A train approaches a stationary observer, the velocity of the train being 1/20 of the velocity of sound. A sharp blast is blown with the whistle of the engine at equal intervals of a second. The interval between the successive blasts as heard by the observer?

• 1/20 sec

• 1/20 min

• 19/20 sec

• 19/20 min

Which of the following equations represents a wave travelling along y-axis?

• y = A sin (x - ωt)

• x = A sin(ky - ωt)

• y = A sin(ky) cosωt

• y = A cos(ky)sinωt

A closed organ pipe and an open organ pipe are tuned to the same fundamental frequency. What is the ratio of their lengths?

• 1 : 2

• 2 : 1

• 2 : 3

• 4 : 3

In a sinusoidal wave, the time required for a particular point to move from maximum displacement to zero displacement is 0.17 sec. The frequency of the wave is :

• 1.47 Hz

• 2.94 Hz

• 0.73 Hz

• 0.36 Hz

When sound waves travel from air to water, which one of the following remains constant?

• Time period

• Frequency

• Velocity

• Wavelength

An organ pipe, opened at both ends, has fundamental frequency of 30 Hz. If one of its ends is closed, then fundamental frequency will be :

• 15 Hz

• 30 Hz

• 45 Hz

• 60 Hz

237.

A stationary police car sounds a siren with a frequency of 990 Hz. If the speed of sound is 30 m/s, an observer, driving towards the car with a speed of 33 m/s, will hear a frequency of

• 891 Hz

• 900 Hz

• 1089 Hz

• 1100 Hz

A wave travelling along positive x-axis is given by y=A sin (ωt -kx). If it is reflected from rigid boundary such that 80% amplitude is reflected then equation of reflected wave is

• y = A sin(ωt - kx)

• y = - 0.8 A sin(ωt + kx)

• y = 0.8 A sin(ωt + kx)

• y=A sin(ωt + 0.8 kx)

Standing waves are formed on a string when interference occurs between two waves  having

• the same amplitude travelling in the same direction with no phase difference between them

• the same amplitude, travelling in the opposite direction with no phase difference between them

• different amplitudes travelling in the same direction

• different amplitudes travelling in the opposite direction

The length of two open organ pipes are l and (l+ Δl) respectively. Neglecting end corrections the beats frequency between them will be approximately

• $\frac{\mathrm{v}}{2\mathrm{l}}$

• $\frac{\mathrm{v}}{4\mathrm{l}}$

• $\frac{\mathrm{v}∆\mathrm{l}}{2{\mathrm{l}}^2}$

• $\frac{\mathrm{v}∆\mathrm{l}}{\mathrm{l}}$