Waves and Sound

  13 Waves and Sound 161 13-1 Wave Motion Vocabulary Wave: Adisturbance in a mediumIn this chapter, you will be working with waves that are periodic or thatrepeat in a regular, rhythmic pattern wave speed  A wavelength BA frequency B or  v     f  The SI unit for wave speed is the meter per second (m/s) The speed of soundin air increases with air temperature For the following exercises, the speed of sound will be written as 3400 m/s All electromagnetic radiation includingradio waves and light waves travel at the speed of light, 300  10 8 m/sThe wavelength of a wave is the distance from one point on a wave to thenext identical point on the same wave, for example, from crest to crest, troughto trough, or condensation to condensation The symbol for wavelength is theGreek letter “lambda,”   The SI unit for wavelength is the meter (m), which is the same unit used forlength in earlier chaptersThe SI unit for frequency is the hertz (Hz) When talking about the broadcastfrequency of a radio station, frequencies of FM radio stations are given inmegahertz, or MHz, and frequencies of AM radio stations are given inkilohertz, or kHz1 MHz  1  10 6 Hzand1 kHz  1  10 3 Hz Solved Examples Example 1: Radio station WCRB in Boston broadcasts at a frequency of 995 MHz What isthe wavelength of the radio waves emitted by WCRB? Given: v  300  10 8 m/s  Unknown:    ?  f   995  10 6 Hz  Original equation: v     f   Solve:      302 m Therefore, the distance from one point on the wave to the next identical pointon the same wave is 302 m Example 2: In California, Clay is surfing on a wave that propels him toward the beachwith a speed of 50 m/s The wave crests are each 20 m apart a) What is thefrequency of the water wave? b) What is the period? a  Given: v  50 m/s  Unknown: f   ?    20 m  Original equation: v     f Solve: f     025 Hzb  Given: f   025 Hz  Unknown: T   ? Original equation: T   Solve: T     40 s One crest comes along every 40 s Practice Exercises Exercise 1: Harriet is told by her doctor that her heart rate is 700 beats per minute If Harriet’s average blood flow in the aorta during systole is 15  10  2 m/s,what is the wavelength of the waves of blood in Harriet’s aorta, created byher beating heart?Answer: Exercise 2: Dogs are able to hear much higher frequencies than humans are capable of detecting For this reason, dog whistles that are inaudible to the human earcan be heard easily by a dog If a dog whistle has a frequencyof 30  10 4 Hz, what is the wavelength of the sound emitted?Answer:    v /  f   (3400 m/s)/(30  10 4 Hz)  0011 m (70 beats/min)/(60 s/min)  117 Hz    v /  f   (15  10  2 m/s)/(117 Hz)  0013 m 1025 Hz1  f  1  f  50 m > s 20 mv l 300  10 8  m > s995  10 6  Hz v f  162Waves and Sound 0013 m0011 m  a)    v /  f   (3400 m/s)/(440 Hz)  0773 m184 m/0773 m  238 full wavelengths  b)  t  d / v  (184 m)/(3400 m/s)  0541 s v     f   (01 m)(4 Hz)  04 m/s Waves and Sound163 04 m/s238 full wavelengths0541 s Exercise 3: While flying to Tucson, Connie’s plane experiences turbulence that causes thecoffee in her cup to oscillate back and forth 4 times each second If the wavesof coffee have a wavelength of 01 m, what is the speed of a wave movingthrough the coffee?Answer: Exercise 4: At a country music festival in New Hampshire, the Oak Ridge Boys areplaying at the end of a crowded 184-m field when Ronny Fairchild hits a noteon the keyboard that has a frequency of 400 Hz a) How many fullwavelengths are there between the stage and the last row of the crowd?  b) How much delay is there between the time a note is played and the time itis heard in the last row?Answer: a Answer: b