17.2 Properties of Mechanical Waves
17-2 C. notes
•
Frequency and period
– Make sure you explain how they are related
– How can you increase the frequency of a wave.
• Wavelength
• Wave speed
– Make sure you explain how the variables are related.
• Amplitude
– Make sure you explain how you can increase the amplitude
of a wave.
17.2 Properties of Mechanical Waves
A surfer needs to
understand the
properties of
waves.
17.2 Properties of Mechanical Waves
II. Properties of Mechanical Waves
A. Frequency and Period
1. Frequency- is the number of complete cycles of a wave in a
given time created by periodic motion.
a. A wave’s frequency equals the frequency of the
vibrating source producing the wave.
17.2 Properties of Mechanical Waves
2. Periodic motion- any motion that repeats at regular time
intervals.
a. Period- The time required for one cycle.
b. the frequency of the wave is measured in cycles per
second, or hertz (Hz).
17.2 Properties of Mechanical Waves
Frequency and Period
A. A wave vibrating at
one cycle per
second has a
frequency of 1.0 Hz.
B. A wave vibrating at
two cycles per
second has a
frequency of 2.0 Hz.
Frequency = 1.0 hertz
One cycle per second
Frequency = 2.0 hertz
Two cycles per second
17.2 Properties of Mechanical Waves
B. Wavelength- the distance between a point on one wave and
the same point on the next cycle of the wave.
1. Increasing the frequency of a wave decreases its
wavelength.
2. For a transverse wave, wavelength is measured between
adjacent crests or between adjacent troughs.
3. For a longitudinal wave, wavelength is the distance
between adjacent compressions or rarefactions.
17.2 Properties of Mechanical Waves
Wavelength
Wavelength can be measured from any point
on a wave to the same point on the next
cycle of the wave.
Long wavelength
Rest
position
Short wavelength
17.2 Properties of Mechanical Waves
C. Wave Speed
1. Formula: Speed = Wavelength X Frequency or
Speed = Wavelength  Wave’s period of vibration
a. wavelength is in meters (m)
b. frequency is in hertz (Hz)
c. the units for speed are meters per second. (m/s)
2. If you assume that waves are traveling at a constant speed,
then wavelength is inversely proportional to frequency.
17.2 Properties of Mechanical Waves
3. Examples:
a. One end of a rope is vibrated to produce a wave
with a wavelength of 0.25 meter. The frequency
of the wave is 3.0 hertz. What is the speed of the
wave?
s= ?
s  0.25  3.0
s
w= .25 m
s  0.75m / s
f= 3.0 Hz
w f
17.2 Properties of Mechanical Waves
b. What is the speed of a wave in a spring if it has a
wavelength of 10 cm and a period of 0.2 s?
w
s
p
w= 10cm
s= ?
p= 0.2s
10
s
0 .2
s  50cm / s
17.2 Properties of Mechanical Waves
4. The speed of a wave can change if it enters a new medium,
or if variables such as pressure and temperature change.
5. For many kinds of waves, the speed of the waves is
roughly constant for a range of different frequencies.
6. The wave with the lower frequency has a longer
wavelength.
17.2 Properties of Mechanical Waves
D. Amplitude – a wave’s maximum displacement of the medium
from its rest position.
1. The more energy a wave has, the greater is its amplitude.
2. The amplitude of a transverse wave is the distance from
the rest position to a crest or a trough.
a. It takes more energy to produce a wave with higher
crests and deeper troughs.
17.2 Properties of Mechanical Waves
Amplitude
The more energy a wave has, the greater is
its amplitude.
High
amplitude
Rest
position
Low
amplitude
17.2 Properties of Mechanical Waves
3. The amplitude of a longitudinal wave is the maximum
displacement of a point from its rest position.
a. The more energy the wave has, the more the medium
will be compressed or displaced.
17.2 Properties of Mechanical Waves
Assessment Questions
1. While wading in shallow waters, six waves crash
into your legs in a 24-second span. What is the
frequency of the waves?
a.
b.
c.
d.
4 Hz
18 Hz
0.25 Hz
2 Hz
17.2 Properties of Mechanical Waves
Assessment Questions
1. While wading in shallow waters, six waves crash
into your legs in a 24-second span. What is the
frequency of the waves?
a.
b.
c.
d.
4 Hz
18 Hz
0.25 Hz
2 Hz
ANS: C
17.2 Properties of Mechanical Waves
Assessment Questions
2. What is the speed of an earthquake wave if it has
a wavelength of 2.3 km and a frequency of 3 Hz?
a.
b.
c.
d.
6.9 km
5.3 km
6.0 km
1.3 km
17.2 Properties of Mechanical Waves
Assessment Questions
2. What is the speed of an earthquake wave if it has
a wavelength of 2.3 km and a frequency of 3 Hz?
a.
b.
c.
d.
6.9 km
5.3 km
6.0 km
1.3 km
ANS: B
17.2 Properties of Mechanical Waves
Assessment Questions
3. Which wave property increases as the energy of a
wave increases?
a.
b.
c.
d.
period
frequency
wavelength
amplitude
17.2 Properties of Mechanical Waves
Assessment Questions
3. Which wave property increases as the energy of a
wave increases?
a.
b.
c.
d.
period
frequency
wavelength
amplitude
ANS: D