Friction (part-5)

FRICTION

Q.11. Define drag.
Ans. The frictional force exerted by fluids (gases and liquids) is called drag. 

Q.12. What are the factors on which frictional force on an object in a fluid depends?
Ans. The frictional force on an object in a fluid depends on its 
  • speed with respect to the fluid
  • shape of the object and
  • the nature of the fluid. 
Q.13. Explain why objects moving in fluids must have special shapes.
Ans. Fluids exert force of friction on objects in motion through them. So, when objects move through fluids, they have to overcome friction acting on them. In this process they lose energy. Efforts are, therefore, made to minimize friction.
As frictional force on an object in a fluid depends on the shape of the object. So, objects are given special shapes that are streamlined shape to minimize friction.

Q.14. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Ans. Seema will have to apply a larger force because frictional force increases with increase in the 'weight' of the body that moves with respect to a surface. So in the case of Seema, the weight of the box is higher and would have more friction that would oppose the motion.

Q.15. Give reasons:
(a)Parts of machines need to be oiled and greased regularly.
Ans. As friction wears out the materials and also produces unnecessary heat which is a waste of energy, this reduces the efficiency of machines. So, Parts of machines need to be oiled and greased regularly because oil and grease are lubricants which help to decrease the friction between the parts of machine.

(b) A pencil will write on paper but not on a glass.
Ans. We know that friction wears out the materials.
The paper is a rough surface having more irregularities which can provide enough friction between the pencil tip and paper to rub off the graphite of the pencil that stick to the paper.
On the other hand, glass is comparatively a very smoother surface than paper. It carries extremely less irregularities than paper. So, it can not provide enough friction between the pencil tip and paper to rub off the graphite of the pencil that can stick to the paper.
Hence, a pencil will write on paper but not on a glass.

(c) Climbing a greasy pole is very difficult.
Ans. Friction is must to have a grip on an object. Grease is a kind of lubricant which reduces friction by filling the grooves of the irregularities. So due to decrease in friction, the grip gets loosen and climbing the greasy pole becomes very difficult.

(d) The handles of motor cycle are covered with a rubber sheet with spikes.
Ans. Friction is must to have a grip on an object. The handles of motor cycles are covered with a rubber sheet with spike to increase friction, so that rider can hold the handle comfortable while riding. This helps rider to have his/ her grip maintained to the handle of the motorcycle and prevents rider from fatal accidents.

(e) Wheel is said to be the greatest invention of mankind.
Ans. The wheel experiences relatively less friction compared to flat objects and also provide a means to travel larger distance with the same force. It is always easier to roll than to slide a body over another. That is the reason it is convenient to pull the luggages fitted with rollers. With the help of wheels, one can transfer heavy load over larger distances which was seemingly impossible to carry earlier. So, Wheel is said to be the greatest invention of mankind. 

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