Talha's Physics Academy
Unit # 3: Dynamics — Friction, Origin, Types & Laws
Lecture Overview
This lecture from Talha's Physics Academy covers the definition of friction, its classical and modern views of origin, different types of friction, and the fundamental laws of friction (Class 11 Physics, Unit 3: Dynamics).
Source Video: Origin, Types and Laws of Friction - Unit 3 Dynamics - #TP11 31
1. Friction
When a solid object moves or attempts to move over the surface of another solid object, its motion is always opposed by a retarding force. This force is called friction. Thus, friction is the force directed opposite to the direction of motion or attempted motion. The frictional force is always parallel to the surface in contact.
2. Origin of Friction
1. Classical View
According to the classical view, when two objects are kept in contact, an interlock forms between the irregular and rough surfaces. To break this interlock, an extra force is needed, which measures the force of friction.
2. Modern View
According to modern theory, friction is due to intermolecular forces of attraction between the surfaces in contact. When two surfaces are put together, the actual (true) area of contact is much less than the apparent area of contact. The pressures at the contact points are very high, pushing molecules very close so that attractive forces weld the surfaces together at contact points (known as cold welding). To break this attachment, an extra force is required.
3. Types of Friction
1. Static Friction
The frictional force that comes into play between two surfaces when one body tends to move on the surface of another body is called static friction.
The maximum value of static friction is called limiting friction.
2. Kinetic (Dynamic) Friction
The frictional force that comes into play between two surfaces when one body actively moves on the surface of another body is called kinetic friction.
There are two types of kinetic friction:
- a) Sliding Friction: The friction that exists between two surfaces when one body is sliding on the other (e.g., friction between a wood block and the road when it slides).
- b) Rolling Friction: The friction that exists between two surfaces when one surface is rolling over the other (e.g., friction between the tire of a moving vehicle and the road).
4. Laws of Friction
- a) The frictional force opposes the relative motion of two surfaces.
- b) The frictional force is parallel to the surfaces in contact.
- c) The frictional force is directly proportional to the normal reaction.
- d) The frictional force is independent of the apparent area of contact.
- e) The frictional force depends upon the nature of the two surfaces in contact and their state of roughness.
- f) The kinetic friction is independent of the relative velocities of the surfaces.
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