S E C T I O N 18 . 3 • Standing Waves in a String Fixed at Both Ends
557
Example 18.5 Changing String Vibration with Water
Interactive
One end of a horizontal string is attached to a vibrating
blade and the other end passes over a pulley as in Figure
18.13a. A sphere of mass 2.00 kg hangs on the end of the
string. The string is vibrating in its second harmonic. A con-
tainer of water is raised under the sphere so that the sphere
is completely submerged. After this is done, the string vi-
brates in its fifth harmonic, as shown in Figure 18.13b. What
is the radius of the sphere?
Solution To conceptualize the problem, imagine what hap-
pens when the sphere is immersed in the water. The buoy-
ant force acts upward on the sphere, reducing the tension in
the string. The change in tension causes a change in the
speed of waves on the string, which in turn causes a change
in the wavelength. This altered wavelength results in the
string vibrating in its fifth normal mode rather than the sec-
ond. We categorize the problem as one in which we will
need to combine our understanding of Newton’s second
law, buoyant forces, and standing waves on strings. We begin
to analyze the problem by studying Figure 18.13a. Newton’s
second law applied to the sphere tells us that the tension in
the string is equal to the weight of the sphere:
&F " T
1
#
mg " 0
T
1
"
mg " (2.00 kg)(9.80 m/s
2
) " 19.6 N
where the subscript 1 is used to indicate initial variables
before we immerse the sphere in water. Once the sphere is
immersed in water, the tension in the string decreases to
T
2
. Applying Newton’s second law to the sphere again in this
situation, we have
T
2
!
B # mg " 0
(1)
B " mg # T
2
The desired quantity, the radius of the sphere, will appear in
the expression for the buoyant force B. Before proceeding
in this direction, however, we must evaluate T
2
. We do this
from the standing wave information. We write the equation
for the frequency of a standing wave on a string (Equation
18.8) twice, once before we immerse the sphere and once
after, and divide the equations:
where the frequency f is the same in both cases, because it
is determined by the vibrating blade. In addition, the linear
mass density + and the length L of the vibrating portion of
the string are the same in both cases. Solving for T
2
, we
have
Substituting this into Equation (1), we can evaluate the
buoyant force on the sphere:
B " mg # T
2
"
19.6 N # 3.14 N " 16.5 N
Finally, expressing the buoyant force (Eq. 14.5) in terms
of the radius of the sphere, we solve for the radius:
B " ,
water
gV
sphere
"
,
water
g ( &r
3
)
To finalize this problem, note that only certain radii of the
sphere will result in the string vibrating in a normal mode.
This is because the speed of waves on the string must be
changed to a value such that the length of the string is an in-
teger multiple of half wavelengths. This is a feature of the
quantization that we introduced earlier in this chapter—the
sphere radii that cause the string to vibrate in a normal
mode are quantized.
7.38 cm
"
7.38 - 10
#
2
m "
r "
√
3
3B
4&,
water
g
"
√
3
3(16.5 N)
4&(1 000 kg/m
3
)(9.80 m/s
2
)
4
3
T
2
"
!
n
1
n
2
"
2
T
1
"
!
2
5
"
2
(19.6
N) " 3.14
N
f "
n
1
2L
√
T
1
+
f "
n
2
2L
√
T
2
+
9:
1 "
n
1
n
2
√
T
1
T
2
You can adjust the mass at the Interactive Worked Example link at http://www.pse6.com.
(b)
(a)
Figure 18.13 (Example 18.5) When the sphere hangs in air,
the string vibrates in its second harmonic. When the sphere is
immersed in water, the string vibrates in its fifth harmonic.