Friction Notes - Class 8 PDF Download

Friction

Force that resists the relative motion of surfaces is called friction. Solid, liquid and air; all types of surface offer the force of friction. The force of friction always applies in the opposite direction of movement. Friction is applied when two surfaces in contact move relative to each other.

Example :–

A moving ball on the ground stops after covering a certain distance. Ball stops after covering a short distance because surface of ground offers friction to the surface of ball in motion. Since, friction acts in the opposite direction of movement of ball, so the ball stops after covering a certain distance.

When a person applies brakes to a moving bicycle, the surface of brakes offers friction to the surface of moving wheel and the bicycle stops because of friction.

Factors that affect friction

A surface offers friction because of irregularities over the surface. A surface may appear smooth but microscopic view shows those irregularities. The irregularities of two surfaces get interlocked when in contact. To remove the interlocking, a surface offers friction when it slides or rolls over another surface.

Thus, rougher surface offers more force of friction.

Friction increases with increase in weight. In the case of increase in weight irregularities between surfaces get interlocked tightly because of pressure, and the surface offers more resistance of friction.

For example –

It is easy to roll or slide a lighter wooden box over ground than a heavier one.

It is easy to drag a mat over ground. But to drag the same mat becomes difficult; with someone sitting over it. This happens because friction increases with increase in weight.

Sliding Friction:

When a surface slides over another surface, the resistance to its motion is called sliding friction. For example; sliding of mat or a box over ground.

Rolling Friction:

When a surface rolls over another surface, the resistance to its motion is called rolling friction. Rolling friction is smaller than sliding friction. This is the cause that to roll anything over the ground is easier than to slide.

Example –

Luggage is fitted with wheels, so that one can roll the luggage easily.

Ball bearings are fitted between hub and axle of bicycles to make the movement easier.

FRICTION: A NECESSARY EVIL

Friction has both harmful and beneficial effects and hence it is called a necessary evil.

One can walk because ground offers friction. In the case of less friction walking becomes almost impossible. Thus, friction is necessary even a simple task like walking.

Example –

A person slips over the banana peel. This happens because banana peels have slippery surface and it offers very less friction to the ground. Thus whenever banana peel comes under the feet of a person, the person may slip and trip over because of less friction.

It is difficult to walk over muddy surface. This is because muddy surface offers very small friction to the surface of feet of a person.

Soles of shoes have grooves on them. Grooved soles offer more friction to the ground which gives better grip while walking. Shoes with worn out soles can be slippery.

The tyres of vehicles have treads for better grip over road. When treads are worn out, the tyres need to be replaced with new one.

Writing with pen is possible because paper offers friction to the tip of pen.

On oily paper, friction is very less and so it is difficult write on such paper.

While writing with chalk over black board, the rough surface of the black board rubs off the chalk because of which some of the particles of chalk get stuck with black board and writing becomes possible.

INCREASE OF FRICTION:

Since, friction is necessary for us, therefore many a time friction is increased to accomplish a task.

Example –

Tyres are treaded to increase friction. Vehicle with treads on tyres offer mores friction which gives better road grip.

Soles of shoes are treaded to increase friction.

Javelin thrower rubs some powder on his palm to get better grip on the javelin.

REDUCTION OF FRICTION:

Sometimes, it is necessary to reduce the undesirable friction. Lubricants make a thin layer over surface and facilitate easy movement. In the engines of vehicles, use of lubricants increases the efficiency as parts are not rubbed directly over one another because of thin layer of lubricant. In many machines, graphite powder or air is used as lubricant.

Example –

While playing carom, talcum powder is sprayed over it to reduce friction.

Lubricant is used to reduce friction in engines, such as engine oil is used in the engine of motorcycle, car, truck, etc. Grease is used over the chain of bicycles.

FLUID FRICTION:

Gas and liquids are considered as fluid. Similar to solid, fluids also offer friction. The friction of fluid increases with the speed of object moving through it; and speed of fluid.

Object which are made to move in fluid are made like boat-shaped to reduce friction offered by fluid. The shape of aeroplane, ship and boat is made streamlined. Birds and fish also have streamlined shape. A shape which is pointed on both ends and wide at the middle is called streamlined shape. Such a shape facilitates easier movement through fluids.

The friction offered by fluids is called drag.