CBSE Class 8 Science Chapter 9 Friction Notes
Friction force is defined as the force that opposes the relative motion between two surfaces of objects when they come into contact.
The direction of the friction force is always opposite to the direction of motion of the moving body.
Read And Learn More CBSE Class 8 Science Notes
Types Of Friction Class 8 CBSE With Examples Spring Balance
It is the device which is used for measuring the force acting on an object. It consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force.
Factors Affecting Friction
The friction force between two surfaces depends on two factors
- The nature of two surfaces.
- The force with which the two surfaces are pressed together
Friction depends on the Nature of the Two Surfaces.
The force of friction is not the same for all surfaces. It depends on the roughness or smoothness of surfaces which are in contact.
When the contact surfaces are smooth, then the friction is veryr less. As the roughness increases, the friction also increases.
Friction Depends on the Force with which the Two Surfaces are Pressed Together
Friction is caused by interlocking of irregularities on the two surfaces of contact. So, even those surfaces which appear very smooth, have a large number of minute irregularities on them.
When two surfaces of objects are pressed together with greater force, it will increase the interlocking of irregularities. Thus, the friction force increases. This will become clearer from the following figure.
How To Reduce Friction Class 8 Science Methods, Types Of Friction
There are generally three types of friction, which are stated below
1. Static Friction
It is the friction that acts between two surfaces when the object starts moving from its state of rest. It is more powerful than other forces. For example, A boy pushes a heavy box, but the box does not move, then he applies more force to the box and the box just tends to start moving. At that instant, the frictional force between the floor and the box is the static friction.
2. Sliding Friction
It is the friction that acts between two surfaces when the object is already in motion or sliding over another surface, Example, skating on snow.
During sliding, an object does not get enough time to lock into the contact points on the floor.
3. Rolling Friction
- It is the friction which comes into play when a body rolls over the surface of another body.
- Example. Wheels of a skateboard roll over a road.
- The direction of rolling friction is opposite to the direction of turning of wheels.
Note: Sliding friction is smaller than static friction but greater than rolling friction.
Friction as a Necessary Evil Class 8 Explanation Friction: A Necessary Evil
The force of friction plays an important role in our day-to-day activities. Sometimes, friction is useful while in some cases, friction is harmful.
Let us learn some advantages and disadvantages of friction.
Advantages of Friction
- Friction enables us to walk easily on the ground When we push the ground backwards while walking, the friction force pushes the foot forward. It prevents us from slipping back.
- Friction enables us to fix the nail into the wall When we hammer a nail into the wall, the force of friction between nail and wall holds the nail tightly in the wall. Thus, we can fix nails in the wall.
- The brakes slow down or stop moving bicycle due to friction If we press the brake lever to apply brakes, the brake pads rub the rims of bicycle wheels. The friction between them slows down the motion of bicycle.
Disadvantages Of Friction
In some cases, friction is harmful to us and can be considered an evil because of the following disadvantages
Wearing away of soles of our shoes takes place due to friction There is a friction between soles of shoes and road during walking. Due to which the soles rub against the road and the tiny pieces of soles keep on breaking off slowly.
Heat is produced due to friction. When we rub our hands together quickly for a few seconds, they become warm because friction between the hands produces heat. Similarly, when we operate a mixer and a grinder lot few minutes, its at becomes hot due to friction.
The efficiency of machines gels reduced due to friction In order to run a machine, some energy is supplied to it. Hue to friction between the parts of machine, some of the energy is wasted in the form of heat. This reduces the efficiency of machines.
Fluid Friction and Drag Force Class 8 Science Methods Of Increasing Friction
The grooves are made on the soles of shoes to increase friction.
These grooves increase the friction with the ground and the shoes get a better grip even on a slippery ground.
The tyres of vehicles are made up of treads to increase friction. A series of patterns made on the surface of tyre is called tread. The tyres are designed with treads which provide a better grip with the ground even on slippery road.
Kabaddi players usually rub their hands with dry soil to increase friction and to get a better grip on their opponents. Gymnasts apply some coarse substance on their hands to increase friction for a better grip.
Friction and Its Effects Class 8 Short Notes: Methods Of Reducing Friction
- With the help ol polishing (lie surface We sprinkle line powder on the carrotn board and then wc polish its surface to make it smooth, so that the striker slides easily on the surface.
- By applying lubricants (like grease or oil) to the rubbing surfaces The substances which reduce friction are called lubricants, e.g. oil , grease, etc.
When the oil, grease or graphite is applied between the moving part of a machine , a thin layer is formed there and moving surfaces do not directly rub against each other.
So, interlocking of irregularities is avoided to a greater extent, thus it reduces friction and the movement becomes easier.
Wheels Reduce Friction
It requires a lot of force or energy to move a heavy bag on the ground as the sliding friction is very large.
However, if we attach wheels or rollers to this heavy bag, then it experiences rolling friction.
In our daily routine, we often observe objects, like trolley bags and attachable fitted with rollers.
It is easier to pull the heavy bag with the attached wheels because rolling friction is smaller than sliding friction.
Real-Life Examples Of Friction For Class 8 Students: Ball Bearing
These are designed to allow moving parts of machines to roll over each other. It consists of metal balls fitted in between metal rings. In bicycles, motorbikes, etc., the ball bearing is inserted between the hub of the wheel and the axle.
When the wheel revolves, the balls of the ball bearing roll, and this reduces the friction. Hence, the wheel rolls smoothly over the axle
Advantages And Disadvantages Of Friction Class 8 Fluid Friction
Those substances which can flow easily are called fluids: liquids and gases. There is a friction whenever an object moves through a fluid, is called fluid friction.
Air and water both exert a friction force when objects move through them.
Note: The frictional force exerted by fluids is also known as drag.
The frictional force exerted by fluid on an object moving through it depends on
- Speed of an object relative to a fluid
- Shape of object
- Nature of fluid (viscosity)
Disadvantages of Fluid Friction
- A fluid friction opposes the motion of the object, which reduces its speed.
- Fluid friction reduces the efficiency of an object. Some energy gets lost in overcoming the fluid friction.
Methods Of Reducing Fluid Friction
The fluid friction can be decreased by giving a special (streamlined) shape to the objects.
Scientists got the idea for streamline shape from the nature. The body shape of birds and fishes help them to reduce frictional force of fluids and make them to move easily.
Note The body shape that offers a small resistance to the flow of fluid around it, is called streamlined shape.
In the figure, we can see that the bird and aeroplane both have a streamlined shape in the middle with two wings and a tall tail. Fishes are also streamlined by nature due to which they move easily In the water.
Force of friction: The force that opposes the relative motion between two surfaces of the object when they come in contact.
Static friction: The friction that acts between two surfaces when the object starts moving from its rest state.
Sliding friction: The friction that acts between two surfaces when the object is already in motion.
Rolling friction: The friction that acts between two surfaces when a body rolls over the surface of another body.
Drag: The frictional force exerted by fluids is called drag.