Friction Complete Notes

Best notes on Friction | Type of friction | Static friction | Limiting friction | Kinetic friction | Rolling friction | Sliding friction | Laws of limiting friction | Angel of Friction | Friction is a necessary evil | Advantages of friction |Disadvantages of friction | Advantages and disadvantages of friction | Friction is an evil | Methods of reducing friction | Methode of Increasing friction | FAQ



Friction is an opposing force that comes into play whenever a body moves or tends to move over the surface of another body. (i.e., relative motion between bodies in contact). This force acts tangentially along with the bodies in contact.

Type of Friction

  1. Static friction
  2. Limiting friction
  3. Kinetic friction

1. Static friction

It is the force of friction that exactly counterbalance the applied force during a stationary state of a body.

It must be noted that static friction does not exist by itself. When there is no applied force, there is no static friction. It comes into play at the moment when there is applied force.

2. Limiting friction

It is the maximum value of the static friction which comes into play when a body just starts moving over the surface of another body.

Laws of Limiting friction

  1. The direction of the force of limiting friction is always opposite to the direction in which the body moves or tends to move.
  2. The limiting friction between two surfaces depends on the nature of surfaces and their state of roughness ( or state of polish).
  3. The limiting force of friction between two surfaces is independent of the area of the surface in contact so long as the normal reaction remains the same.
  4. The limiting friction acts tangentially along the surface of contact of two bodies.
  5. The limiting friction is directly proportional to the normal reaction R between two surfaces.

3. Kinetic friction

The force of friction that comes into play when a body actually moves over the surface of another body.

  • Kinetic friction is less than the limiting friction.
  • Kinetic friction is independent of the relative speed between the bodies provided the speed is small.
  • Friction depends upon the nature of the contact surface. Due to roughness, the friction increases.
  • The force of kinetic friction is directly proportional to the normal reaction R between two surfaces.

Types of Kinetic Friction

(i) Sliding Friction: Force of friction that comes into play when a body slides over the surface of another body. For example, when a wooden block is pulled or pushed over the ground then sliding friction comes into play.

(ii) Rolling Friction: Force of friction that comes into play when a body rolls over the surface of another body. For example when a wheel rolls over the road, then rolling friction comes into play. (Rolling friction is much less than sliding friction)

Angel of Friction

The angle of friction between any two surfaces in contact is the angle at which the resultant force of limiting friction (fsmax.) and normal reaction R make with normal reaction R.

Let the body of weight W = mg lies on a horizontal surface. The weight W of the body is balanced by normal reaction R. P is the applied force, fs (max) is the limiting force of friction. Completing parallelogram OABC. The resultant of OA(R) and OB (fsmax.) is represented by OC(R’). Finally, the angle between OC(R’) and OA(R) is called as the angle of friction (θ).

Why friction is a necessary evil?

Friction is called a necessary evil. It is a necessity because we cannot do anything without it. At the same time it is an evil because it involves unnecessary waste of energy. The following facts make the point clear :

Advantages of friction (Friction is a necessity)

  1. Walking will not be possible without friction. Our foot pressing the ground will only slip.
  2. No two bodies will stick other if there is no friction.
  3. The brakes of the vehicles will not work without friction.
  4. Writing on the blackboard or on a paper will also not be possible without friction.
  5. Nuts and bolts for holding the part of machinery together will not work.
  6. It is due to friction between matchstick and matchbox that the matchstick catches fire.
  7. Cleaning with sandpaper will not be possible without friction.

Disadvantages of friction

  1. Friction causes wear and tear on the part of machinery in contact. Thus their lifetime reduces.
  2. Frictional forces result in the production of heat, which causes damage to the machinery. Hence we conclude that friction is a necessary evil.
  3. Force of friction restricts the speed of moving vehicles like buses, trucks, cars etc.
  4. A significant amount of energy of any moving object is wasted in the form of heat to overcome friction.
  5. Friction always opposes the relative motion between any two surfaces in contact and extra energy is required to overcome friction. So friction involves the wastage of a large amount of energy. Hence output power is always less than the input power.

Methods of reducing friction

  1. By polishing: Polishing makes the surfaces smoothers. Therefore friction reduces.
  2. By lubrication: Lubrication like oil, grease etc. fill-up the irregularities of the surface making them smoother.
  3. By streamlining: Friction due to air is considerably reduced by streamlining the shape of the body moving through the air.
  4. By using ball bearing: Since rolling friction is much smaller than sliding friction, so we convert sliding friction to rolling friction. This is done by using ball bearings.

Methode of Increasing friction

  1. On a rainy day, some sands are thrown on the slippery ground. This increases friction.
  2. Similarly, sand is spread on a track covered with snow.
  3. Proper threading of tyres also increases the force of friction between the tyres and the road.

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