Homework for October 14 - University of Minnesota



Homework for October 14

The whole homework is worth 8 points. (0.4 for every right answer, 0.2 for every attempted answer).You can just turn in a sheet with the answers.

1) The total distance traveled by a point on a string one complete cycle as a sinusoidal wave passes by is ______ times the amplitude of the wave.

a) half

b) one

c) two

d) three

e) four

2) Two different sound waves are traveling in air. Wave 2 has twice the frequency of wave 1. What is the ratio of their wavelengths (1/(2?

a) 0.5

b) 0.25

c) 2:

[pic]

d) 4

3) You are standing at one end of a long boat pier. There is an explosion at the other end of the pier. You hear three distinct sound waves from this explosion, arising from sound traveling through the water, through the air and through a steel rail along the pier. In which order did they arrive?

a) Air, water, steel

b) Air, steel, water

c) Steel, air, water

d) Steel, water, air: fastest arrives first

e) Water, steel, air

f) Water, air, steel

4) A tuning fork emits a 340 Hz tone. How will the sound wave change as it travels into a colder region of air?

a) the frequency of the transmitted wave will be lower but the wavelength will be longer

b) the frequency of the transmitted wave will remain the same but the wavelength will be shorter

The frequency is impressed on the wave by the source and will not change during transmission. The speed of sound is lower in cold air. Since v=(f, and f can not change, ( will be decreased.

c) the frequency of the transmitted wave will be greater but the wavelength will be longer

5) You hear a thunder clap 7.5 s after you see the lightning flash. How far away was the lightning strike?

a) 45 m

b) 7000 m

c) 2500 m [pic]

d) 3.7 km

e) 0.02 m

6) The law of superposition deals with two waves passing a point and the adding of their

a) Displacements

b) wavelengths

c) frequencies

d) all of the preceding

e) only two of the above are correct

7) Constructive interference is a result of :

a) the positive amplitude of one wave adding to the negative amplitude of a second wave

b) the positive amplitude of one wave adding to the positive amplitude of a second wave

c) when the distance from each wave to the observer is the same.

d) A node in one wave adding to an anti-node in a second wave.

e) two of the above answers are true

8) A wave of amplitude A interferes with another wave of the same kind whose frequency is different but whose amplitude is also A. The resulting wave

a) has an amplitude 2A

b) varies in amplitude between 0 and A

c) varies in amplitude between 0 and 2A (you get beats)

d) varies in amplitude between A and 2A

e) none of the above is correct

9) For 440 Hz sound in air at 20oC, which of the following path differences would result in constructive interference?

[pic]

a) 11 cm

b) 39 cm

c) 117 cm

d) 156 cm (that’s twice the wave length)

10) Two stereo speakers are separated by a distance of 4.30 m. You are 2.80 m directly in front of one speaker, as shown. What is the lowest frequency of the sound that would result in constructive interference?

a) 663 Hz

b) 442 Hz

c) 221 Hz

d) 294 HZ

e) 147 Hz (the pathlength difference is 2.3 m. A wave

With that wavelength has a frequency of 147 Hz.

11) In order to produce beats, the two sound waves should have:

a) the same amplitude

b) slightly different amplitudes

c) the same frequency

d) slightly different frequencies

e) two of the above must be true to get beats

12) Two guitar strings are plucked simultaneously and twelve beats are heard in three seconds. One string is vibrating at 370 Hz, and the other is vibrating at

a) 358 Hz

b) 368 Hz

c) 373 Hz

d) 370.3 Hz

Sorry, I messed up. The beats have a frequency of 4 Hertz, and possible answers would be 374 Hz and 366 Hz.

13) Two tuning forks, one with frequency 440 Hz, the other with frequency 438 HZ, are sounded simultaneously. How often does the loudness of the resulting sound rise and fall each second?

a) 2 times per second

b) 4 times per second

c) 778 times per second

d) 1 time per second

14) You are listening to a mosquito in your room. If the pitch of the sound decreases, this means that:

a) the mosquito is moving towards you

b) the mosquito is moving away from you

c) you are moving towards the mosquito

d) you are moving away from the mosquito

e) either a or c

f) either b or d

15) The speed of sound in a certain situation is determined by

a) the frequency of the wave

b) the wavelength of the wave

c) the medium in which the wave propagates

d) two of the above

e) all three of the above

16) If the person could see a molecule of air in front of a speaker, he would see it:

a) move vertically up and down

b) move horizontally side to side

c) gradually move away from the speaker

d) gradually move towards the speaker

17) Constructive interference is a result of :

a) the positive amplitude of one wave adding to the negative amplitude of a second wave

b) the positive amplitude of one wave adding to the positive amplitude of a second wave

c) when the distance from each wave to the observer is the same.

d) A node in one wave adding to an anti-node in a second wave.

e) two of the above answers are true

18) An explosion at the other end of a long steel pipe is heard 1.87 s later in air (v=340 m/s) than through the metal of the pipe(v=5100 m/s). How long is the pipe?

a) 28 m

b) 8900 m

c) 9500 m

d) 635 m

e) 681 m

[pic]

19) A student is watching the waves rolling out from a 60-m long wave pool. There is a wave arriving every 3 s. At any given time, four wave crests are visible along the length of the pool. Determine the speed of the waves.

T=3 s, f=0.33Hz

(=60m/4=15m

V=(f=15m/3s=5m/s

-----------------------

2.80 m

4.30 m

(((2.82+4.32)=5.1 m

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