Page 116 - Physics - XI

Q10. Why are the ball-bearings put in the glycerine before making them fall through glycerine?
Ans. This is done to avoid the formation of air bubbles around the ball-bearings.

Q11. Why do bubble rises up in water?
Ans. The terminal velocity v is given as
2r ( g )
2
v =

for air bubble (ρ) < σ (water)
then v becomes negative, hence, air bubble rises up due to lesser density than water.

Q12. What is the eff ect of temperature on the coeffi cient of viscosity of liquid? (NCERT)
Ans. The coeffi cient of viscosity decreases with increase in temperature because the forces of cohesion
between the liquid molecules decreases with increase in temperature.
Q13. Do all the raindrops, strike the ground with the same velocity irrespective of their size? (NCERT)
Ans. We know that the terminal velocity is directly proportional to the square of radius, but as per the
question, the velocity is independent of the size of the raindrop. Therefore, all the raindrops strike
the ground with the same velocity irrespective of their size.

Q14. Is Stokeʼs law applicable to the body of shapes other than spherical? (NCERT)
Ans. According to Stokeʼs law, viscous force F = 6 πηrv is only applicable for a spherical body. So, we
can say that Stoke's law is only applicable to the body of a spherical shape.

COMPETENCY-BASED QUESTIONS
COMPETENCY-BASED QUESTIONS

Multiple Choice Questions
Q1. A rectangular fi lm of liquid is extended from (4 cm × 2 cm) to (5 cm × 4 cm). If the work done
is 3 × 10 J, the value of surface tension of the liquid is
–4
a. 0.2 Nm b. 8.0 Nm c. 0.125 Nm d. 0.250 Nm –1
–1
–1
–1
Q2. A soap bubble, having radius of 1 mm, is blown from a detergent solution having a surface
tension of 2.5 × 10 N/m. The pressure inside the bubble equals at a point z below the free
–2
0
surface of water in a container. Taking g = 10 m/s , density of water = 10 kg/m , the value of z is
2
3
3
0
a. 1 cm b. 10 cm c. 100 cm d. 0.5 cm
Q3. The bulk modulus of a spherical object is ̒Bʼ. If it is subjected to uniform pressure 'p', the
fractional decrease in radius is
a. p b. B c. 3 p d. p
B 3 p B 3 B
Q4. A small sphere of radius 'r' falls from rest in a viscous liquid. As a result, heat is produced due

to viscous force. The rate of production of heat when the sphere attains its terminal velocity is
proportional to
a. r b. r c. r d. r 4
5
3
2

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