JEE Physics Notes: Friction

Introduction to Friction

Friction is a force that opposes the relative motion between two surfaces in contact. It plays an essential role in everyday life, from walking on the ground to driving vehicles. While friction helps in movement and grip, it also leads to energy loss in mechanical systems.

Types of Friction

Static Friction

Static friction is the force that prevents an object from moving when a force is applied. It acts between two surfaces that are not moving relative to each other. This friction increases with the applied force until it reaches its maximum limit, beyond which motion begins.

Kinetic Friction

Once an object starts moving, static friction is replaced by kinetic friction. This type of friction acts between two surfaces in relative motion. It is usually lower than static friction, meaning less force is needed to keep an object moving than to start its motion.

Rolling Friction

Rolling friction occurs when an object, such as a wheel or a ball, rolls over a surface. It is much smaller than static and kinetic friction, which is why wheels and ball bearings are used in vehicles and machinery to reduce friction.

Laws of Friction

• Friction depends on the nature of the surfaces in contact.
• Static friction is usually greater than kinetic friction.
• Friction does not depend on the surface area in contact, as long as the normal force remains the same.
• Friction acts in a direction opposite to the applied force or motion.

Factors Affecting Friction

• The roughness of the surfaces in contact.
• The nature of the materials in contact.
• The presence of lubricants, which reduce friction.
• The normal force pressing the surfaces together.

Angle of Friction

The angle of friction is the angle made by the resultant of the normal force and frictional force with the normal force itself. A higher angle of friction indicates higher resistance to motion.

Angle of Repose

The angle of repose is the maximum angle at which an object placed on an inclined surface remains at rest without sliding down. It depends on the friction between the object and the surface.

Methods to Reduce Friction

• Using lubricants like oil and grease to make surfaces smoother.
• Polishing surfaces to reduce roughness.
• Using ball bearings and rollers to replace sliding friction with rolling friction.
• Applying air cushions or magnetic levitation to reduce contact between surfaces.

Applications of Friction

• Walking and running rely on friction between our feet and the ground.
• Brakes in vehicles use friction to stop motion.
• Nails and screws hold objects together due to friction.
• Matches ignite due to friction when struck against a rough surface.

Friction is an essential concept in physics, influencing motion, energy efficiency, and mechanical operations. Understanding friction is key to solving many problems in JEE Physics.