6) What length of pendulum is needed to create a period of 11.7 seconds?

8). A boat is riding the waves. If 5.0 waves go past the boat in a 4.2 minute period, what is the period of one wave (in seconds)?

9) If the speed of sound is 335 m/s and a hiker hears an echo from a cliff 4.38 seconds after he shouts, how far away is the cliff?

11). A car is traveling 25.6 m/s towards a stationary sound (whistle) that has a frequency of 980 Hz. If the speed of sound is 340 m/s, what is the apparent frequency of the whistle as heard by the driver of the car?