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# Long Answers - Friction, Science, Class 8 Class 8 Notes | EduRev

## Class 8 : Long Answers - Friction, Science, Class 8 Class 8 Notes | EduRev

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Or Explain in details why friction is called necessary evil.
Ans.
(i) We cannot write with a pen or pencil, if there were no friction.
(ii) We cannot write on the blackboard with a chalk, if there were no friction.
(iii) If an object started moving, it would never stop, if there were no friction.
(iv) We cannot walk on the road without friction.
(v) We cannot fix a nail on the wall or tie a knot without friction.

(i) It wears out the materials whether they are screws, ball bearings or soles of shoes.
(ii) It causes damage to the parts of machines.
(iii) Friction also produces heat. When a machine is operated, heat is generated that causes wastage of energy.
(iv) The tyres of cars, buses and trucks, etc., also worn out due to friction.
(v) Friction reduces the speed due to which more force is required.
In this way, we see that friction is harmful as well as useful so it is called necessary evil.

Q. 2. Write some methods used to reduce friction.
Ans.
Methods to reduce friction:
(i) By using fine powder on the surfaces like on carrom board.
(ii) By using oils in the tools or in machine parts.
(iii) By using grease between the moving parts of the machines.
(iv) By using graphite powder between moving parts of a machine.
(v) By using air cushion between the moving parts in some machines where oils or lubricants may not be advisable.

Q. 3. Explain various types of frictions.
Ans. There are following types of frictions:
(i) Static friction (ii) Sliding friction (iii) Rolling friction

(i) Static friction: The friction which is produced between two surfaces in contact with each other at the position of rest is called static friction. This type of friction is more powerful than others. It is not easy to overcome the static friction. This type of friction needs a lot of energy to overcome.

(ii) Sliding friction: The friction which arises when two objects in contact are in motion is called sliding friction. It is lesser than static friction but more than rolling friction. In this type of friction irregularities of surfaces do not get enough time lock with each other as objects are already in motion.

(iii) Rolling friction: The friction which arises when two bodies are rolling on each other is called rolling friction. Rolling movement of both objects do not allow the locking of irregularities, so it is the smallest friction out of three types of frictions.

Q. 4. What is fluid friction? Write the factors on which fluid friction depends.
Ans.
The gases and liquids are called fluids. The friction exerted by fluids on an object is called fluid friction. The fluid friction is also called drag. The factors on which fluid friction depends are as following:

(i) The fluid friction of an object depends on its speed with respect to fluid.

(ii) The fluid friction of an object also depends on the shape of the object.

(iii) The frictional force due to fluids also depends on the nature of the fluid.

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