The Friction

Friction is defined as the force which opposes the relative motion between the two surfaces that are in contact with each other. The word friction is derived from the Latin word “fricare” which means to rub.

In the above activity, a force is acting on the book opposing its motion. This force is called the force of friction. This force acts between the surface of the book and the table. If you apply the force along the left, friction acts along the right and vice-versa. So, the force of friction always opposes the motion of the object. Rub your palms together for sometime. Now, feel them on your cheeks. You will feel warm. The friction produced by rubbing your palms is the cause of this heat.

Thus, we can say that the friction has the following two properties:

  • Friction is a force that always opposes the motion of a moving object. It also prevents a stationary object from moving.
  • Friction produces heat.


Friction is a Contact force, i.e., it occurs when two surfaces are in contact and one surface tries to move or moves on other surface.

  • Friction depends on the nature of the surfaces, i.e., smoothness or roughness of surfaces.

Frictionperfectly smooth. Even a table top which appears to be smooth has little bumps or irregularities in it (when observed through a magnifying glass). When two surfaces with minute irregularities come in contact, they interlock and oppose the motion. The rough surface has larger irregularities. Rougher the surface more is the force of friction.

The above activity also shows that smoother a surface is, lesser is the force of friction.

  • Friction also depends on the mass of the object. For example, if you want to push an empty box, you will need some force. But if you put some weight in that box, you will need more force to push it. Thus, we can say that force of friction increases as the weight of the object increases.

Friction will also increase if the two surfaces in contact are pressed harder. This can be experienced by dragging a mat alone and when a person is pressing onto it.


‘There are three types of frictional force’

Static friction

It is the force acting between two surfaces when they are not moving relative to each other, even though an external force is applied on them. To move an object, static friction must be overcome.

Sliding friction

It is the force acting between the two surfaces when object slides on another object. When an object slides, the contact points on its surface do not get enough time to lock into the contact points of the surface on which it is_ sliding. So, sliding friction is slightly smaller than the static friction.

Rolling friction

It is the force acting between two surfaces when an object rolls over another object. Rolling friction is always less than the sliding friction. Thus, when heavy objects are to be moved, we use rollers to make the movement easier. Vehicles like cars, trucks, bicycles, etc. are provided with wheels to make their movement easier.

In space, there is no frictional resistance to motion as there no air. A moving body will keep on moving in space in a straight line unless an external force is applied. As rolling friction is less than the sliding friction, ball bearings are used in machines to reduce friction by replacing sliding will rolling.


Fluids, that include liquids and gases, exert force of friction when objects move through them. This frictional force is called drag. The drag depends on the shape of the object its speed with respect to the fluid and the nature of the fluid. Objects lose energy while trying to overcome the frictional force when they are moving through a fluid. Thus, they are given a special shape called a streamlined shape, that helps them to pass through fluids easily. For example, boats, ships, automobiles, airplanes, etc. are given streamlined shapes to cut easily through water and air (respectively). In nature, birds and fish have streamlined shapes.


  • Friction between our feet and the grow: enables us to walk. Without it, we would slip and fall.
  • Friction between the wheels and the brakes slows down or stops the motion of a vehicle. If there were no friction, an object in motion would never stop.
  • Cars and buses are able to move only due to friction between the road and the tyres.
  • Friction enables us to write. We cannot even hold a pencil or any other objects in our hands without friction.
  • Friction helps in lighting a matchstick.
  • . Friction holds a nail in a wall.

All the above points show that friction is of great help to us in our daily life.


Friction is also called a necessary evil since along with many advantages, it has the following disadvantages also.

  • Friction opposes motion and stops moving objects.
  • Machine parts get heated due to friction and thus, get damaged. Due to this, they have to be replaced or repaired from time to time.
  • Friction causes wear and tear of materials like screws, ball- bearings, soles of shoes, tyres of vehicles, etc.
  • Friction causes wastage of energy. For example, while driving a car on a rough road, the engine exerts more force in moving the car due to friction between the road and the wheels. This leads to heating up of the engine and wastage of energy.


In some cases, friction is desirable. Methods to increase friction involves making the surface rough for better grip.

  • Tyres of vehicles are treaded with many grooves on their surface. These treads improve the grip of tyres on the road by increasing the friction between the tyres and the road.
  • Soles of shoes of athletes and mountaineers are spiked, for better grip and to prevents slipping.
  • Pavements and playing areas, such as basketball courts are made slightly rough to prevent slipping.
  • Friction is increased by using brake pads in the brake system of bicycles and automobiles. When we press the brake, these pads arrest the motion of the wheels due to friction.
  • Kabaddi players rub their hands with soil for a better grip on their opponents.
  • Gymnasts rub some coarse substance on their hands to increases friction for better grip.


In cases where friction is undesirable, it is reduced by making the surface smooth and slippery.

By using ball bearings

Ball bearings are small steel or lead balls introduced in between the two moving surfaces to reduce friction. For example, use of ball bearings in bicycles, ceiling fans, and other moving parts of machines.

By using lubricants

The use of lubricants remove the unevenness of the two surfaces in contact, by forming a thin smooth layer between them. This helps the surfaces to move easily over one another oil, grease, graphite powder, etc., are some common lubricants. In some machines, an air cushion between tts moving parts is used to reduce friction. We put fine talcum powder on the carrom board to reduce friction.

By polishing the surfaces

Polishing the surfaces make them smooth and thus, reduces friction. Varnish and electroplating are done to polish the surface and make it smooth.

By using wheels or rollers

Rolling friction is less than the sliding friction, Heavy luggage like a suitcase, are fitted with rollers or wheels so that they can be moved easily.

By streamlining

For objects that move through fluids (liquids or gases), a streamlined shape reduces friction and allows them to move easily. For example, aircraft and automobiles have streamlined shapes to move through the air with minimum friction.

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