21.

Two wires of same material and length are stretched by the same force. Their masses are in the ratio 4 : 3. The ratio of their elongation will be

• 16 : 9

• 9 : 16

• 4 : 3

• 3 : 4

Surface tension of a detergent solution is 2.8 x 10^-2 Nm^-1. The work done in blowing a bubble of diameter 1 cm will be nearly

• 1.24 x 10^-4 J

• 1.76 × 10^-5 J

• 1.8 × 10^-3 J

• 2.4 x 10^-4 J

A particle executes SHM of amplitude 4 cm and time period 2 s. Then, the time taken by it to move from positive extreme position to half the amplitude is

• $\frac{1}{3} \mathrm{s}$

• $\frac{1}{2} \mathrm{s}$

• $\frac{1}{4} \mathrm{s}$

• 2 s

An organ pipe open at one end is vibrating in second harmonic and is in resonance with another pipe open at both the ends and vibrating in second overtone. The ratio of length of two pipes is

• $\frac{1}{2}$

• $\frac{1}{4}$

• $\frac{3}{4}$

• $\frac{3}{2}$

A wire suspended vertically from one of its ends is stretched by attaching a weight 100 N to the lower end. The wire stretches by 1mm. The elastic energy stored in the wire is

• 0.02 J

• 0.05 J

• 0.1 J

• 0.2 J

A circular metal plate of radius 5 cm rests on a layer of centre oil 2mm thick whose coefficient of viscosity is 15.5 dyne cm^-2 s. The value of shearing stress when a horizontal force move the plate with a speed of 5 cm/s will be

• 387.5 dyne cm^-2

• 328 dyne cm^-2

• 250 dyne cm^-2

• 428 dyne cm^-2

The surface tension and vapour pressure of water at 20°C is 7.28x10^-2 Nm^-1 and 2.33 x 10³ Pa respectively. The radius of the smallest spherical droplet which can form without evaporating is

• 6.25 x 10^-5 m

• 6.25 x 10^-3 m

• 7.45 x 10^-3 m

• 8.9 x 10^-4 m

During adiabatic expansion, the volume of air increases by 4%. The percentage decrease in the pressure will be

• 3.2 %

• 5.6 %

• 1.5 %

• 4.8 %

A simple pendulum of length l₁ has a time period of 2 s and another pendulum of length l₂ has a time period of 1 s. Then, the time period of pendulum having length l₁ − l₂ will be

• 2 s

• 1.2 s

• 3 s

• $\sqrt{3}$ s

Two sound waves with wavelengths 4 m and 5 m respectively each propagating in a gas with velocity 340 m/s. The number of beats per second will be

• 12 Hz

• 15 Hz

• 17 Hz

• 11 Hz