# Solved > 21) Light from a laser forms a 1.31:2007510 …

21) Light from a laser forms a 1.31 mm diameter spot on a wall. If the light intensity in the spot is 3.69 x 104 W/m2, what is the power output of the laser?

A) 49.7 mW

B) 30.8 mW

C) 41.7 mW

D) 57.7 mW

22) The intensity of sunlight impinging on Earth is about 1.4 kW/m2. What is the power emitted by the Sun in the spectrum of frequencies? (Earth-Sun distance = 1.5 × 108 km, Earth radius = 6.4 × 103 km.)

A) 4.0 × 1026 W

B) 3.2 × 1022 W

C) 7.2 × 1014 W

D) 7.6 × 108 W

23) For a sound wave in air of frequency 3500 Hz and average intensity 1 W/m2 (pain threshold) what is the maximum change in pressure (pressure amplitude) of the wave?

A) 28.6 N/m2

B) 816 N/m2

C) 20.2 N/m2

D) 3.18 × 10-6 N/m2

24) A glass window is installed in the window space and the intensity level of the sound entering through the window is reduced from 72 dB to 47 dB. The factor, by which the acoustic power entering the chamber is reduced by the glass window, is closest to:

A) 3.2 × 10-3

B) 2.2 × 10-2

C) 5.6 × 10-2

D) 1.0 × 10-1

E) 1.5 × 10-1

25) What is the intensity level in decibels of a sound whose intensity is 10-7 W/m2?

A) 20 dB

B) 30 dB

C) 40 dB

D) 50 dB

E) 60 dB

Situation 15.1

A train is approaching a signal tower at a speed of 40 m/s. The train engineer sounds the 1000 Hz whistle and a switchman in the tower responds by sounding the 1200 Hz siren. The air is still and the speed of sound is 340 m/s.

26) In Situation 15.1, the frequency of the train whistle tone, heard by the switchman, in SI units, is given by:

A) 1000

B) 300/340 × 1000

C) 340/300 × 1000

D) 340/380 × 1000

E) 380/340 × 1000

27) In Situation 15.1, the frequency of the tower siren tone, heard by the engineer, in SI units, is closest to:

A) 1200

B) 300/340 × 1200

C) 340/300 × 1200

D) 340/380 × 1200

E) 380/340 × 1200

28) Find the wavelength of a train whistle heard by a fixed observer as the train moves toward the observer with a velocity of 50.0 m/s. A wind blows at 5.0 m/s from the observer to the train. The whistle has a natural frequency of 500.0 Hz.

A) 0.55 m

B) 0.77 m

C) 0.75 m

D) 0.57 m

29) The sound from a truck moving at 17.00 m/s reflects from a cliff and is heard by an observer on the truck. What is the change in frequency for a wave with initial wavelength γ = 0.1500 m?

A) 245 Hz

B) 2281 Hz

C) 2066 Hz

D) 2400 Hz

30) The siren of an ambulance wails at 1395 Hz when the ambulance is stationary. What frequency will you hear after this ambulance passes you while traveling at 33.60 m/s? The speed of sound under the prevailing conditions is 343.0 m/s.

A) 1271 Hz

B) 1547 Hz

C) 1325 Hz

D) 1465 Hz

31) The howler monkey is the loudest land animal and can be heard up to a distance of 2.5 km. Assume the acoustic output of a howler to be uniform in all directions. The acoustic power emitted by the howler, in mW, is closest to:

A) 0.079

B) 0.026

C) 0.26

D) 0.79

E) 2.6

32) The howler monkey is the loudest land animal and can be heard up to a distance of 9.8 km. Assume the acoustic output of a howler to be uniform in all directions. The distance at which the intensity level of a howler’s call is 25 dB, in SI units, is closest to:

A) 550

B) 660

C) 830

D) 940

E) 1100

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