Unit 6:-
Forces And Machines
In our daily life, we experience different types of forces every time. We require force to stop or move things. A book lying on a table can be moved by applying force. Similarly, in cricket ground, player stops or tries to stop a ball.
We use force when we play, walk, pedal a bicycle, open a door or even write on a paper. We cannot see a force but we can see it effects. There are different kinds of force.
In this unit, we will study a kind of force i.e., friction. We will also learn about gravitational force, inertia and simple machines.
6.1 Friction
When a body is rubbed or dragged on the surface of another body, it produces an opposite force. This force stops or tries to stop the movement of these bodies and is called as friction. Friction slows down the motion and always works in the opposite direction of the motion of the objects.
Causes of Friction
Every surface has some pores in it. If we observe a very smooth surface with the help of microscope, we see many small pores in it.
It indicates that surfaces of all the objects have crests and troughs. When we drag an object onto another, these crests and troughs get entangled with each other. This creates resistance in the movement of the object. This resistance is called friction. The larger the pores the more rough will be the surface that causes more friction.
When we apply a force on a ball, it starts to roll. The force pushes the bat forward but the crests and troughs of the surfaces of ball and ground interlock with each other. As a result, the ball first slows down and then eventually stop due to friction.
Liquid and gases also produce friction. For example, when a swimmer swims, he faces the friction of water. Similarly, an aeroplane experiences air friction while flying in the air.
Friction also produces heat energy, For example, when we rub our hand they get warmer. This is due to the presence of friction between our hands.
6.2 Advantages and disadvantages of Frictions
Advantages of Friction
Friction helps us to walk on Earth. When we walk, there is a friction between our feet or shoes and the ground. That is why we do not slip and fall.
Friction keeps wheels of vehicles on the road. Without friction, we cannot stop a car by applying brakes.
We cannot write, if there is no friction between paper and pencil. Due to friction carbon particles of the pencil leave a mark on the paper.
Birds cannot fly if there is no air resistance.The reaction of the life pushing wind enables the birds to fly.
Friction enables us to hold different objects. For example, we can hold a spoon, pencil, book, box, etc. due to friction. On the other hand, it is difficult to hold an ice cube because it has less friction.
We can't nail wood or walls without fircation.it is friction which holds the nail.
Disadvantages of Friction
Although friction is very important in our daily life, but it also has some disadvantages.
The main disadvantage of friction is that it generates heat in different parts of the machines. thus, some useful energy is lost as thermal energy.
Friction also causes wear and tear in parts of machines.
Friction slows the speed of moving objects. The engines of Cars and machines use more fuel due to friction.
Friction also worns out the surfaces of tyres and soles of shoes.
Fiction also causes noise between machines parts.
6.3 Methods to Reduce Friction
Smooth have less friction than rough surfaces. Therefore, we can reduce friction either by the surfaces smooth or adding a layer of liquid to it. For this purpose, different methods are used. Some methods are discussed as follows:
i:-Polishing of surfaces:
Polishing the surfaces of the objects reduces their roughness and hence reduces friction.
ii:-Using lubricants:
Friction can be reduced by oiling and lubricating of the moving parts of machines. Proper lubrication of the sliding parts of the machine reduces friction.
iii:-Use of wheels or rollers:
The friction of a rolling object is always less than the friction of a sliding object. This is because the rolling objects (trolleys, carts etc.) have rollers or wheels which reduce friction and make our work easier.
iv:- Use of ball bearing:
There are small balls which allow the surfaces to move (roll) easily over one another. The friction is reduced by using ball bearings.
V:-Streamlining the bodies:
can be reduced by streamlining the bodies of the objects. The fronts of fast moving trains and aeroplanes are made long or wedge like shaped.
Similarly, jet rocket, car, and fish etc., reduce friction due to their point shapes. Such special shapes are called streamlined shapes.
