Problems
573
spacings of the first two frets. Calculate the distance be-
tween the last two frets.
24. The top string of a guitar has a fundamental frequency of
330 Hz when it is allowed to vibrate as a whole, along all of
its 64.0-cm length from the neck to the bridge. A fret is
provided for limiting vibration to just the lower two-thirds
of the string. (a) If the string is pressed down at this fret
and plucked, what is the new fundamental frequency?
(b) What If? The guitarist can play a “natural harmonic”
by gently touching the string at the location of this fret
and plucking the string at about one sixth of the way along
its length from the bridge. What frequency will be heard
then?
25. A string of length L, mass per unit length +, and tension T
is vibrating at its fundamental frequency. What effect will
the following have on the fundamental frequency? (a) The
length of the string is doubled, with all other factors held
constant. (b) The mass per unit length is doubled, with all
other factors held constant. (c) The tension is doubled,
with all other factors held constant.
26. A 60.000-cm guitar string under a tension of 50.000 N has
a mass per unit length of 0.100 00 g/cm. What is the high-
est resonant frequency that can be heard by a person capa-
ble of hearing frequencies up to 20 000 Hz?
A cello A-string vibrates in its first normal mode with a fre-
quency of 220 Hz. The vibrating segment is 70.0 cm long
and has a mass of 1.20 g. (a) Find the tension in the string.
(b) Determine the frequency of vibration when the string
vibrates in three segments.
28.
A violin string has a length of 0.350 m and is tuned to con-
cert G, with f
G
"
392 Hz. Where must the violinist place
her finger to play concert A, with f
A
"
440 Hz? If this posi-
tion is to remain correct to half the width of a finger (that
is, to within 0.600 cm), what is the maximum allowable
percentage change in the string tension?
29.
Review problem. A sphere of mass M is supported by a
string that passes over a light horizontal rod of length L
(Fig. P18.29). Given that the angle is / and that f repre-
sents the fundamental frequency of standing waves in the
portion of the string above the rod, determine the mass of
this portion of the string.
27.
30.
Review problem. A copper cylinder hangs at the bottom of
a steel wire of negligible mass. The top end of the wire is
fixed. When the wire is struck, it emits sound with a funda-
mental frequency of 300 Hz. If the copper cylinder is then
submerged in water so that half its volume is below the
water line, determine the new fundamental frequency.
31.
A standing-wave pattern is observed in a thin wire with a
length of 3.00 m. The equation of the wave is
y " (0.002 m) sin(&x) cos(100&t)
where x is in meters and t is in seconds. (a) How many
loops does this pattern exhibit? (b) What is the fundamen-
tal frequency of vibration of the wire? (c) What If? If the
original frequency is held constant and the tension in the
wire is increased by a factor of 9, how many loops are pres-
ent in the new pattern?
Section 18.4 Resonance
32. The chains suspending a child’s swing are 2.00 m long. At
what frequency should a big brother push to make the
child swing with largest amplitude?
33. An earthquake can produce a seiche in a lake, in which the
water sloshes back and forth from end to end with remark-
ably large amplitude and long period. Consider a seiche
produced in a rectangular farm pond, as in the cross-sec-
tional view of Figure P18.33. (The figure is not drawn to
scale.) Suppose that the pond is 9.15 m long and of uni-
form width and depth. You measure that a pulse produced
at one end reaches the other end in 2.50 s. (a) What is the
wave speed? (b) To produce the seiche, several people
stand on the bank at one end and paddle together with
snow shovels, moving them in simple harmonic motion.
What should be the frequency of this motion?
L
M
θ
Figure P18.29
Figure P18.33
34.
The Bay of Fundy, Nova Scotia, has the highest tides in the
world, as suggested in the photographs on page 452. As-
sume that in mid-ocean and at the mouth of the bay, the
Moon’s gravity gradient and the Earth’s rotation make the
water surface oscillate with an amplitude of a few centime-
ters and a period of 12 h 24 min. At the head of the bay,
the amplitude is several meters. Argue for or against the