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Class 9, Science, Chapter-8, Lecture-4, Friction (Notes)

FRICTION: 

The force which always opposes the motion of one body over another body in contact with it and acts tangentially along the surface of contact is called the force of friction.

  •  Force of friction is always retarding.
  •  Force of friction always acts tangentially along the surface of contact.

CAUSE OF FRICTION:

  1. Interlocking between the surfaces in contact due to their roughness
  2. Force of adhesion (the force of attraction between different types of molecules)

FACTORS AFFECTING FRICTION:

  1. The roughness of surface
  2. The area of contact
  3. The weight of the sliding body.

TYPES OF FRICTION:

  1. Static Friction: ­
    The force of friction which completely balances the applied force so that the body on which force is applied does not move at all is known as static friction.
  2. Dynamic Friction
    The force of friction which acts on a body when it moves over another body is termed as the dynamic friction.
    Two types of dynamic frictions are:
  • Sliding Friction
    The force of friction which acts on a body when it slides over another body is termed as the sliding friction.
  • Rolling Friction
    The force of friction which acts on a body when it rolls over another body is termed as the rolling friction.

LIMITING FRICTION:

The maximum value of the static friction which comes into play when a body is just going to start sliding over the surface of another body is called limiting friction.

ADVANTAGES OF FRICTION:

  1. Enables us to walk
  2. Helps in writing with pen or chalk
  3. Helps in applying brakes
  4. Helps to drive machines.

DISADVANTAGES:

  1. Friction produces heat which damages machines
  2. Loss of efficiency of machines.
  3. Wear and tear of machine parts.

Friction is a necessary evil. 

Justification:
Friction causes loss of efficiency of machines, wear and tear of machine parts and heating of machine parts. So ‘friction is an evil’.
However, ‘friction is necessary’ as it enables us to walk, helps in writing with chalk, in applying brakes etc.
Therefore, friction is a necessary evil.

METHODS OF REDUCING FRICTION:

  1. Polishing
    Polishing reduces the friction between two surfaces by reducing the roughness of the surfaces.
  2. Using Ball bearing
    Ball bearing reduces friction by converting sliding friction into rolling friction
  3. Streamlining
  4. Lubrication-
    Lubricant reduces friction by filling the gaps present on the surfaces.
  • By applying oil or grease
  • By applying powder
  • By using soap solution