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Force for Class 8 CBSE

Let’s start with very important and interesting topic Force for Class 8 CBSE students. When we start reading any chapter first we should read introduction of that chapter. Try to understand what we learn form that?

INTRODUCTION

In every time in a day we use force to perform our daily activity. Like we use force to

  • Open a window
  • Pick up the books
  • Brush our teeth
  • Squeeze out toothpaste from a tube
  • Turn on a tap and so on.

    Force Class 8 CBSE

Even we stick on earth because the earth is exerting a force on us all time. It is pulling all of us and all things on earth towards it by gravitational force. Not only that in fact we uses force for every single action of ours.

What we learn from this topic regarding Force for Class 8 CBSE students?

From this topic Force Class 8 CBSE, we learn lots of new thing. We make a table for that.

Sl noTopic
1           Definition of force           
2Unit of force
3Effects of force
4Types of force
5Mass and weight
6Resultant of force

 

Definition of force

We can define force as a “force is an action of one body on the other”. As per syllabus of Force for Class 8 CBSE, we can define as simple as “force is a push or a pull on other object“.

We should take a note on that. When more than one forces are applied on an object in the same direction, we get the net force by adding the magnitude of those forces. And when more than one forces are applied on an object in the opposite direction the net force is the difference in the magnitude of forces and acts in the direction of larger force.

UNIT OF FORCE

(i) The SI unit of force is called Newton and its symbol is N (kg × m/sec2)

(ii) C.G.S unit of force is dynes (gm cm/s2)

1 Newton = 105 dynes,

1 N = kg – m/s2

= 1(1000 gm) (100 cm/s2)

= 105 gm–cm/s2

So that 1 N = 105 dyne.

EFFECT OF FORCE

 

1 . FORCE CAN MOVE A BODY LYING AT REST

If we apply force on an object lying at rest, it can start moving.

Example:

(a) Kicking a stationary football.

(b) Lifting a book kept on a table top.

(c) Hitting a stationary ball with a bat.

2 . FORCE CAN STOP A MOVING BODY

If we apply force on moving object, it can stop moving.

Example :

(a) A goalkeeper stopping a football moving towards the goal post.

(b) A moving bicycle coming to rest on applying brakes. Here the frictional force between the brake lining of the brake shoe and the bicycle wheel rim, acts to stop the bicycle.

3 . FORCE CAN CHANGE THE SPEED OF A MOVING BODY

If we apply force on moving object, speed of that object either increase or decrease.

Force Class 8 CBSE

Example.

To decrease speed, force is applied in a direction which is opposite to the motion of the body. If we apply force in the direction of motion of the body, it tends to increase the speed of the body. For example, if your friend is riding a bicycle and you push the bicycle in the same direction. The speed of bicycle will increase. On the other hand if you pull the bicycle, i.e., you apply force against the direction of motion, the speed of bicycle decreases.

4 . FORCE CAN CHANGE THE DIRECTION OF A MOVING BODY

If we apply force on moving object, it can change the direction of moving object.

Example :

(a) A carom counter changes its direction after a collision.

(b) When a batsman hits a ball, he changes the direction of the ball.

(c) When a stone is rotated in a circular path, the direction of motion of the stone change continuously. The force acting on the stone towards the center of the circular path is responsible for changing the direction of the stone.

(d) A football player hitting a ball coming towards him, towards the goal post.

5 . FORCE CAN CHANGE THE SHAPE AND SIZE OF AN OBJECT

If we apply force on an object, it can change the shape and size of that object.

Example :

(a) When we squeeze a toothpaste tube, it gets flattened.

(b) When we stretch a rubber band, its shape and size changes.

(c) Force exerted to stretch a spring Force

(d) On stretching a spring, its length changes.

6 . FORCE CAN MAKE A BODY ROTATE

If we apply force on an object, it can make that object rotate.

Example :

(a) When electric current is passed through a motor of a ceiling fan, forces are produced in the dynamo (or motor) which makes the fan to rotate.

(b) For closing a door you apply a push.

TYPES OF FORCE

As per syllabus of Force for Class 8 CBSE, We can categories the force mainly two types.

1.Contact force

2.Non-contact force

(A) CONTACT FORCE :-

When a force involves direct contact of two bodies we call the force a contact force. When a force involves direct contact of two bodies we call the force a contact force.

(i) FRICTIONAL FORCE :–

The force which acts to reduce relative motion between the surfaces of contact is called the frictional force.

Suppose a block is moving on a horizontal surface with a velocity υ. The darkened line at the bottom of the block is the surface of contact of the block with floor. We notice that this surface of contact is moving towards right with respect to the horizontal surface. To reducing this relative motion, frictional force acts. Frictional force is exerted by the floor on the block in a direction opposite to velocity.

(ii) NORMAL FORCE :

The force acting on a body perpendicular to the surface of contact is called a normal force.

(iii) MUSCULAR FORCE :

The force resulting due to the action of muscles is known as the muscular force. Animals also make use of muscular force to carry out their physical activities and other tasks. Animals like bullocks, horses, donkey and camels are used to perform various tasks for us. In performing these tasks they use muscular force. Since muscular force can be applied only when it is in contact with an object. This force is also known as Biological force.

(iv) MECHANICAL FORCE :

The force exerted by a machine is called mechanical force. Machines do not produce force by themselves. In order to produce force they need energy from other sources. As a example mechanical force produced by a car engine, and mechanical force produced by the turbines in a hydroelectric power station.

(B) NON-CONTACT FORCE

The force which a body applies on another body when the two bodies are not in contact with each other is called non-contact force.

(i) GRAVITATIONAL FORCE :

The force of attraction which one body exerts on the other because of their masses is called gravitational force. Gravitational force is the weakest force in nature and still this force is responsible for the revolution of earth around the sun.

(ii) ELECTROSTATIC FORCE :

Force which results due to the repulsion of similar charges or attraction of opposite charges is called electrostatic force. If we rub a plastic object like a pen or a comb with hair and bring it close to tiny bits of paper, the bits of paper get attracted to the plastic object. This is due to electric force. Tiny particles of dust and smoke can also be attracted by electrostatic force. This method is used in air purifiers and in factories to purify air in chimneys, before letting it escape into the atmosphere.

(iii) MAGNETIC FORCE :

This is the force exerted by magnets on each other and on some magnetic metals like iron and nickel. Since magnets attract iron, magnets are used to separate waste iron object form garbage dumps so that they can be recycled. Magnetic force and electrostatic force are inter-related and are together called electro-magnetic force.

 

RESULTANT FORCE

As per syllabus of Force for Class 8 CBSE, if a single force acting on a body produces the same acceleration as produced by a number of forces, then single force can called the resultant of these individual forces. The resultant force may also called the net force.

Ex. Suppose an almirah is being pushes by two persons A and B. Let force applied by A be F1 = 2N and that by B be F2 = 3N. These two forces together will result in some displacement of the almirah in a given time. In another situation, a person C pushing the same almirah kept initially in the position as that of the previous situation. The person C is applying a force F = 5N in the same direction as that of A and B.

BALANCED FORCES

When the force acting on a body cancel out the effects of each other in such a way that the resulting force is zero, then these forces can said to be balanced

Ex. Let us consider a rigid almirah kept at rest on a horizontal surface. Let two forces F1 and F2 act simultaneously on the almirah from opposite directions. If the two forces are equal, the effects produced by one force get cancelled by the effect produced by the other. The net force or the resultant force is then zero. The almirah continues to remain at rest. When a numbers of forces acting on a body do not cause any change in its state of rest, or of uniform motion is a straight line then the force can said to be balanced forces.

UNBALANCED FORCE

If number of forces acting on a body produce an acceleration in the body, then the forces acting are called unbalanced.

Suppose F1 = 5 newton and F2 = 3 Newton. In this case a resultant force is acting on the almirah towards the direction of F.

Let see how you learn of Force for Class 8 CBSE.

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Subham
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Subham

Subham

Hi myself Subham Dutta, having 15+ years experience in filed of teaching. I love to teach and try to build foundation of students. Try to make them imagine what they learn.

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