Page 115 - Physics - XI

2. The ball-bearings should be dropped gently.
3. The ball-bearings should fall along the central axis of glass cylinder.

4. The ball-bearings should be thoroughly coated with the given viscous liquid.
5. Diameter of the ball-bearings should be measured in two mutually perpendicular directions.
6. The liquid should be highly viscous.

Sources of Error
1. The ball-bearings may not be perfectly spherical.
2. The liquid may not have uniform density.

3. The observed velocity may not be constant.

VIVA VOCE

Q1. What is viscosity?
Ans. The property of fl uids in motion due to which the fl uids tend to oppose the relative motion between
the fl uid layers is called viscosity.
Q2. What is the dimensional formula of coeffi cient of viscosity?

2
Ans. We know η = FA/ MLTL 2
dv dx LT L 1
1
/
= [ML T ]
–1
–1
Q3. What is the cause of viscosity?
Ans. Viscosity is due to the cohesive forces between molecules of the liquid.

Q4. What is the coeffi cient of viscosity?
Ans. It is defi ned as the tangential force required to maintain a unit velocity gradient between two layers
each of the unit area.
Q5. Give the units of coeffi cient of viscosity.
Ans. In CGS system, its unit is poise and in S.I. system, its unit is pascal second.

Q6. Which is more viscous—glycerins or water?
Ans. Glycerine
Q7. State Stoke's law.
Ans. When a small sphere of radius r moving with a uniform velocity v in vertically downward direction
in a viscous medium of coeffi cient of viscosity η, it experiences an opposing force F which is given
by F = 6 πηrv.
This is called Stoke's law.

Q8. Defi ne terminal velocity.
Ans. The maximum constant velocity acquired by the body falling freely in the viscous medium is called
terminal velocity.
Q9. What is the eff ect of temperature on viscosity of liquids?
Ans. In general, the viscosity of liquid decreases with rise in temperature.

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