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PH202 Recitation Week 09
Problem Set
Winter 2015
Ryan Scheirer
Email: scheirer@onid.orst.edu
My Website: http://people.oregonstate.edu/~scheirer/PH202_REC.html
Problem 01
Figure 1.a shows the displacement of particles in a string as a function of position x at time t = 0s. Figure
1.b shows the displacement of the particle at x=0 as a function of time.
(a) Determine the wavelength, period, wavenumber, frequency and amplitude.
(b) Write the equation for the displacement of this traveling wave as a function of position and time.
Problem 02
Autofocus features on cameras can be classified into two categories, active and passive. Passive autofocusing
utilizes software to determine how to adjust the lenses in the camera, most smart phones use this technique.
While active autofocusing determine the distance to the object using very high frequency (ultrasonic) sound
waves, or infrared laser light. In 2014 the first smart phone LG G3 was released with the capability to
use active autofocuing with infrared light. (a) Assume standard atmospheric conditions, what is the total
time it takes for an active sound pulse camera to emit a sound and receive it if the object of interest is 10
meters away? (b) How long would it take for a light pulse to travel the same distance?
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Problem 03
You decide to drop a stone off a cliff of unknown height. The splash of the object hitting the water below
is heard 3.5 seconds after you dropped the stone. How high is the cliff?
Problem 04
While playing an intense game of Robot Roller-Derby Disco Dodgeball, I realized that the sound intensity
level was quite high, around 90 dB. If the area of my eardrum is about 5.0 × 10−5 m2 , (a) how much energy
is absorbed by the eardrum per second? (b) At this rate, how many hours of RRDDD would I have to play
for my eardrum to receive a total energy of 1.0 J?
Problem 05
Two sound sources each emit power uniformly in all directions. There are no reflections. Both sources
are located on the x axis, one at the origin and the other at x= 186 m. The source at the origin emits
four times more power than the other source. (a) Find the two positions along the x-axis where the two
sounds are equal in intensity. (b) The source located at the origin emits 400 W, what are the sound level
intensities of the two locations found in part a?
Problem 06
In the figure below, the power power of source 1 is 10 W. What is the sound level intensity at the origin?
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