NEET Notes: Friction

Introduction to Friction

Friction is a resistive force that opposes the relative motion between two surfaces in contact. It plays a crucial role in everyday activities, from walking to driving, and has significant applications in mechanics. Understanding friction is essential for solving problems related to motion, work, and energy in the NEET exam.

Types of Friction

Friction can be classified into different types based on the nature of motion and the surfaces in contact.

• Static Friction: The force that resists the initiation of motion between two stationary surfaces. It is always equal to or greater than the applied force until motion begins.

• Kinetic (Dynamic) Friction: The force that opposes the relative motion of two moving surfaces. It is generally less than static friction.

• Rolling Friction: The force resisting the rolling motion of an object over a surface. It is significantly smaller than static and kinetic friction.

• Fluid Friction (Viscous Force): The resistance experienced by an object moving through a fluid (liquid or gas). It depends on the shape, speed, and viscosity of the fluid.

Causes of Friction

Friction arises due to several factors, including:

• Surface Roughness: Irregularities at the microscopic level cause interlocking between surfaces.

• Intermolecular Forces: Adhesion between molecules of different surfaces contributes to frictional resistance.

• Deformation of Surfaces: Softer materials deform under pressure, increasing resistance.

Laws of Friction

Friction follows fundamental laws that help in analyzing its effects in mechanics.

1. First Law of Friction: Friction acts tangentially to the contact surface and opposes relative motion.

2. Second Law of Friction: The force of friction is directly proportional to the normal force acting between two surfaces.

3. Third Law of Friction: The coefficient of kinetic friction is always less than or equal to the coefficient of static friction.

4. Fourth Law of Friction: Friction is independent of the apparent area of contact between two surfaces.

Coefficient of Friction

The coefficient of friction (μ) is a dimensionless quantity representing the ratio of the force of friction to the normal force.

• Static Friction Coefficient (μs): Determines the maximum friction before motion starts.

• Kinetic Friction Coefficient (μk): Defines the resistance during motion, which is usually lower than μs.

Angle of Friction and Angle of Repose

• Angle of Friction: The angle (θ) between the resultant of normal force and frictional force with the normal to the surface. It is given by tanθ = μ.

• Angle of Repose: The minimum angle of inclination at which an object placed on an inclined plane just starts sliding due to gravity. It is also given by tanθ = μ.

Rolling Friction vs. Sliding Friction

• Rolling Friction: Lower than sliding friction because rolling motion reduces direct surface interaction.

• Sliding Friction: Higher than rolling friction as more surface area is involved in resistance.

Applications of Friction

Friction has various practical applications in daily life and engineering.

• Walking and Running: Friction between the feet and the ground prevents slipping.

• Braking in Vehicles: Increases resistance between tires and roads, allowing controlled stopping.

• Matchstick Ignition: Friction between the matchstick and rough surface produces heat.

• Machines and Engines: Controlled friction ensures efficient operation of gears and mechanical components.

Methods to Reduce Friction

In many cases, excessive friction leads to energy loss and wear. Methods to reduce friction include:

• Lubrication: Applying oil or grease to reduce surface roughness.

• Polishing Surfaces: Smoothening surfaces minimizes interlocking.

• Using Ball Bearings: Rolling motion reduces direct surface contact.

• Streamlining Shapes: Reducing air and fluid resistance improves motion efficiency.

Methods to Increase Friction

In some applications, increasing friction is beneficial to prevent slipping and enhance grip.

• Using Rough Surfaces: Increases interlocking between contact points.

• Adding Treads on Tires: Provides better traction on roads.

• Using Dry Surfaces: Removing lubrication enhances resistance.

• Applying Powder or Chalk: Increases grip in sports activities.

Friction in Circular Motion

Friction plays a critical role in objects moving in circular paths, such as vehicles on curved roads.

• Centripetal Force Contribution: Static friction provides the necessary centripetal force for circular motion.

• Banking of Roads: Friction, along with banking angle, helps vehicles maintain stability while turning.

Energy Dissipation Due to Friction

Friction converts mechanical energy into heat energy, leading to energy loss in machines. This is an important factor in designing efficient mechanical systems.

• Heat Generation: Excessive friction causes overheating in machines.

• Wear and Tear: Continuous friction leads to material degradation over time.

• Noise Production: Vibrations due to friction generate unwanted noise in moving parts.

Conclusion

Friction is a fundamental force in mechanics that influences motion, energy transfer, and practical applications in real life. A strong understanding of frictional concepts is essential for solving NEET physics problems related to mechanics, motion, and energy dissipation.