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12. Friction

This chapter talks about types of friction, its advantages and disadvantages and methods to reduce friction.

The force which opposes the motion of an object on the surface of another object is called force of friction. If you apply the force along the left, friction acts along the right & vice-versa.

12.1     Factors affecting Friction

Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces which appear very smooth have a large number of minute irregularities on them. Irregularities on the two surfaces lock into one another. When we attempt to move any surface, we have to apply a force to overcome interlocking or rough surfaces, there are a larger number of irregularities, so the force of friction is greater if a rough surface is involved.

12.2     Types of Friction

12.2.1 Static Friction

The force which opposes the motion when the body just starts moving from rest is called Static Friction. Static Friction equals to most required to a body from rest.

12.2.2.     Sliding Friction

The force which opposes the motion when the body is sliding on other surface is called Sliding Friction. Sliding Friction is less than Static Friction. Sliding friction is most required to maintain the speed of a body.

12.2.3.     Rolling Friction

The force which opposes the motion when the body rolls on another surface is called rolling Friction. It is the least among all the three. Rolling Friction equals to most required to roll a body.

12.3     Friction : A necessary evil

The force which opposes the motion of an object on the surface of another object is called Force of Friction. It has its advantages & disadvantages as well. So, it is said to be a necessary evil.

12.3.1 Advantages of Friction

  • We are able to hold things.
  • We are able to walk.
  • We are able to write.
  • We can start or stop or change the direction of motion due to friction.
  • Without friction no building could be constructed.
  • We can write on the black board.

12.3.2 Disadvantages of friction

  • It causes wear & tear of machine parts
  • It wears out screws, ball bearings, soles or tyres of vehicles.
  • Friction can also produce heat & causes wastage of energy which reduces efficiency of machines.

12.4     Increasing & Reducing Friction

Friction, as we understood in the previous section, is a necessary evil. It is undesired in many situations, whereas friction is desired in some situations.

12.4.1 Methods of Increasing Friction

Friction can be increased by making the surface rough because rough surfaces have more irregularities, so interlocking of irregularities is also more. Examples:

  • Grooves are present or tyres or soles of shoes to provide better grip on the ground because grooves make the surfaces rough & increase friction.
  • Kabaddi players rub their hands with soil for a better grip of their opponents.
  • Gymnasts apply a coarse substance on their hands to increase friction for a better grip.

12.4.2 Methods of Reducing Friction

We can reduce friction by making the surfaces smooth because on smooth surfaces irregularities are less & so interlocking between irregularities is also less. Examples:

  • By sprinkling fire powder on the carom board.
  • Oil, grease & graphite is applied between the moving parts of a machine, so that a thin layer is formed there & moving surfaces do not directly rub against each other.
  • Few drops of oil are poured on the hinges of a door, the door moves smoothly.

12.4.3 Lubricants reduce Friction

The slippery substances which reduce friction are called Lubricants. Eg. Oil, Grease, Graphite. The lubricants form a thin layer between the moving parts of a machine. Surfaces do not directly rub against each other. So, interlocking of irregularities is avoided to a great extent.

However, in some machines, it may not be advisable to use oil as lubricant. An air cushion between the moving parts is used to reduce friction.

It is important to remember that we cannot reduce friction to zero by polishing surfaces or using large amount of lubricants.

Note: Friction can never be totally eliminated as there is no perfectly smooth surface.

12.4.4 Wheels Reduce Friction

Rolling reduces friction. It is always easier to roll than to slide a body over another. Eg. It is easier to pull luggage filled with rollers. Rolling Friction is smaller than the sliding friction. Sliding has been replaced in most machines by rolling by the use of ball bearings. Eg Axles of ceiling fans & bicycles.

12.5     Fluid Friction

In Science, the common name of gases and liquids is fluids. Fluids exert frictional force called Drag on objects in motion through them.

The frictional force on an object in a fluid depends on its speed with respect to the fluid. The frictional force also depends on the shape of the object & the nature of the fluid.

When objects move through fluids, they have to overcome friction acting on them. In this process, they lose energy, therefore efforts are made to minimize friction.

That is why, objects are given special shapes (stream lined shape). Examples:

  • Aeroplanes have shape similar to birds.
  • Boats have shape similar to fishes.
  • All vehicles are designed to have shapes which reduce fluid friction.