# DAV Class 8 Science Chapter 5 Notes – Friction

These DAV Class 8 Science Notes and DAV Class 8 Science Chapter 5 Notes – Friction act as excellent revision resources, particularly in preparation for board exams.

## Friction Class 8 DAV Notes

→ Friction: When an object moves, or tends to move, over the surface of another object, there is a force acting between the two surfaces in contact. This force is called force of friction or friction.

→ Characteristics of Friction: The force of friction always opposes, or tends to oppose, any relative motion between the two surfaces in contact. The force of friction always works in the opposite direction of motion.

→ Cause of Friction: The irregularities of one surface get interlocked with the irregularities of another surface. This interlocking of irregularities of two surfaces in contact is the cause of friction.

→ Factors Affecting Friction:

• Nature of two surfaces in contact.
• Forces pressing the two surfaces together.

→ Types of Friction:
→ Static Friction: When a body is at rest and a force is applied on that body, the friction which comes into play is called static friction. Enough force needs to be applied to overcome static friction so that the object can come into motion. The maximum value of static friction is called limiting friction.

→ Sliding Friction: When a body is in motion on a surface, the friction which comes into play is called sliding friction. Sliding friction is always less than limiting friction for a given system of two surfaces.

→ Rolling Friction: When a body rolls over a surface, the force of friction which comes into play is called rolling friction. Rolling friction is always less than sliding friction.
Static Friction > Sliding Friction > Rolling Friction

→ Friction, A Necessary Evil:
Friction is necessary for doing most of our daily activities. So, friction is a friend. But, on the other hand, friction leads to wear and tear of many things and decreases efficiency. So, friction is an evil. Due to this, friction is called a necessary evil.

→ Friction, A Necessity:

• Walking is possible because of friction between the ground and our feet. It is impossible to walk over a frictionless surface. This is the reason, we tend to slip on a polished marble floor or on a wet floor or on icy surface or on banana peel.
• Wheels are able to roll and vehicles are able to move safely because of friction. Lack of friction will result in reduced road grip of tyres on the road and may result in accidents.
• We are able to do many day-to-day activities, because of friction. Opening a bottle cap, writing with pen and pencil, etc. are possible because of friction.

→ Friction, An Evil:
Friction consumes substantial part of useful energy. Some of the effort is wasted in overcoming friction while doing an activity.

Friction results in wear and tear of machine parts, tyres, soles of shoes, etc. Thus, friction results in monetary loss.

→ Methods of Increasing Friction:
Surface is made rough for increasing friction. For example the handlebars of bicycle have rubber covering to increase friction. Tyres have treads so that friction can be increased to improve the road grip. Soles of shoes have grooves over them to increase friction. Sportspersons use spikes in their shoes for better grip while running.

→ Methods of Reducing Friction:

• Polishing: A surface is polished to make it smooth so that friction can be reduced.
• Lubrication: Oil or grease is applied on machine parts to reduce friction. This helps in smooth movement of machine parts and reduces wear and tear.
• Ball bearings: Ball bearings are used in rotating machine parts. We know that rolling friction is less than sliding friction. That is why, ball bearings help in reducing friction.

→ Fluid Friction:
When an object moves through a fluid (liquid or gas), the friction offered by the fluid is called fluid friction. Fluid friction is also known as drag.

→ Factors affecting fluid friction:

• Nature of fluid.
• Shape of the moving object.
• Speed of the moving object with respect to the fluid.

→ Streamlined Body: A body which is spindle-shaped is called streamlined. A streamlined body has to encounter least amount of friction, while moving through a fluid. Birds and fishes have streamlined bodies. Similarly, aeroplanes, boats and ships have streamlined bodies.

→ Lubricant : Substances which are applied to reduce friction.

→ Streamlined : A spindle-shaped body is called streamlined.

→ Drag : Fluid friction is called drag.

→ Ball bearing : A device which is fitted in moving parts of machines to reduce friction by changing sliding friction into rolling friction.