Friction Class 8 Notes

Friction is the force acting along the two surfaces in contact that opposes their relative motion. For example, it acts along our slippers while we walk.

Friction is also called as Force of Friction.

The cause of the friction includes:
1. Roughness of the surfaces: The hills and the valleys of the roughness along the surfaces gets interlocked.
2. Force of adhesion between two surfaces: The force of attraction between atoms and molecules of surfaces in contact causes friction.

Friction depends on the following factors:
1. Normal force acting between the two surfaces. Higher normal force means higher friction.
2. Nature of the surface i.e. roughness of the surface. Friction force is higher between rough surfaces. If the surface is smooth the friction is low.
3. However, force of friction does not depend on area of the surfaces in contact.

The friction that acts between the surfaces of a solid and a fluid (liquid or gas) as the solid moves through it is drag. The drag exerted by a liquid on a solid is greater than the drag exerted by a gas on it.

Drag on a solid due to a fluid depends on following factors:
1. Speed of the solid with respect to the fluid – Higher is the speed, more will be the drag,
2. Shape of the solid – Streamlined shaped solid experiences least drag,
3. Nature of the fluid – Drag by mustard oil is higher than drag by water.

There are three kinds of friction. Static friction, Sliding or Kinetic friction and Rolling friction.

Static friction is the friction force acting between any two surfaces which are at rest with respect to each other. It is self-adjusting force.
For example: if a student pulls a heavy box and it does not move, then the friction between box and the land is static friction.

Static friction acting between two surfaces changes its values depending on the applied force. When applied force increases, the friction increases too. If it decreases, the static friction decreases too. That’s why, the static friction is self-adjusting force.

The maximum static friction force that acts between two surfaces is limiting friction. If a force higher than limiting friction is applied, there will be motion between the two surfaces.

Sliding or Kinetic friction is the friction force acting between the two surfaces which are sliding over each other. For example, the force of friction between a box and the land when the box is dragged over the land.

Rolling friction is the force of friction when one of the surfaces in contact rolls over the another. For example, the friction force when the wheel of a cycle rolls over the road.

Limiting friction is the strongest friction. Rolling friction has the least value. Sliding friction has the value in between them.
Limiting Friction > Sliding Friction > Rolling friction

Friction is very useful in our day-to-day life. Some of its importance are as follows:
It helps us walking,
It helps in fixing nails and screws on wooden board or walls,
Movement of wheels over the road are due to friction,
Writing on paper is possible only due to friction,
Brakes are impossible without friction.

Friction is wasteful also. It causes wear and tear of moving parts of the machines, just like brake shoes.
It also causes loss of energy as heat. Thus, it reduces efficiency of the machines.

A person cannot walk on oily surfaces. This is so because due to the oiliness of the surface there will be no friction, and without friction no one can walk.

Friction can be increased by increasing the roughness of the surfaces in the contact. The way to increase friction depends on the situations. For examples:
1. Wrestlers and Kabaddi players rub soil on their hands to increase their grips
2. Soles of shoes and Tyres of vehicles are corrugated,
3. Brake pads/lining are made rough.

Friction is decreased by polishing the surfaces. It reduces the roughness. For example:
1. By applying oil or grease on the surfaces (by lubrication) as in machines,
2. By sprinkling a soft, slippery powder on the surfaces as in carom-board,
3. By using ball-bearings or roller- bearings or wheels as in trolley bags,
4. Streamlining the body reduces friction in the fluids (drag)

Shapes of birds, boats, airplanes, fishes etc. are streamlined shapes. Such shapes reduce the drag (friction in fluids) acting on these bodies when they move in air or water.

Meteors move with very high speed. When they pass through earth’s atmosphere, the friction (drag) causes heating. The heating is so high that the meteors burn out. Thus, we are saved from meteors.