Page 64 - Physics - XI

Q10. Does μ depend upon the area of two surfaces in contact?
L
Ans. No, it is independent of the area of two surfaces in contact.

Q11. Why is pulley kept frictionless?
Ans. It is done to keep the weight of pan and weights in it to be fully eff ective. If there is friction in pulley,
it will reduce its eff ective value.

Q12. What is static friction?
Ans. The opposing force which comes into play when one body tends to move over the surface of another
body is called static friction.

Q13. Which one is greater — dynamic or static friction?
Ans. Static friction is slightly greater than dynamic friction.
Q14. Defi ne the angle of friction.
Ans. The angle which the resultant of the normal reaction and force of limiting friction makes with the
normal reaction is called angle of friction.

Q15. What is the angle of repose?
Ans. It is the minimum angle of inclination of a plane with the horizontal such that a body placed on the
plane just begins to slide down.

Q16. What is the shape of graph between limiting friction and normal reaction?
Ans. It is a straight line which gives that limiting friction is directly proportional to the normal reaction.
Q17. How can we reduce friction?
Ans. (i) By using lubricants such as oil, grease, etc.

(ii) By using ball-bearings.

Q18. How does a body start moving over the surface of another body when the force of friction is
always there to oppose it?
Ans. The force of friction has the maximum value called limiting friction when the applied force acting
on the body is greater than the limiting friction the motion starts.

Q19. Which one is greater — sliding or rolling friction?
Ans. Rolling friction is always less than sliding friction.

Q20. Why is friction called as a necessary evil?
Ans. Due to the presence of friction in the moving parts of the machine, the effi ciency of the machine is
considerably reduced. So, it is an evil. But in many cases, it is a big necessity. Without friction, we
cannot walk, we cannot use brakes of automobiles. We cannot write and stitch the clothes without
friction. Friction is very much needed in our day-to-day life. Hence, friction is called as a necessary
evil.

Q21. Why do we slip on a muddy road?
Ans. On a muddy road, a thin layer of water is formed between the feet and the road. This thin layer
decreases the friction between road and feet. The decrease in friction makes us slip.

Q22. Why wheels are made circular?
Ans. To minimise the rolling friction.

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