Transcript Do Now 8/28/12 - North Hills Preparatory

March 24 – 27, 2015
What is a Wave?
Watch the two waves.
What do you
see?
How do the
particles move?
The particles
oscillate - they
are NOT carried
across the screen
by the page.
KEY POINT:
Waves carry
ENERGY not
MATTER
Transverse wave
Longitudinal wave
What is a Wave?
Definition:
A wave is a
disturbance that
carries energy
through matter
or space without
transferring
matter.
The energy carried
by a wave can be
substantial – think
of earthquakes,
tsunamis, or the
heat and light of
the sun.
Transverse wave
Longitudinal wave
Transverse wave
The particles of the medium oscillate perpendicular to
the direction of energy transfer/propagation of the
wave.
Examples:
• earthquake secondary waves,
• waves on a stringed musical instrument
• waves on the rope,
• EM waves: light, radio waves, microwaves…
Longitudinal wave
The particles of the medium oscillate parallel to the
direction of energy transfer/propagation of the
wave.
Creates areas of increased and decreased density
• Areas of high density / pressure = compression
• Areas of low density / pressure = rarefaction
Examples
• Sound waves
• Earthquake p waves
• Compression in a spring
Wave characteristics
Wavelength (λ)


The length of one complete wave cycle of a wave
measured in meters
Phase
• Two points on a wave that occur in the same position within the
wave cycle – that is, they are one or more whole wavelengths
apart -- are said to in phase.
• Points in a wave medium can be anywhere from 0° to 360° out of
phase with each other.
• A trough and a crest are 180° out of phase with each other.
Wave characteristics
Amplitude, A
● the maximum displacement of a particle from its equilibrium
position.
● measured in m
● the greater the amplitude, the greater the energy of the wave
● if a wave doesn’t lose energy, then its amplitude remains
constant; if the wave does lose energy then the amplitude
decreases over time (damping)
Wave characteristics
Period, T
● is the time taken for one complete wave to pass any given point.
● measured in seconds
Frequency, f
● is the number of waves passing any given point per second.
● measured in cycles per seconds or Hz
f=
1
T
T=
1
f
Wave characteristics
Wave speed, v
● The speed at which wavefronts pass a stationary observer.
● Depends only on the medium through which a wave travels!
Waves travel faster through
material that is stiffer and
through material that is less
dense.
Speed of sound in:
air: 343 m/s
helium: 1005 m/s
water: 1500 m/s
bone: 3000 m/s
steel rod: 5000 m/s
glass: 4500 m/s
Mechanical vs. EM waves
Mechanical Waves
● Only propagate (transmit energy) through a medium
● Make the particles of the medium oscillate at frequency of the
wave
Examples: waves on a string, sound waves, earthquakes, etc.,
EM (electromagnetic) waves
● Can propagate through a vacuum –or- through a medium
● Do not cause the particles of the medium to oscillate
● Made up of changing electric and magnetic fields
● always occur as transverse waves
● In a vacuum, EM waves travel at the speed of light
c ≈ 3 x 108 m/s
(EM waves travel more slowly through a medium)
Wave equation
A wave generator was used to generate waves of different
frequencies in a rope. Two different tensions of rope where
used. The wave speed and wavelength were measured.
Use the data in the table to answer the following questions
a) Which variable(s) affected wave speed?
b) How are wavelength, frequency, and speed related?
a) Only tension (changing the
stiffness of the medium)
affects speed.
b) λ = (v/f)
where,
λ = wavelength, m
v = wave speed, m/s
f = frequency, Hertz (Hz)
Check your Understanding
Show:
1) A arrives first
2) B arrives first
3) Same time
4) Impossible to determine
Wave speed depends only
on the medium!
Check your Understanding
c) Wave speed depends only on
the medium!
Check your Understanding
2
seconds!
You do
A sound wave has a frequency of 192 Hz and travels the
length of a football field, 91.4 m, in 0.271 s.
a)
What is the speed of the wave?
b)
What is the wavelength of the wave?
c)
What is the period of the wave?
d)
If the frequency were changed to 442 Hz, what would
be the new wavelength and period?
a)v = x/t = 337 m/s
b)λ = v/f = 1.76 m
c)T = 1/f = 0.00521 s
d)λ = v/f = 0.762 m; T = 1/f = 0.00226 s
You do
A sound wave produced by a clock chime is heard
515 m away 1.5 s later.
(a)What is the speed of sound in the air there?
(b) The sound wave has a frequency of 436 Hz. What is
the period of the wave?
(c) What is the wave's wavelength?
You do
A hiker shouts toward a vertical cliff 465 m away. The echo is
heard 2.75 s later.
(a) What is the speed of sound in air there?
(b) The wavelength of the sound is 0.75 m. What is the
frequency of the wave?
(c) What is its period?