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**1) A 50-cm-long wire with a mass of 1.0 g and a tension of 440 N passes across the open top of a vertical tube partially filled with water. The wire, which is fixed at both ends, is bowed at the center so as to vibrate at its fundamental frequency and generate a sound wave. The water level in the tube is slowly lowered until the sound wave from the wire sets up a standing wave in the tube. It is then lowered another 36.0 cm until the next standing wave is detected. Use this information to determine the speed of sound in air.**

I got that the linear density = 1x10^-3/0.5 = 2x10^-3 kg/m

And so the speed on the string is sqrt(tension/linear density) = 469.04m/s

Now how can I proceed? I looked through all examples in the textbook, but I still can't figure out what to do...

**2) Two radio antennas are 100 m apart along a north-south line. They broadcast identical radio waves at a frequency of 3.0 MHz. Your job is to monitor the signal strength with a handheld receiver. To get to your first measuring point, you walk 800 m east from the midpoint between the antennas, then 600 m north. What is the phase difference between the waves at this point?**

I found that the path difference is 79.964 m, and lambda = speed/frequency = 3x10^8/3x10^6 = 100m

Now what can I do? Can someone give me a hint?

Any help is appreciated! Thanks!