roofner (bar784) – Practice Problems 2 Solutions – Weathers – (17104)
1
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printout
should
have
36
questions.
Multiplechoice questions may continue on
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before answering.
001
(part 1 of 3) 10.0 points
You dip your finger into a pan of water twice
each second, producing waves with crests that
are separated by 0
.
12 m.
Determine the frequency of these water
waves.
Correct answer: 2 Hz.
Explanation:
Let :
n
= 2 vibrations
and
Δ
t
= 1 s
.
f
=
number of vibrations
Δ
t
=
2
1 s
=
2 Hz
.
002
(part 2 of 3) 10.0 points
Determine the period of these water waves.
Correct answer: 0
.
5 s.
Explanation:
T
=
1
f
=
1
2 Hz
=
0
.
5 s
.
003
(part 3 of 3) 10.0 points
Determine the speed of these water waves.
Correct answer: 0
.
24 m
/
s.
Explanation:
Let :
λ
= 0
.
12 m
.
v
=
f λ
= (2 Hz)(0
.
12 m) =
0
.
24 m
/
s
.
keywords:
004
10.0 points
The distance between two successive crests of
a certain transverse wave is 1
.
26 m.
Eight
crests pass a given point along the direction
of travel every 10
.
5 s.
Calculate the wave speed.
Correct answer: 0
.
96 m
/
s.
Explanation:
Let :
λ
= 1
.
26 m
,
n
= 8 crests
,
and
Δ
t
= 10
.
5 s
.
v
=
f λ
=
n
Δ
t
λ
=
8
10
.
5 s
(1
.
26 m)
=
0
.
96 m
/
s
.
keywords:
005
10.0 points
Earthquakes produce two kinds of seismic
waves:
The
longitudinal
primary
waves
(called
P
waves) and the transverse
secondary
waves
(called
S
waves). Both S waves and P waves
travel through Earth’s crust and mantle, but
at different speeds; the P waves are always
faster than the S waves, but their exact speeds
depend on depth and location. For the pur
pose of this exercise, we assume the P wave’s
speed to be
v
P
= 9560 m
/
s while the S waves
travel at a slower speed of
v
S
= 4960 m
/
s.
Suppose a seismic station detects a P wave
and then Δ
t
= 93
.
3 s later detects an S wave.
How far away is the earthquake center?
Correct answer: 961
.
753 km.
Explanation:
Suppose the earthquake happens at time
t
= 0 at some distance
d
.
The P wave and
the S wave are both emitted at the same
time
t
= 0, but they arrive at different times,
respectively
t
P
=
d/v
P
and
t
S
=
d/v
S
. The
S wave is slower, so it arrives later than the
P wave, the time difference being
Δ
t
=
d
v
S
−
d
v
P
=
d
(
v
P
−
v
S
)
v
P
v
S
.
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roofner (bar784) – Practice Problems 2 Solutions – Weathers – (17104)
2
Consequently, given this time difference and
the two waves’ speeds
v
P
and
v
S
, we find the
earthquake center being
d
=
v
P
v
S
Δ
t
v
P
−
v
S
=
961
.
753 km
.
away from the seismic station.
keywords:
006
10.0 points
A wave is sent along a long spring by mov
ing the left end rapidly to the right and keep
ing it there. The figure shows the wave pulse
at QR  part RS of the long spring is as yet
undisturbed.
P
S
P
Q
R
S
Which of the graphs correctly shows the
relation between the original position
x
of
each point of the spring and the corresponding
displacement Δ
x
of that point?
(Displacements to the right are positive.)
1.
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 Fall '08
 Weathers
 mechanics, Frequency, Correct Answer

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