PHYS 201 TEST
#5 12/12/08 DR. HOLMES NAME
Do all six problems. All answers should be
in MKS units unless otherwise indicated. Show your work for partial credit.
1) A mass of 600 grams is suspended from a
spring which stretches the spring 9 cm from its original unstretched
position due to the weight.
a) What is the spring constant, k, of the
spring?
65 Nt/m
b) If this mass is stretched 2 cm
from the equilibrium position and then released, it will oscillate. What will
be the period, T, of its oscillation?
0.60 sec
c) ) If the amplitude is increased, that is, the spring is
stretched more than 2 cm from the equilibrium position and then released, will
the period of oscillation [increase, stay the same, or decrease]?
stay the same
d If a second identical spring is used in series with the first
spring so that the 600 gram mass stretches this “new” spring 18 cm (twice the
original 9 cm) from its original position
(assume amplitude remains at 2 cm
and mass remains at 600 kg), will the period of oscillation [increase, stay the
same, or decrease]?
increase
e) If the original spring is used (from
parts a&b), but the mass that is oscillating in increased from 600 grams to
1,200 grams, assuming the amplitude remains at 2 cm, will the period of
oscillation [increase, stay the same, or decrease]?
increase.
2) A certain piano string has a length of
0.84 meters and a mass density of 0.38 grams/meter. A tension of 19 Nt. is
applied to it.
a) What will be the speed of the wave on
this string?
233.6 m/s
b) What will be the fundamental frequency
when it is plucked?
133 Hz.
c) If a fundamental frequency of 240
cycles/sec is desired, what should the tension in the string be?
61.8 Nt.
d) If a different string of the same length
but a smaller mass density is used, and if the original tension of 19 Nt is
applied, will the fundamental frequency for this string be [higher, the same,
or lower] than for the string in part (b)?
higher.
3) a) What is the speed of sound in air that
is at a temperature of -20oF ?
313 m/s = 701 mph.
b) What is the wavelength of a 280 Hz sound
wave in this air?
1.12 m
c) What is the frequency of a 28 cm
wavelength sound wave in this air?
1117 Hz
d) If the temperature of this air were raised
and the frequency of the sound wave were kept the same, would the wavelength of
the sound wave [increase, stay the same, or decrease] ?
increase
e) If helium (molar mass of helium is 4
gm/mole, and helium is a monatomic gas, not a diatomic gas like N2
and O2) were used instead of air, and if the temperature and
wavelength of the sound were the same, would the frequency be [lower, the same,
or higher than] the frequency of this sound in air?
higher .
4) A source of sound (assume it is a point source)
emits a power of 372 nanoWatts (sound, not electrical). What is the intensity
at a distance of 1 meter from the source
(a) in Watts/m² ? 2.96 x 10-8
W/m2 (b) in db ? 44.7 dB
If the source is 8 meters away, what will
the intensity be
(c) in Watts/m² ? 4.62 x 10-10
W/m2 (d) in db ? 26.7 dB
If the power is doubled from 372 nanoWatts
to 744 nanoWatts, what will be the intensity 8 meter away now:
e) in Watts/m2 ? 9.24 x 10-10
W/m2 (f) in db ? 29.7 dB
5) For this problem, assume the speed of
sound in the air around the train and car is 340 m/s. All answers must be at least to the nearest
Hz. Do not round your answers more than
that.
A train
moving East at a speed of 24 m/s approaches a person in a stopped car.
a) If
the train emits a sound of frequency 4,000 Hz, what will the car observer measure
for the frequency? (Assume no wind.)
4,303.8
Hz
b) After the train passes the car and starts heading
away (still going East), what will the car observer measure for the frequency?
(Again assume no wind.)
3,736.3 Hz
c) The car now goes 32 m/s East and approaches
the rear of the moving train (which is still going 24 m/s East). If the train again blows its horn (4,000 Hz
with respect to the train), what will the person in the car measure for the
frequency of the horn? (Again assume no wind.)
4,087.9 Hz
d) ) If
there is an West wind of 15 m/s (from the West blowing toward the East) for
part c, how much will the frequency change from part c due to the effect of the
wind? [a plus difference means the frequency with the wind will be higher, a
minus means it will be lower] ?
+3.79 Hz .