When are beats generated in two interfering waves?

Aray diagram for a refracted light day is shown. what does the dashed line represent

What if? if the temperature near the cliff suddenly falls to 0°c, reducing the speed of sound to 331 m/s, what would the initial speed of the rock have to be (in m/s) for the soccer player to hear the sound of the splash 2.90 s after kicking the rock?

Aboxcar traveling at 12 m/s approaches a string of 5 identical boxcars sitting stationary on the track. the moving boxcar collides and links with the stationary cars and they all move off together along the track. what is the final speed of the cars immediately after the collision? (you may take the mass of each boxcar to be 18,537 kg.)