11- Force and Pressure | Part 2 | Science | NCERT | Class 8

Hello students, in this blog you can find important summary notes from the Chapter 11, Science , NCERT Book of Class 8. These notes can be used for preparation of various school Competitive Exams, Olympiads and also developing strong Fundamentals.

21. Muscular force : The force resulting due to the action of muscles is known as the muscular force.

22. Contact force : Since muscular force can be applied only when it is in contact with an object, it is also called a contact force.

23. Force of friction : The force responsible for changing the state of motion of objects is the force of friction.
– Force of friction between the surface of the ball and the ground that brings the moving ball to rest.
– Friction between water and the boat brings it to a stop once you stop rowing.

24. The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion.

25. The force of friction arises due to contact between surfaces, hence it is an example of a contact force.

26. Non-contact force : A magnet exerts a force on another magnet without being in contact with it. This force exerted by a magnet is an example of a non-contact force.
Example : The force exerted by a magnet on a piece of iron.

27. Charged body : A straw is said to have acquired electrostatic charge after it has been rubbed with a sheet of paper. Such a straw is an example of a charged body.

28. Electrostatic force : The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

29. Electrostatic force is a non-contact force.

30. Force of gravity : This is an attractive force.
Objects or things fall towards the earth because it pulls them. This force is called the force of gravity, or gravity.
Examples :
– Water flows towards the ground as soon as we open a tap.
– Water in rivers flows downward.

31. Gravitational force : Every object in the universe, whether small or large, exerts a force on every other object. This force is known as the gravitational force.

32. Pressure : The force acting on a unit area of a surface is called pressure.

Porters place a round piece of cloth on their heads, when they have to carry heavy loads :
∗ By doing this they increase the area of contact of the load with their head.
∗ So, the pressure on their head is reduced and they find it easier to carry the load.

The area of the pointed end of the nail is much smaller than that of its head, :
∗ the smaller the area, larger the pressure on a surface for the same force.
∗ the same force, therefore, produces a pressure sufficient to push the pointed end of the nail into the wooden plank.

33. Pressure Exerted by Liquids and Gases
– Pressure exerted by water at the bottom of the container depends on the height of its column
– A liquid exerts pressure on the walls of the container
– Liquids exert equal pressure at the same depth
– Gases, too, exert pressure on the walls of their container.

34. Atmospheric Pressure :
– There is air all around us. This envelop of air is known as the atmosphere.
– The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by this air is known
as atmospheric pressure.
– Atmospheric pressure is the force of gravity on air in a column of unit area.
– The pressure inside our bodies is also equal to the atmospheric pressure and balances the pressure from outside.

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To read Part 1: Click here

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