Transcript WAVES Chapter Twenty-Three: Waves 23.1 Harmonic Motion 23.2 Properties of Waves 23.3 Wave Motion.

WAVES
Chapter Twenty-Three: Waves
23.1 Harmonic Motion
23.2 Properties of Waves
23.3 Wave Motion
Chapter 23.1 Learning Goals
Identify examples of simple
oscillators.
Compare and contrast harmonic
motion with linear and curved motion.
Apply a rule to determine the
frequency and period of an oscillator.
Investigation 23A
Harmonic Motion
Key Question:
How do we describe the
back and forth motion
of a pendulum?
23.1 Harmonic motion
A. Linear motion
gets us from one
place to another.
B. Harmonic motion
is motion that
repeats over and
over.
23.1 Harmonic motion
A pendulum is a device that swings
back and force.
A cycle is one unit of harmonic motion.
23.1 Oscillators
An oscillator is a
physical system that
has repeating cycles
or harmonic motion.
Systems that oscillate
move back and forth
around a center or
equilibrium position.
23.1 Oscillators
A restoring force is any force that
always acts to pull a system back
toward equilibrium.
23.1 Harmonic motion
Harmonic motion can be fast or slow,
but speed constantly changes during
its cycle.
We use period and frequency to
describe how quickly cycles repeat
themselves.
The time for one cycle to occur is
called a period.
23.1 Harmonic motion
The frequency is the
number of complete
cycles per second.
Frequency and period are
inversely related.
One cycle per second is
called a hertz,
abbreviated (Hz).
Solving Problems
The period of an
oscillator is 2 minutes.
What is the frequency
of this oscillator in
hertz?
Solving Problems
1. Looking for:
 …frequency in hertz
2. Given
 …period = 2 min
3. Relationships:
 …60 s = 1 min
 … f = 1/T
4. Solution
 … f = 1/120 s
f = .008 Hz
23.1 Amplitude
Amplitude describes
the “size” of a cycle.
The amplitude is the
maximum distance
the oscillator moves
away from its
equilibrium position.
23.1 Amplitude
The amplitude of a water wave is found by
measuring the distance between the
highest and lowest points on the wave.
The amplitude is half this distance.
23.1 Amplitude
A pendulum with
an amplitude of 20
degrees swings 20
degrees away
from the center in
either direction.
23.1 Damping
Friction slows a pendulum down, just
as it slows all motion.
Damping is the gradual loss of
amplitude.
23.1 Graphs of harmonic motion
A graph is a good
way to show
harmonic motion
because you can
quickly recognize
cycles.
Graphs of linear
motion do not
show cycles.
23.1 Natural frequency
and resonance
The natural
frequency is the
frequency (or period)
at which a system
naturally oscillates.
Every system that
oscillates has a
natural frequency.
23.1 Natural frequency
and resonance
You can get a swing
moving by pushing it
at the right time
every cycle.
A force that is
repeated over and
over is called a
periodic force.
23.1 Natural frequency
and resonance
Resonance happens
when a periodic force
has the same
frequency as the
natural frequency.
When each push adds
to the next one, the
amplitude of the
motion grows.
Chapter Twenty-Three: Waves
23.1 Harmonic Motion
23.2 Properties of Waves
23.3 Wave Motion
Chapter 23.2 Learning Goals
Describe the properties and
behavior of waves.
Calculate the speed of waves.
Identify the parts of a wave.
Investigation 23B
Natural Frequency and Resonance
Key Question:
 What is resonance and why is it important?
23.2 Waves
A wave is an oscillation that travels from
one place to another.
If you poke a floating ball, it oscillates up
and down.
The oscillation spreads outward from
where it started.
23.2 Waves
When you drop a ball into water,
some of the water is pushed aside
and raised by the ball.
23.2 Waves
Waves are a
traveling form of
energy because they
can change motion.
Waves also carry
information, such as
sound, pictures, or
even numbers.
23.2 Frequency, amplitude, and
wavelength
You can think of a wave as a moving series
of high points and low points.
 A crest is the high point of the wave.
 A trough is the low point.
23.2 Frequency
The frequency of a wave is the rate
at which every point on the wave
moves up and down.
Frequency means “how often”.
23.2 Amplitude
The amplitude of a water wave is the
maximum height the wave rises
above the level surface.
23.2 Wavelength
Wavelength is the distance from any point
on a wave to the same point on the next
cycle of the wave.
The distance between one crest and the
next crest is a wavelength.
23.2 The speed of waves
The speed of a water wave is how fast
the wave spreads, NOT how fast the
water surface moves up and down or how
fast the dropped ball moves in the water.
How do we measure the wave speed?
23.2 The speed
of waves
A wave moves one
wavelength in each
cycle.
Since a cycle takes one
period, the speed of the
wave is the wavelength
divided by the period.
23.2 The speed of waves
The speed is the distance traveled (one
wavelength) divided by the time it takes
(one period).
We usually calculate the speed of a wave
by multiplying wavelength by frequency.
Solving Problems
The wavelength of a wave on a string
is 1 meter and its speed is 5 m/s.
Calculate the frequency and the period
of the wave.
Solving Problems
1. Looking for:
 …frequency in hertz
 …period in seconds
2. Given
 … = 1 m; s = 5 m/s
3. Relationships:
 s = f x  or f = s ÷ 
 f = 1/T or T = 1/f
4. Solution
 f = 5 m/s ÷1 m = 5 cycles/s
 T = 1/5 cycles/s = .2 s
f = 5 Hz
T = .2 s
Chapter Twenty-Three: Waves
23.1 Harmonic Motion
23.2 Properties of Waves
23.3 Wave Motion
Chapter 23.3 Learning Goals
Distinguish between transverse
and longitudinal waves.
Demonstrate an understanding of
wave interactions.
Distinguish between constructive
and destructive interference.
23.3 Wave Motion
A wave front is the
leading edge of a moving
wave which is considered
to be the crest for
purposes of modeling.
The crests of a plane wave
look like parallel lines.
The crests of a circular
wave are circles.
23.3 Four wave interactions
 When a wave encounters
a surface, four
interactions can occur:
1. reflection,
2. refraction,
3. diffraction, or
4. absorption.
23.3 Wave interactions
A boundary is an edge or surface
where things change.
Reflection, refraction, and diffraction
usually occur at boundaries.
23.3 Wave interactions
Diffraction usually
changes the direction
and shape of the
wave.
When a plane wave
passes through a
small hole diffraction
turns it into a circular
wave.
23.3 Transverse and longitudinal
waves
A wave pulse is a short ‘burst’ of a
traveling wave.
It is sometimes easier to see the motion of
wave pulses than it is to see long waves
with many oscillations.
23.3 Transverse waves
The oscillations of a transverse wave
are not in the direction the wave
moves.
23.3 Longitudinal waves
The oscillations of a longitudinal wave
are in the same direction that the
wave moves.
23.3 Constructive interference
Constructive interference happens when
waves add up to make a larger amplitude.
Suppose you make two wave pulses on a
stretched string.
One comes from the left and the other
comes from the right.
When the waves meet, they combine to
make a single large pulse.
23.3 Destructive interference
What happens when one pulse is on top of
the string and the other is on the bottom?
When the pulses meet in the middle, they
cancel each other out.
During destructive interference, waves add
up to make a wave with smaller or zero
amplitude.
Investigation 23C
Waves in Motion
Key Question:
How do waves move?
Cell Phones: How they work
The process that allows a cell phone to communicate
is the same as for a radio or walkie-talkie. All of
these devices use electromagnetic waves of within a
specific frequency range to send information.