12- Friction | Science | NCERT | Class 8

Hello students, in this blog you can find important summary notes from the Chapter 12, Science , NCERT Book of Class 8. These notes can be used for preparation of various school Competitive Exams, Olympiads and also developing strong Fundamentals.

1. Force of friction : It is a force acting on any body that opposes its motion.

2. The force of friction always opposes the applied force.

3. Spring balance is a device used for measuring the force acting on an object.

4. Friction is caused by the irregularities on the two surfaces in contact.

5. Irregularities on the two surfaces lock into one another. When we attempt to move any surface, we have to apply a force to overcome this interlocking.

6. Friction is caused by the interlocking of irregularities in the two surfaces.

7. The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction.

8. The force required to keep the object moving with the same speed is a measure of sliding friction.

9. The sliding friction is slightly smaller than the static friction.

10 . We find it somewhat easier to move the box already in motion than to get it started.

11. If an object started moving, it would never stop if there were no friction.

12. Friction can also produce heat.

13. When 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.

14. Lubricants : The substances which reduce friction are called lubricants.

15. Friction can never be entirely eliminated. No surface is perfectly smooth.

16. Rolling friction : When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.

17. Rolling reduces friction. It is always easier to roll than to slide a body over another, hence it is convenient to pull luggage fitted with rollers.

18. Since the rolling friction is smaller than the sliding friction, sliding is replaced in most machines by rolling by the use of ball bearings.
Examples: Use of ball bearings between hubs and the axles of ceiling fans and bicycles.

19. Fluid Friction :

Air is very light and thin, yet it exerts frictional force on objects moving through it.

Water and other liquids exert force of friction when objects move through them.

Fluids : In science, the common name of gases and liquids is fluids.

– Fluids exert force of friction on objects in motion through them.

Drag : The frictional force exerted by fluids is also called drag.

– The frictional force on an object in a fluid depends on :
∗ its speed with respect to the fluid
∗ on the shape of the object
∗ the nature of the fluid

– All vehicles are designed to have shapes which reduce fluid friction.



  1. Thank you sir,
    This notes are very useful for me.
    And after reading the notes I understand things easily thank you once again.

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