Mechanical Waves Types of Wave Describing Waves The Wave Equation

Mechanical WavesTypes of Wave

Describing WavesThe Wave Equation

What is a wave?

A disturbanceThat propagates through space and timeAnd transfers energya.k.a. progressive wave or travelling wave

Disturbance

dropping a stone in the water

plucking a guitar string

blowing through a pipe

An Unusual Disturbance

Types of Wave

1 Mechanical Wave• Produced by a disturbance• Transmitted by particles of the medium vibrating to and fro

• Examples: sound, water waves

Types of Wave2 Electromagnetic Wave

• Produced by a disturbance• Does not need a medium to

transmit it

Let us concentrate on Mechanical Waves!

Medium

Water (liquid)

String (solid)

Air (gas)

Transverse Waves

• direction of disturbance is perpendicular to the direction of travel of the wave

• Examples: • String instruments• Earthquake s-waves• Light

Diagram of a transverse wave

Equilibrium position

crest

trough

Longitudinal Waves• direction of disturbance is parallel to the

direction of travel of the wave• Examples:

• Sound waves• Ultrasound• Earthquake p-waves

Diagram of a longitudinal wavecompression

rarefaction

Describing Waves

WavelengthAmplitudePeriodFrequencySpeed

Describing Wavesdi

spla

cem

ent

wavelength

amplitude

positiontime

period

• Wavelength• Equivalent to one complete vibration• Distance from crest to crest, or trough

to trough, OR• Distance from two identical successive

points on a graph• Unit: meter, m• Symbol: lambda, λ

• Amplitude• Maximum displacement of vibrating

particles• Unit: meter, m• Symbol: A

• Period• Time for one complete vibration• Time corresponding to one wavelength• Unit: second, s• Symbol: T

• Frequency• Number of vibrations (or waves) in one

second• Reciprocal of period, 1/T• Units:

• vibrations per second, s-1• cycles per second, s-1•Hertz, Hz

• Symbol: ƒ

Wave Speed•Distance moved by any point on the wave in one second•Product of wavelength and frequency•Units: ms-1

•Symbol: v

Displacement (m)

Distance (m)

0.5

-0.5

What is the amplitude in metres and the wavelength in metres?

Displacement (m)

Time (s)

0.2 0.4 0.6 0.8 1.0

0.5

-0.5

For the same wave, what is the period in seconds and the frequency in Hertz?

What is the speed of this wave?

20 ms

What is the period and frequency of this wave?

1. A water wave travels through a pond with a speed of 1ms-1 and a frequency of 5Hz. What is the wavelength of the waves?

2. The speed of sound is 330ms-1 (in air). When Ricky hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?

3. Purple light has a wavelength of around 6x10-7m. If its frequency is 5x1014 Hz what is the speed of light?

4. Red light travels at the same speed. Work out its frequency if its wavelength is 4x10-7m.