PHYC 110
Physical Science
Name _______________
Date ________________
Exercise 11: Waves and Sound
speed of sound vsound = 340 m/s
speed of light c = 3.0x108 m/s
1. A pendulum makes 12 complete swings in 8.0 seconds.
a. What is its frequency?
f = 12 cycles/8 seconds = 1.5 cycles/second or 1.5 Hz
b. What is its period?
f = 1/T
1.5 = 1/T, T = 0.67 s
2. A live APU track meet is being fed to DirecTV Los Angeles for local programming. If
the distance from the DirecTV broadcasting station to the satellite is 36x103 km, show
that the time delay to the viewer is about ¼ second, as the link goes up to the satellite and
down to the viewer. The signal travels at the speed of light. Neglect any delays with the
surface link from APU to DirecTV.
d = 33x103 km = 33x106 meters (convert to meters) (distance up)
total distance = 66x106 meters (goes up and down)
v = 3x108 m/s
v = d/t or t = d/v
t = 66x106/3.0x108 = 22x10–2 = 0.22 s.
3. With one violin playing, the sound level at a certain place is measured as 50 dB. If ten
violins play equally loudly, what will the sound level most likely be at this place?
Ten times louder. Lone = 105 above minimum threshold of hearing
New loudness L x 10 = 105 x 101 = 106 or 60 db.
4. On a recent physical education-sponsored adventure education program, students went
hiking at Devil's Head State Park. At one point, Jeremy let out a holler which reflected
off a nearby rocky cliff and was detected as an echo 1.80 seconds later. Determine the
distance to the rocky cliffs. Assume a speed of sound of 340 m/s.
d = vt
v = 340 m/s
t = 1.80
d = 612 meters, this is twice the distance to the cliff (to the cliff and back), so the cliff is
306 meters away.
5. Strong winds can apply a significant enough force to tall skyscrapers to set them into a
back-and-forth motion. The amplitudes of these motions are greater at the higher floors
and barely observable for the lower floors. It is said that one can even observe the
vibrational motion of the Sears Tower in Chicago on a windy day. As the Sears Tower
vibrates back and forth, it makes about 8.6 vibrations in 60 seconds. Determine the period
of vibration of the Sears Tower.
T = 60 seconds/8.6 vibrations = 7.0 seconds