Properties of Waves Properties of Waves

Mechanical Waves Mechanical Waves

Mechanical wave requires a medium – such as rope, air, water or soil

The wave begins at a source, this causes molecules in the medium to move up and down, right and left, or in and out around their equilibrium position.

Transverse and Longitudinal Transverse and Longitudinal WavesWaves

Wave pulse – a wave pulse is produce from the source, it makes a single disturbance that travels through the medium.

Periodic wave – if the source vibrates repeatedly it produces a periodic wave.

Transverse and Longitudinal Transverse and Longitudinal WavesWaves

A transverse wave – the wave moves left or right, the particles in the medium move up and down

Transverse and Longitudinal Transverse and Longitudinal WavesWaves

A longitudinal wave – the wave moves left or right and the particles in the spring move left or right. The same direction as the wave.

WavesWaves

Amplitude – the amplitude of a wave is the distance from the equilibrium, or , average, position to the peak or trough.

Waves Waves

Wavelength – the length of the repeating pattern. We use the Greek letter Lambda λ

Waves Waves

Frequency – the rate of occurrence of the event. For a wave the frequency is the number of waves (or cycles) per unit of time that passes a point in that medium(material).

Amplitude and WavelengthAmplitude and Wavelength

Water waves pass by a 16 ft. fishing boat. There are exactly 12 crests from front to back of the boat. The vertical distance between crest and trough is 7 inches. What is the amplitude and wavelength of the water waves?

Amplitude – the amplitude of a wave is the distance from the equilibrium, or , average, position to the peak or trough.

2A=7 A=3.5

WavelengthWavelength

Since 12 crests extend horizontally between the front and the back of the boat, there are 11 crest to crest distances. The wavelength λ is one crest to crest distance.

11 λ = 16ft. λ = 16ft/11

λ =1.45ft.

Frequency Frequency

Using a stopwatch for the water wave in our last example, you determine that 3 wave crests pass the bow(front) of the boat in 2.6 seconds. What is the frequency and period of the waves?

Frequency Frequency

There are 2 wave cycles between three crests.

f = Number of cycles

time interval

=2cycles/2.6 seconds

=0.77cycles/second

=0.77Hz

PeriodPeriod

The wave period is the reciprocal of the frequency.

T =1/f

=1/0.77cycles/s

=1.3 s/cycle By convention the cycle is dropped when

reporting the wave period

Sound waves Sound waves

Sound is a longitudinal mechanical wave. The source of a wave vibrates rapidly causing pressure variations in the medium.

A guitar string, a drum head, and your vocal cords are all sources of sound waves.

Their vibrations causes sound to move through air.

Doppler effect Doppler effect

is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves. For waves that propagate in a wave medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted