Class 8 Science Chapter 9 Question Answers - Friction

Q1: Explain why friction is a necessary evil?
Ans: Friction is very important for us it helps. It allows us to walk comfortably,It allows us to grip and catch other objects,things do not move from their places because of the friction, Speeds of moving objects can be controlled due to friction only.
But apart from this friction is considered as an evil because it causes wear and tear in objects of our daily use including machines. It makes movement of heavy objects very difficult.We need to spend a lot of time and money for the maintenance of various machines in order to keep their friction low.
Q2: Force of friction increases when two surfaces are pressed harder? Justify the statement.

Ans: Friction is caused by interlocking of irregularities in the two surfaces, thus it is quite obvious that the Force of friction increases when two surfaces are pressed harder, this can be experienced by dragging a mat when no one is sitting on it, and when a person is sitting on it.
Q3: Explain increasing and reducing friction.

Ans: Soles of shoes are grooved so that a better grip is provided to the surface and thus can move safely and freely on floors. The tyres of vehicles are treaded to provide better grip with the ground. Powder is sprinkled on the carom board to reduce friction and increase efficiency. Friction can never be entirely eliminated, because no surface is perfectly smooth, there are always some irregularities present in it.

Q4: Explain the advantages and the disadvantages of friction.
                                                 Or 
Explain in details why friction is called necessary evil.
Ans. Advantages of friction:
(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.

Disadvantages:
(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.
Q5: 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.
Q6: 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.

Q7: 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.