CBSE Class 8 Science Force And Pressure Notes Set B

CBSE Class 8 Science Force and Pressure Chapter Notes. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.

FORCE & PRESSURE

INTRODUCTION
We use force all the time. We use force to open a door, to pick up the school bag, to brush our teeth, to squeeze out toothpaste from a tube, to turn on a tap and so on. Even the earth is exerting a force on us all time. It is pulling all of us and all things on earth towards it.In fact we use force for every single action of ours! You have studied about force in previous classes. Do you remember what a force is ?

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NOTE :
• Force has a sense of direction also. It means that we can apply the same amount of force in different direction, producing different effects.
• Without source force cannot be possible.

UNIT OF FORCE
(i) The SI unit (in standard international system) 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

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(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
Example :

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(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
Ex. 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

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4. FORCE CAN CHANGE THE DIRECTION OF A MOVING BODY
Example :
(a) A carrom 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 centre of the circular path is responsible for changing the direction ofthe stone.
(d) A football player hitting a ball coming towards him, towards the goal post.

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