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> To be answered by 2/3 after reading chap2.2 and 2.3
 
 
Q1. An ambulance truck is heading for a wall 90m away. The sound wave of the siren bounces off the wall and comes back to the driver. The siren produces sounds at the frequency of 15/s, the truck is moving at 45m/s. What frequency of sound does the driver hear?(Aside from the wave that comes straight from the siren.)
 
Q2. Take a glass bowl or cup and tap it. (gently! I wouldn't be too rough with mom's special cup!) Try a little harder. Note the difference of the sounds. Try filling some water in the cup and see what happens.
 
 

A1. I'm not going to use the usual doppler effect interperation for this because if I do, I will have to calculate twice. Once with the wall( It is supposed to be a listener.) and once with the driver. So, I'm going to make a different aprouch. At normal periods, the speed of sound is 340m/s. In this instance the truck's siren is moving at 45m/s in the same direction as the sound. More over, the 385m/s sound bounces off the wall and back to the fast aproaching truck. Since they are moving towards eachother, the driver can think of it as the sound traveling toward a standing truck at the speed of 430m/s. Speed equals frequency multiplied by wavelength. The wavelengh wouldn't change, but the wave would cover the distance faster, so the frequency would increase. 15/s multiplied by 430/340 equals about 18.97/s.

A2. Tapping harder would only increase the loudness of the noise. If you try to increase the frequency you could try tapping it a lot faster but I don't recommend it. Your arm would hurt too much, so you could fill water in the cup to give it a different frequency. The more water in the cup, the more slowly it will viberate, and so it will have a smaller frequency.