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

Transverse and Longitudinal Waves 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.

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

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

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 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

Wavelength 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 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 There are 2 wave cycles between three crests. f = Number of cycles time interval =2cycles/2.6 seconds =0.77cycles/second =0.77Hz

Period 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 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 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 transmittedfrequencywavelengthwave soundvelocity