Science Class 8 Friction : A necessary evil

                                       Friction : A necessary evil           

Q1. When the cutting edge of a knife is put against a fast-rotating stone to sharpen it, sparks are seen to fly. Explain the reason.

Ans. Friction generates heat. When the cutting edge of a knife comes in contact with a fast rotating stone, heat is produced due to friction between the grinding stone and the cutting edge of the knife. As the friction is very large, a large amount of heat is produced and sparks are seen flying while sharpening the cutting edge of the knife.

Q2. We have two identical metal sheets.One of them is rubbed with sand paper and other with ordinary paper. The one rubbed with sand paper shines more than the other. Give reason.

Ans. The friction between the sand paper and the metal sheet is very large as compared to that between the ordinary paper and metal sheet. The sand paper is able to remove the outer dull layer from the metal sheet more effectively. Therefore, the metal sheet rubbed with sand paper shines more. The metal sheet rubbed with ordinary paper does not shine much.

Q3. While travelling on a rickshaw, you might have experienced that if the seat cover is very smooth, you tend to slip when brakes are applied suddenly. Explain.

Ans. Friction in a contact force and is due to roughness of the surfaces. If the seat cover of a rickshaw is very smooth, then the friction between our body and the seat would be very small. We would tend to slip when the brakes of the rickshaw are suddenly applied.

Q4. You might have noticed that when used for a long time, slippers with rubber soles become slippery. Explain the reason.

Ans. The rubber soles of new slippers are treaded to increase the friction. When slipperss have been used for long time, the treads wear off and the surface of the soles become smooth. As a result, the friction between the rubber soles and the floor decreases and the slippers become slippery.

Explaination : Friction depends on the nature of surfaces in contact and for a given pair of surfaces, it depends on the roughness/smoothness of the two surfaces.

Q5. Is there a force of friction between the wheels of a moving train and iron rails? If yes, name the type of friction. If an air cushion can be introduced between the wheel and the rail, what effect will it have on the friction?

Ans. Yes, the force of friction between the wheels of a moving train and iron rails is rolling friction. If an air cushion is introduced between the wheels and the rails, the friction between the two surfaces will decrease.

Q6. Cartilage is present in the joints of our body, which helps in their smooth movement. With advancing age, this cartilage wears off. How would this affect the movement of joints?

Ans . The wearing off of cartilage will increase the friction between the joints of our body. As a result, the movement of joints will become difficult with advancing age. It may also lead to joint pains.

Q7. The handle of a cricket bat or a badminton racket is usually rough. Explain the reason.

Ans. The handle of a cricket bad or a badminton racket is usually made rough to make a better grip. Roughness of handle increases the friction between the handle of the bat/racket and the hands of the player.

Explanation : Friction depends on the nature of surfaces in contact and for a given pair of surfaces, it depends on the roughness/smoothness of the two surfaces. If the handle of the bat/racket is smooth, it may slip out of the hands of the player.

Q8 Explain why the surface of mortar and pestle ( silbatta) used for grinding is etched again after prolonged use?

Ans. The surface of mortar and pestle used for grinding is etched again after a prolonged. Use to increase the friction at the time of grinding. Etching makes the mortar and pestle more effective for grinding again.

Q9. A marble is allowed to roll down an inclined plane from a fixed height, At the foot of the inclined plane, it moves on a horizontal surface (a)covered with silk cloth(b)covered with a layer of sand and (c)covered with a glass sheet. On which surface will the marble move the shortest distance. Give reason for your answer.

Ans. The distance covered by the marble will depend upon the nature of the surface on which it is moving.The marble will cover the shortest distance on the surface covered with a layer of sand. The layer of sand has a rough surface and offers the maximum friction for the motion of the marble. Silk cloth and glass sheet are comparatively and will offer lesser friction.