This requires a medium!

Transcription

1 Unit 7: Waves Mechanical Wave a disturbance in matter that carries energy from one place to another This requires a medium! 1

2 Types of Mechanical Waves 1. Transverse Wave a wave that causes matter to vibrate at right angles to the direction in which the wave travels Parts of a transverse wave * Rest Position the center of the wave * Crest highest point above the rest position * Trough lowest point below the rest position 2

3 2. Longitudinal Wave a wave in which the vibration of the medium is parallel to the direction of the wave Parts of a longitudinal wave * Compression an area where the particles of the medium are close together * Rarefaction an area where the particles of the medium are spread out 3

4 3. Surface Wave a wave that travels along a surface separating two media * The particles of the medium undergo a circular motion Properties of Mechanical Waves 4

5 * Amplitude the maximum amount of displacement of a wave (from rest to crest or rest to trough) * The more energy a wave has the greater its amplitude * Wavelength distance between a point on a wave and the same point on the next cycle of a wave (from crest to crest or trough to trough) 5

6 * Period the time between complete cycles on a wave (time between crests) * Measured in seconds * Frequency number of complete cycles per second * measured in Hertz (waves per second) * Increasing the frequency of a wave decreases the wavelength 6

7 * Wave Speed how fast a given wave travels * Usually pretty constant, but can change with changes in medium, temperature, or pressure Wave Calculations in seconds period = in Hz 1 frequency wavelength = in distance/time speed frequency in distance wavespeed = wavelength frequency in Hz in distance/time 7

8 A wave has a frequency of 4 Hz. What is it s period? frequency = 4 Hz period period = 1 frequency period = 1 / 4Hz period = 0.25 seconds A power line sways in the breeze at 4 m/s with a frequency of 3.2 Hz. What is the wavelength of this wave? speed = 4 m/s frequency = 3.2 Hz wavelength wavelength = speed frequency wavelength = 4 m/s / 3.2 Hz wavelength = 1.25 m 8

9 The tail of a kite swings with a wavelength of 0.1 meters and a frequency of 18 Hz. What is the speed of this wave? wavelength = 0.1 m frequency = 18 Hz wave speed wavespeed = wavelength frequency wave speed = 0.1 m x 18 Hz wave speed = 1.8 m/s Sound Waves - longitudinal waves that travel through a medium 9

10 Speed of Sound * In dry air, sound travels at 342 m/s * It travels fastest in solids because the particles are more tightly packed together Intensity the rate at which a sound wave s energy flows through a certain area * Effected by the wave s amplitude and distance from its source * Measured in decibels (db) which is a logarithmic scale * Loudness a person s physical response to the intensity of sound 10

11 Pitch how high or low a sound is sensed * Perceived as tone * The frequency of a sound wave is directly related to pitch * A musical instrument often changes sound wave frequency by changing the length of tubing through which the air is moving 11

12 Uses For Sound * Most humans can hear frequencies between 20Hz and 20,000Hz * Sounds higher than 20,000 Hz are called ultrasound * Sounds lower than 20Hz are called infrasound * SONAR (Sound Navigation and Ranging) uses sound waves to determine locations and distances 12

13 * Ultrasound has medical applications that make images of internal anatomy The Doppler Effect * A change in sound frequency due to the motion of the sound source, motion of the receiver, or both. * As a sound source approaches, the pitch gets higher * As a sound source moves away, the pitch gets lower 13

14 Hearing The ear has three major regions 1. Outer Ear gathers and focuses the sound 2. Middle Ear receives sound and amplifies the vibrations, contains the ear drum 3. Inner Ear uses nerve endings to send messages to the brain, contains the cochlea Music * Most musical instruments use resonance to amplify sound * Resonance the response of a standing wave to another wave of the same frequency * This can produce a dramatic increase in amplitude 14

15 3/1/16 * When one sound wave forces another material to vibrate because they have a matching natural frequency, the two objects will resonate * Ex: Tacoma Bridge * Concert halls use resonance to improve sound to the listener * Reflecting panels are use to prevent dead spots * Absorbing tiles are used to reduce unwanted reflections 15

16 Interference - the phenomenon that occurs when two waves meet while traveling along the same medium Types of Interference 1. Constructive Interference (sometimes called reinforcement) when two or more waves combine to produce a wave of greater amplitude * When 2 crests or 2 troughs meet * The wave created has a higher intensity than the original waves 16

17 2. Destructive Interference (sometimes called cancellation) when waves combine to make sounds of a lesser amplitude * When the crest of a wave meets the trough of another * The wave produced has a lower intensity than the original waves. Standing waves a wave that appears to stay in one place, results from the interference between a wave and its reflection 17