Page 65 - Physics - XI

Q23. Why tyres are made of rubber, but not steel or iron?
Ans. The coeffi cient of friction between rubber and road is smaller than iron or steel and road.

Q24. On the basis of your observations, fi nd the relation between limiting friction and the mass of
sliding body. (NCERT)
Ans. The limiting frictional force can be expressed as,
F = µ R
L L
Where µ is the coeffi cient of limiting friction and R is the normal reaction on the sliding body.
L
Now,
R = mg

Where m is the mass of the sliding body. So,
F = µ R = mgR
L L
This is the required relation between limiting friction and the mass of sliding body.

Q25. Why do we not choose a spherical body to study the limiting friction between the two surfaces?
(NCERT)
Ans. We know that friction is a force which resists the relative motion between the two surfaces in
contact. So, in the case of a sphere which rolls without slipping over a horizontal plane will suff er
no friction.
So, at every instant, there is just one point of contact between the body and the plane and this point
has no motion related to the plane. So, this is the reason for not choosing a spherical body to study
limiting friction between the two surfaces.

Q26. Why should the horizontal surfaces be clean and dry? (NCERT)
Ans. The presence of dust particles between the wooden block and the horizontal plane surface can aff ect
the friction which in turn leads to errors in the observations.

Q27. Why should the portion of thread between the moving body and pulley be horizontal? (NCERT)
Ans. The portion of the thread between the moving body and pulley should be horizontal. If it would not
be as that, then only a component of tension in the thread would act as the force to move the block.

Q28. Why is it essential in this experiment to ensure that the surface on which the block moves
should be horizontal? (NCERT)
Ans. Because on a horizontal surface, the weight of the block is equal to the normal reaction.

Q29. Comment on the statement: “The friction between two surfaces can never be zero.” (NCERT)
Ans. Friction can be reduced by polishing surfaces or using large amount of lubricants such as oil, water,
or grease, but we cannot reduce friction to zero. Friction exists at even atomic levels too. We cannot
completely eliminate friction.

Q30. In this experiment, usually unpolished surfaces are preferred, why? (NCERT)
Ans. Friction depends upon the nature and the material of the surfaces in contact. So, if the surface is
unpolished, then it will cause more friction.

Q31. What do you understand by self-adjusting nature of force of friction? (NCERT)
Ans. Static friction opposes the movement of the body. Its value is equal to the applied force till the body
remains in a static position, i.e. it is self-adjustable.

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