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Lesson 11-7: Wave Motion

Lesson 11-8: Types of Waves: Transverse and

Longitudinal

TRANSITION VIDEO

Physics of Waves

INTRODUCTORY VIDEOS

Cool Waves

Bad Waves

Properties of Waves

OBJECTIVES

1. Understand the difference between wave motion and

the motion of the particles that comprise the wave.

2. Know that a wave consists of oscillations that move

without carrying matter with them.

3. Know that travelling waves transport/transfer

energy.

4. Describe the source of a wave.

5. Identify and explain the different terms associated

with waves.

6. Use the relationship between velocity, wavelength,

frequency, and period to solve problems.

7. Differentiate between transverse and longitudinal

waves.

8. Solve problems involving wave velocity and

elasticity.

WAVE MOTION

1. What is a wave?

a) Video: Pulses and Waves

b) 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

c) Waves occur because something is disturbed

d) 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.

2. Wave pulses

a) rope tricks

i) half-pulse vs. full pulse

b) Wave pulses travel with a given speed

i) speed is distance divided by time

ii) period is defined as the time it takes one

complete pulse (one wavelength) to pass a

given point

iii) so wave speed can be defined as,

or, since

iv) wave speed in a rope is determined by the

tension in the rope and mass per unit length,

μ = m/L

v) the speed of the wave is determined by the

properties of the medium and not by how

the wave is created,

vi) i.e. independent of shape or how fast you

produced it

3. 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)

4. Harmonic waves

a) exhibit SHM

b) period (s) – time to complete one full wave, unit

is s

c) wavelength ( ) – length of a wave (surprise!)

d) velocity (v) – equal to one wavelength (m) per

the period (s)

e) frequency ( ) – number of waves per unit time,

equal to 1/T, unit is s-1

or Hz

,

,

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)

5. 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

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

,

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

6. 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

ii) electromagnetic waves

c) Longitudinal -- waves in which the disturbance

is along the direction of energy transfer

i) dominos

ii) sound waves

iii) energy propagated through a series of

compressions (region of higher than normal

density) and expansions or rarefactions

(region of lower than normal density)

iv) When wave movement is dependent on the

medium (like for sound [compression]

waves), displacement is given in terms of

density:

or in terms of pressure

d) velocity for a longitudinal wave looks like that

of a transverse wave

i) down a long solid rod (E is elastic modulus)

ii) in a liquid or gas (B is bulk modulus)

7. Displacement

a) is a function of distance and time for any wave

regardless of whether the displacement is

vertical (transverse) or horizontal (longitudinal)

b) graphs of displacement (y) versus position (x) let

us determine wavelength,

c) graphs of displacement (y) versus time (t) let us

determine period, T

d) Points on the wave with maximum displacement

are call crests and those at the minimum

displacement are called troughs

e) 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

Summary

1. Do you understand the difference between wave

motion and the motion of the particles that comprise

the wave?

2. Do you know that a wave consists of oscillations that

move without carrying matter with them?

3. Do you know that travelling waves transport /

transfer energy?

4. Can you describe the source of a wave?

5. Do you identify and explain the different terms

associated with waves?

6. Can you use the relationship between velocity,

wavelength, frequency, and period to solve

problems?

7. Can you differentiate between transverse and

longitudinal waves?

8. Can you solve problems involving wave velocity and

elasticity?

Questions?

Look Ahead Video

Oscillating Vortex Flows

Homework

#34-42

Optional Reading

University Physics, Chapter 13

Holt Physics, Chapter 11