Class 8 NCERT Science, Chapter 12 Friction - CBSE Notes and CBSE Guess Questions


CBSE Class 8, NCERT Science

Chapter 12, FRICTION

CBSE Notes - CBSE Guide - Solutions of CBSE Sample Questions

Keep in Mind
=> Friction is the force equal and opposite to the relative motion between the two surfaces in contact.
=> Friction has no relation with area of contact (exposed to the force of friction).
=> Sliding friction is less than the static friction.
=> Friction is a necessary evil.  
Definitions / Terms
Static Friction
Static Friction is the friction which is produced between two surfaces in contact with each other at the position of rest. This friction is more powerful than any other type of friction. It is most difficult to overcome the static friction because at this position, the irregularities are most effectively pressed against each other. This type of friction needs a lot of energy to overcome.
Sliding Friction
This type of friction arises when two objects in contact are in motion. It is lesser than Static friction. This friction is lesser because the irregularities of both surfaces do not get enough time to get locked with each other as the objects are already in motion.     
Rolling Friction
Rolling friction is the friction which comes in effect when two bodies are rolling on each other. Rolling movement of both objects don’t allow the locking of irregularities, thus it is the smallest force (friction) of all the three types.
It is the term used to describe the frictional force which is exerted by the fluids on the bodies moving in it. This friction needs a lot of energy to overcome so, bodies of such objects which move in fluids are streamlined to counter the friction offered by fluids.   
Solutions of CBSE Sample Questions
Question.1: Fill in the blanks:
1)      Friction always works in _____ direction of motion of the surface.
2)      Friction depends on the relative ______ between two surfaces.
3)      _______ is also produced with friction.
4)      Shape of the objects moving in the fluid is _____ to overcome friction exerted by the liquids.
5)      _______ are used in machines to protect their surfaces from wear due to friction.
6)      Soapy floor is slippery due to ______ friction.
7)      Ball bearings are used to convert static friction into _______ friction.
8)      Sliding friction is ______ than the static friction.
9)      Friction increases with the increase in _______.
10)   _____ type of friction is the most powerful among all types of frictions.
Question.2: What causes friction between two surfaces?
Question.3: In which direction does friction work?
Question.4: What is done to tires of the vehicles to reduce the friction? 
Question.5: What does friction produce?
Question.6: Name any machine where ball bearing is used to reduce friction.
Question.7: What are the factors of friction?
Question.8: Can we eliminate friction completely?
Question.9: What causes chalk to write on the black-board?
Question.10: How does the nature of surface effect friction?
Question.11: It is easy to move an object on a sliding surface (inclined surface) than on a plane surface. Why?
Question.12: Why is it difficult to move an object from its static position?
Question.13: Why friction is called a necessary evil ?
Question.14: Why do we need to decrease friction?
Question.15: Explain the different types of friction.

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