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Devil [pic] Physics
Lesson 4.2:
Traveling Wave Characteristics
TRANSITION VIDEOS
Physics of Waves
INTRODUCTORY VIDEOS
Cool Waves
Bad Waves
Video: Properties of Waves
OBJECTIVES
1. IB Assessment Statements for Topic 4.4., Wave Characteristics
4.4.1. Describe a wave pulse and a continuous progressive (travelling) wave.
4.4.2. State that progressive (travelling) waves transfer energy.
4.4.3. Describe and give examples of transverse and of longitudinal waves.
4.4.4. Describe waves in two dimensions, including the concepts of wavefronts and of rays.
4.4.5. Describe the terms crest, trough, compression and rarefaction.
4.4.6. Define the terms displacement, amplitude, frequency, period, wavelength, wave speed and intensity.
4.4.7. Draw and explain displacement-time graphs and displacement-position graphs for transverse and for longitudinal waves.
4.4.8. Derive and apply the relationship between wave speed, wavelength and frequency.
4.4.9. State that all electromagnetic waves travel with the same speed in free space, and recall the orders of magnitude of the wavelengths of the principal radiations in the electromagnetic spectrum.
2. Objectives: By the end of this class you should be able to:
a) state what is meant by wave motion
b) distinguish between longitudinal and transverse waves
c) define amplitude, wavelength, period, and frequency and state the relationship between them, [pic]
d) state what is meant by crest and trough and identify these on a graph
e) find amplitude and period from a displacement-time graph
f) find amplitude and wavelength from a displacement-position graph
g) understand the meaning of the terms wavefront and ray
h) use [pic]
3. What is a wave?
a) Video: Pulses and Waves
b) A wave is a way of transferring energy and momentum from one place to another, but without the actual large-scale motion of a material body
c) Examples:
i) Life is a beach
ii) Light from the sun
iii) Soprano breaking a glass
iv) Light travels in a vacuum – no medium required – an electromagnetic wave
v) Sound and water require a medium – mechanical waves
d) Waves occur because something is disturbed
e) A wave is a disturbance that travels, transferring energy and momentum from one place to another, but without the actual large-scale motion of a material body. The direction of energy transfer is the direction of propagation of the wave.
4. Transverse and Longitudinal Waves
a) Video: Longitudinal and Transverse Waves
b) Transverse – waves in which the disturbance is at right angles to the direction of energy transfer
i) rope tricks
[pic]
ii) electromagnetic waves
[pic]
c) Longitudinal -- waves in which the disturbance is along the direction of energy transfer
i) dominos
ii) sound waves
[pic]
d) energy propagated through a series of compressions (region of higher than normal density) and rarefactions (region of lower than normal density)
[pic]
5. Wave pulses
a) rope tricks
i) half-pulse vs. full pulse
[pic]
ii) wave pulses travel with a given speed determined by the tension in the rope and mass per unit length, μ = m/L
[pic]
iii) the speed of the wave is determined by the properties of the medium and not by how the wave is created,
iv) i.e. independent of shape or how fast you produced it
6. Travelling waves
a) those that take more than one step before dribbling (N/A NBA)
b) when pulses are repetitiously produced
c) if the wave driver demonstrates SHM, the wave produced will exhibit SHM and the wave will look like a sine wave (also called a harmonic wave)
7. Harmonic waves
a) exhibit SHM
[pic]
b) period (s) – time to complete one full wave, unit is s
c) wavelength ([pic]) – length of a wave (surprise!)
d) velocity (v) – equal to one wavelength (m) per the period (s)
[pic]
e) frequency ([pic]) – number of waves per unit time, equal to 1/T, unit is s-1 or Hz
[pic] , [pic] , [pic]
The marvels of Algebra!!!
f) displacement (y) – height obtained above or below the undisturbed point due to the disturbance, it is a function of the distance (x) and time (t)
i) When wave movement is dependent on the medium (like for sound [compression] waves), displacement is given in terms of density:
[pic]
or in terms of pressure
[pic]
g) displacement is a function of distance and time for any wave regardless of whether the displacement is vertical (transverse) or horizontal (longitudinal)
h) graphs of displacement versus position let us determine wavelength, [pic]
[pic]
i) graphs of displacement versus time let us determine period, T
[pic]
j) Points on the wave with maximum displacement are call crests and those at the minimum displacement are called troughs
k) In a travelling wave, the crests will steadily move forward until the period is reached which is where the wave will look like it originally did when it was disturbed
[pic]
8. Example Questions:
a) A radio station emits at a frequency of 90.8 MHz. What is the wavelength of the waves emitted?
Since they are electromagnetic waves, they travel at the speed of light, 3x108 m/s
f = 90.8x106 s-1
[pic]
[pic]
b) A sound wave of frequency 450Hz is emitted from A and travels towards B, a distance of 150m away. Take the speed of sound to be 341 m s-1 . How many wavelengths fit in the distance from A to B?
f = 450 s-1
v = 341 m s-1
[pic] , [pic]
[pic]
[pic]
c) The noise of thunder is heard 3 s after the flash of lightning. How far away is the place where the lightning struck? (Use 340 m s-1 for the speed of sound)
Since light travels so fast, you can assume you saw it instantaneously.
v x t = d
(340 m s-1)(3 s) = 1020 m
9. Wavefronts and Rays
[pic]
a) If you consider a wave to be moving horizontally, a wavefront is a plane perpendicular to the wave and perpendicular to the direction the wave is travelling
b) Rays are a pretty good baseball team
c) They are also lines perpendicular to the wavefronts in the direction the wave is travelling
[pic]
d) Examples:
i) The waves on the beach would have a rectangular wavefront
ii) If you drop a stone in the water, the wavefront would be cylindrical
iii) Light waves from a point source would have spherical wavefronts
e) Example Question: Why do waves from a rock thrown into a lake eventually die out?
[pic]
[pic]
[pic]
Period 5 -- STOPPED HERE
Summary
1. Objectives. Are you able to:
f) state what is meant by wave motion
g) distinguish between longitudinal and transverse waves
h) define amplitude, wavelength, period, and frequency and state the relationship between them, [pic]
i) state what is meant by crest and trough and identify these on a graph
j) find amplitude and period from a displacement-time graph
k) find amplitude and wavelength from a displacement-position graph
l) understand the meaning of the terms wavefront and ray
m) use [pic]
Questions?
Look Ahead Video
Oscillating Vortex Flows
Lesson 4.2: Homework
Part A, #1-10
Part B, #11-20
Optional Reading
Giancoli, Chapter 11, Sections 1-6
University Physics, Chapter 13
Holt Physics, Chapter 11
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