resistive force that arises when two surfaces come into contact and attempt to move relative to each other. It acts parallel to the contact surface and opposes motion.

What is the relationship between applied force, frictional force, and normal force?

• Applied Force: The external force applied to an object to move it.
• Frictional Force: The opposing force that resists motion.
• Normal Force: The perpendicular force exerted by a surface against the object’s weight.
  The frictional force is directly proportional to the normal force and acts opposite to the applied force.

What factors affect the amount of friction?

• Surface roughness
• Nature of materials in contact
• Normal force (heavier objects experience more friction)

Why does friction oppose motion?

Friction opposes motion to maintain equilibrium by countering any relative movement or its tendency. It resists motion until the applied force exceeds its maximum limit.

What is the “force of limiting friction”?

It is the maximum resistive force that must be overcome to initiate motion. Once this limit is exceeded, the object moves, and kinetic friction takes over.

How is the force of limiting friction calculated?

It is calculated using the formula:

Here,

• = force of limiting friction
• = coefficient of limiting friction
• = normal reaction force

What is the coefficient of limiting friction?

It is the ratio between the force of limiting friction and the normal reaction force, denoted by . It is a dimensionless quantity that depends on material properties and surface conditions.

How does friction work on a microscopic level?

Even smooth surfaces have microscopic asperities (peaks and valleys). These asperities interlock or form “coldwelds” and resist the motion. In rough surfaces, irregularities further hinder movement.

What are the advantages and disadvantages of friction in daily life?

Advantages

Enables walking, gripping objects, writing, and vehicle braking.

Disadvantages

Causes wear and tear, reduces efficiency, and generates unwanted heat.

How can friction be reduced?

Friction can be reduced by:

• Using lubricants
• Polishing surfaces
• Using rolling elements like wheels or ball bearings