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(II) A police car sounding a siren with a frequency of 1280 $\mathrm{Hz}$ is traveling at 120.0 $\mathrm{km} / \mathrm{h}$ . (a) What frequencies does an observer standing next to the road hear as the car approaches and as it recedes? (b) What frequencies are heard in a car traveling at 90.0 $\mathrm{km} / \mathrm{h}$ in the opposite direction before and after passing the police car? (c) The police car passes a car traveling in the same direction at 80.0 $\mathrm{km} / \mathrm{h}$ .What two frequencies are heard in this car?

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Physics 101 Mechanics

Chapter 16

Sound

Periodic Motion

Mechanical Waves

Sound and Hearing

Simon Fraser University

University of Sheffield

McMaster University

Lectures

08:15

In physics, sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Humans can only hear sound waves as distinct pitches when the frequency lies between about 20 Hz and 20 kHz. Sound above 20 kHz is known as ultrasound and has different physical properties from sound below 20 kHz. Sound waves below 20 Hz are called infrasound. Different species have different hearing ranges. In terms of frequency, the range of ultrasound, infrasound and other upper limits is called the ultrasound.

04:49

In physics, a traveling wave is a wave that propogates without a constant shape, but rather one that changes shape as it moves. In other words, its shape changes as a function of time.

10:07

A police car sounding a si…

03:23

03:54

03:13

04:35

A police car is traveling …

03:11

A stationary police car em…

02:58

04:48

04:40

A driver travels northboun…

here. We have a kiss in the first case in part A Ah, the observer is especially. So Observer is especially, uh, stationery, stationery. And, um, a source is approaching. So we have a source. Right? So our source is approaching. Um, So I approaching approaching before approaching our right shortcut. Then we can write a frequency. So sorry. I already wrote a frequency of four source moving tours. Source moving toward MT. Eye right. Can be written as a frequency. Oh, we're, um one minus speed off the source S O s. Alright. Right here for the speed of source. Or, um, this speed off the sound, which is yes. Then frequency we have here is a 12. 80. If he's a 12 80 or one minus force is having a speed 1 20 kilometers per hour, which, if we can work into a meter per second, then it becomes 33.33 33 Um, as usual, their speed off a sound is a three 43 3 43 meter per second. So, um, by dividing these two numbers than the frequency off the source moving towards we get here is 14. 20 14 20 Hertz, um, frequency for a source which is moving and approaching, so approaching. Hey, I'm right here. Well, Britain is f or one plus the speed of the source source or the speed off the sound that is equal to buy the plugging the value off 12. 80 and 33.3 tree for the speed off the force or ah, the sound speed. Then the answer you get here is 11 Ah, 70 hearts. This is a frequency for source moving approaching, um, in part b for the case when both observer and source are moving So observer and source of moving and we can write the frequency, uh, for approaching is equal to frequency times. Um, speed of the sound, yes, the speed of the observer or or the speed of the sound minus the speed off the source. And this gives us, um, we can put the value, um, by converting a 90 kilometers per hour into um meter per second, which is 25 meters per second. Replied the values here, which is so f is a 12 80 as we rode previously. Speed off the sound is 3 43 pass. Ah, they were lost. He or the speed off observer is a 25 meter per second and working from kill me to put our and for a second Oh, we're, um the speed off the sound 3 43 Subtracting the source speed, which is 33 going to three. Three tree meter per second. And by doing your mats, the non. So we get here is a 15 20 hertz, 20 hertz. This is for a frequency approaching now. Frequency for receding. Um, let me write it here. Frequency for receiving weaken Right here. So right, R e for a sitting, we can write. This formula is f into of yes, minus the speed off observer or mmm, speed off the sound plus the speed off the source. So as our c I would like for source and, um as we did previously by putting the values for ah, speed of the sound and speed of the observer and speed of the source. The answer we get here for receding frequency is 1080 hurts A de hurts. So this was a part B in part C. Both source and observer are moving. So we have a situation in which source An observer I'm in. Then we can write the frequency off. Police car Police car approaching Fortune will be equal to our frequency times. Um, speed off the sound minus speed of the observer will be divided by the speed off the sound The minus speed of the source. We can, uh, pluck the values as we did previously. Um, for the speed off observer only here we have is frequency, as usual, is a 12 12 80 12 80 a zoo jewel. The speed of sound is a 3 43 minus the speed off observer which is a 22 point to tool to to hear, or the speed of the sound, which is again three for three minus the speed off the source 33 point 33 33 Then doing a simple mats deriding these numbers than the police car Frequency approaching will be 13 30 herds, herds the inn next case where we have a police car which is receding. The police car receding ari leased car receding can be written as f into, uh, speed off the sont plus the speed off observer or the speed of the sound. Plus the speed off the source. We can substitute the values as we did above. Then the answer we get for the police car receding frequency is 12 40 hurts 12 40 hurts hearts end off the problem. Thank you for watching.

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