Class 8 Science


Friction: When one surface is moving over another surface, a force comes into play and opposes their relative motion. This force is called friction or force of friction. Friction always opposes motion because it acts in opposite direction to motion. Force of friction arises due to contact between two surfaces. If there is no contact between two surfaces there would be no friction.

Factors Affecting Friction:

Irregularities on surfaces: Friction is created because of irregularities on surfaces. The irregularities on surfaces result in interlocking of two surfaces. The interlocking between two surfaces creates friction. Hence, the force of friction will be greater in case of rough surface than in case of smooth surface.

Pressure Between two Surfaces: The force of friction increases with increase in pressure between two surfaces. You must have seen that moving a heavy box is difficult compared to moving a light box. Pressure is higher in case of a heavy box.

Types of Friction:

  • Static Friction: The minimum amount of force that is required to overcome the force of friction is called static friction. In other words, the minimum force that is required to make an object moving is called static friction.
  • Sliding Friction: Once an object starts moving on a surface, some force is required to keep the object moving. The minimum force that is required to keep a moving object in motion at a constant speed is called sliding friction. Sliding friction is always less than static friction. Due to this, it is easier to push a moving box compared to pushing a static box.
  • Rolling Friction: When an object rolls over a surface, the friction created is called rolling friction. Rolling friction is less than sliding friction.

Static friction > Sliding friction > Rolling friction

Friction: A Necessary Evil

Friction is all pervasive. Even a simple task; like writing on a page; cannot be done without friction. Walking on road won’t be possible because friction. But friction causes wear and tear of materials, and wastage of energy. Thus, friction is beneficial as well as harmful. Hence, friction is called a necessary evil.

Benefits of Friction

  • It is not possible to even walk on the road; in the absence of friction. Walking on ice is almost impossible because of lack of friction.
  • Friction is necessary to keep a ladder against a wall. The ladder will not find grip against the wall in the absence of friction.
  • It is very difficult to drive on a wet road because of less friction. A thin film of water is made over the surface of road. This thin film of water greatly reduces friction. Due to this, the driver finds it difficult to control the vehicle while driving on a wet road.

Harms of Friction

  • It results in wear and tear of shoes and tyres, machine parts, clothes, etc. It causes monetary loss.
  • Heat is generated by friction, which results in wastage of energy. Moreover, a significant amount of energy is wasted in overcoming friction.

Ways to Increase Friction

  • Rubber grips are put on bicycle handles, electrical tools and on many other tools. Increased friction helps in giving better grip on these objects.
  • A surface can be made rough with the help of sand paper; in order to increase friction.
  • Soles of shoes are made rough to increase friction. This helps in providing better grip on surface while walking.
  • Treads of tyres are deliberately made so that a good road grip can be achieved with the help of friction.
  • Holding a bottle cap with a thick towel helps in increasing friction. Increased friction makes it easier to open the bottle cap.

Ways to Reduce Friction

  • Lubricating oil and grease are used in machines to reduce friction between moving parts.
  • Some machines use air cushion for reducing friction because use of oil is not ideal in such machines.
  • Graphite powder is used in some machines to reduce friction.
  • Ball bearing coverts sliding friction into rolling friction. You have read that rolling friction is less than sliding friction. Thus, ball bearing helps in reducing friction.

Fluid Friction

When an object moves through a fluid, the fluids create friction. Friction created by a fluid is called fluid friction or drag. Force of friction is directly proportional to the density of a fluid. Hence, friction in oil will be greater than in water. Similarly, friction in water will be greater than that in air.

Streamlined Shape: A shape which is wide in middle and tapered at ends is called a streamlined shape. Such a shape reduces drag or fluid friction. Hence, a streamlined body can easily move through a fluid. Birds and fish are naturally endowed with streamlined body. Boats and airplanes are also made streamlined so that they can easily move through air or water.