Friction | Science 8th | Chapter 14

Science: Jammu and Kashmir | jandkncert |

Friction | Science 8th | Chapter 14 |

1. Answer the following questions:

(i) Why are the worn-out tyres, discarded?

Ans. When the tyres are, worn out, their groves disappear due to the constant use; this reduces the friction between tyres and road. The reduced friction increased the speed of vehicles, resulting in the chances of accidents, that is why worn-out tyres are discarded.

(ii) Why do carom coins move faster on the carom board when dusted with talcum powder?

Ans. When carom board is, dusted with talcum powder, the friction between carom coins and the surface of carom board reduces, thus the carom coins move faster on carom board.

(iii) Why is the surface of the conveyor belt, made rough?

Ans. Conveyor belts are used, in the mechanical machines at railway stations or airports to carry passenger’s luggage. The belts are made rough to increase the friction between belt and pulley so that to slow down the speed of the machine. Due to slow speed, no luggage slides over another and this makes passengers easy to collect his luggage.

(iv) Why is the sewing machine often, oiled?

Ans. Constant use of the sewing machine makes the surfaces of its moving parts irregular that increases the friction resulting in its slow speed and more energy wastage. A less viscous oil acts as a lubricant in the machine that makes the surfaces smooth reducing the friction, less energy usage but more and faster work. That is why sewing machine is often oiled.

(v) Why do new automobile tyres have deep grooves?

Ans. The tyres with deep grooves are the kind of irregular surfaces, since the friction is, caused due to the interlocking irregularities of two surfaces, the friction between the tyres and the road increases; this slows down the speed of the vehicle, resulting in the fewer chances of accidents. It also helps apply the brakes safely.

(vi) Why does a ball rolling on the ground slowdown?

Ans. When the ball rolls on the ground a part of the ball and the ground surface gets continuously deformed thereby increasing the rolling friction between the two. The increase in the friction reduces the speed of the ball, since a limiting force is, applied to the ball it slows down after moving a small distance.

(vii) Why are the boats and aeroplane, given special shape?

Ans. The boats and aeroplane is given a special shape so that friction between water (in case of the boat) or air (in the case of an aeroplane) is reduced to a large extent. This also offers the least resistance to the air and water and help them to move or fly with a lot of speed.

(viii) Why do meteors burn on entering into the atmosphere?

Ans. The meteors enter the earth at a very high speed. At such speeds, the friction due to air is extremely high. Due to this high friction, the temperature of the meteor rises to such a high degree that they catch fire and burn down while entering into the atmosphere.

(ix) Why do painters use sandpapers in polishing doors?

Ans. Sand papers are, used to make the surfaces smooth. In other words, it helps a painter held the paint while polishing, with the help of sliding friction.

(x) Why is it easier to tie a knot with cotton string as compared to silk string?

Ans. The cotton string has a rough surface while a silk string has a smooth surface. While we knot a tie, the friction between two end surfaces of cotton string increases, this makes it easy to knot a tie with cotton string as compared to silk string, which has the reduced friction between two smooth end surfaces.

2. Fill in the blank spaces by choosing words from the list given below:

List: movement, sliding, streamlined, static, opposite

(i) Friction always acts in the direction opposite to the direction of applied force.

(ii) Static friction is a self-adjusting force.

(iii) Friction is very useful as it helps in the movement of the bodies.

(iv) The sliding friction is 10 times the rolling friction.

(v) The boats and aeroplanes are streamlined so as to reduce fluid friction.

3. Statements given below are incorrect. Write the correct statements.

(i) Sliding friction is slightly more than the limiting friction.  True

(ii) The conveyor belts are, made rough, in order to decrease friction.        False

(iii) The friction between two surfaces decreases with the increase in the weight of as a body.        False

(iv) The friction offered by the wheels is called sliding friction.        True

(v) The friction increases with the increases in the area of contact at the two surfaces.     True

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