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Vibrations, Waves and Sound
Unit 7: Vibrations, Waves & Sound
Chapter 19: Waves19.1 Waves
19.2 The Motion of Waves
19.3 Wave Interference and Energy
19.1 Investigation: Waves in Motion
Key Question:
How do waves move?
Objectives:
Explain how waves move.
Compare and contrast transverse and longitudinal waves.
Use knowledge of longitudinal and transverse waves to describe water waves.
WavesA 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.
Why learn about waves?
Waves carry useful information and energy.
Waves are all around us:— light from the stoplight— ripples in a puddle of— electricity flowing in wires— radio and television and
cell phone transmissions
Recognizing waves around you
Waves are present:— when you see a vibration that
moves.— when something makes or
responds to sound.— when something makes or
responds to light.— when technology allows us to “see
through” objects.— when information travels through
the air (or space) without wires.
WavesWaves are a traveling
form of energy because they can change motion.
Waves also carry information, such as sound, pictures, or even numbers.
Wave pulses
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.
Transverse wavesA transverse wave has its oscillations
perpendicular to the direction the wave moves. A
A wave pulse along a rope attached to a wall moves left to right, while the boys hand moves up and down.
Longitudinal waves
The oscillations of a longitudinal wave are in the same direction that the wave moves.
Longitudinal waves A sharp push-pull on the
end of the spring results in a traveling wave pulse as portions of the spring compress, then relax.
Sound waves are longitudinal waves.
Like a wave pulse on a spring, air molecules oscillate back and forth as sound travels.
Frequency, amplitude, and wavelengthYou 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.
Frequency
The frequency of a wave is the rate at which every point on the wave moves up and down.
Frequency means “how often”.
Amplitude
The amplitude of a water wave is the maximum height the wave rises above the level surface.
WavelengthWavelength 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.
The speed of wavesThe 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?
The speed of wavesA 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.
The speed of wavesThe 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.
A wave has a wavelength of 0.5 meters, and its frequency is 40 hertz. What is the speed of the wave?
Calculating wave speed
1. Looking for: …speed of the wave.
2. Given: …wavelength (0.5 m) and frequency (40 Hz).
3. Relationships: Use formula: speed = ƒ x
4. Solution: …speed = 40 Hz × 0.5 m = 40 1/s × 0.5 m
speed = 20 m/s
Cooking with waves A microwave heats food by
transferring wave energy to the food.
The magnetron is a device in a microwave oven that creates a wave with electricity.
The wave vibrates inside the cooking space at 2.5 gigahertz- the frequency best absorbed by water.
Standing waves on a stringA wave that is confined between boundaries is
called a standing wave.With all waves, resonance and natural
frequency are dependent on reflections from boundaries of the system containing the wave.
Standing Waves and Harmonics
The standing wave with the longest wavelength is called the fundamental.
The fundamental has the lowest frequency in a series of standing waves called harmonics.
The first five standing wave patterns of a vibrating string shows that patterns occur at multiples of the fundamental frequency.
Standing wavesStanding waves have nodes and antinodes.
A node is a point where the string stays at its equilibrium position.
An antinode is a point where the wave is as far as it gets from equilibrium.
Standing waves
It is easy to measure the wavelength of a standing wave on a string.
Two harmonics equals one wave!
Unit 7: Vibrations, Waves & Sound
Chapter 19: Waves19.1 Waves
19.2 The Motion of Waves
19.3 Wave Interference and Energy
19.2 Investigation: Resonance and Standing WavesKey Question:
How do we make and control waves?
Objectives: Describe how frequency, wavelength, and speed are
related. Measure the wavelength and frequency of a vibrating
string. Recognize and apply the concept of harmonics in
resonant systems. Define natural frequency and apply methods for changing
the natural frequency of a system.
Waves propagationWaves propagate, which means they spread
out from where they begin.
When you drop a ball into water, some of the water is pushed aside and raised by the ball.
Wave motionA 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.
Four wave interactions
1. reflection,
2. refraction,
3. diffraction, or
4. absorption.
When a wave encounters a surface, four interactions can occur:
Diffraction
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.
Unit 7: Vibrations, Waves & Sound
Chapter 19: Waves19.1 Waves
19.2 The Motion of Waves
19.3 Wave Interference and Energy
19.3 Investigation: Exploring Standing Wave PropertiesKey Question:
How does changing the tension affect a vibrating string?
Objectives: Apply an understanding of inertia and restoring force to
describe the effects of increasing the tension of a vibrating string.
Explain how a string’s tension and mass affects its frequency and amplitude.
Describe how wave properties are applicable to musical instruments.
Superposition principle Interference happens when two or more
waves mix together.When more than one wave is present, the
total oscillation of any point is the sum of the oscillations from each individual wave.
Noise canceling headphones Specialized headphones can
create “anti-noise.”
A microphone in the headphone samples the noise and generates anti-noise, or sound that is 180 degrees out of phase with the noise.
The anti-noise uses superposition to reduce or muffle noise.
Constructive interferenceConstructive 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.
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.
Resonance and lightA wave has to be caught in a system with
boundaries to show resonance.
Catch light between two perfect mirrors and we can get resonance of light waves.
This is exactly how a laser works!
Resonance and elastic string
Resonance elastic strings is created by adding new pulses so that each adds to the reflected pulse in constructive interference.
Waves and energy
The energy of a wave is proportional to its frequency.
Higher frequency means higher energy.
Waves and energyThe energy of a wave
is also proportional to its amplitude.
Given two standing waves of the same frequency, the wave with the larger amplitude has more energy.
Waves that Shake the Ground
On January 12, 2010, a 7.0 magnitude earthquake struck the Caribbean nation of Haiti.
Its capital, Port-au-Prince, was nearly destroyed.
Many government buildings, schools, hospitals, and businesses collapsed.
The powerful earthquake was caused by waves traveling through Earth.
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