WAVES AND VIBRATIONS NOTES

A wave is a vibration moving in time and space   It cannot exist in one place but must extend from one place to another Wave motion is a way to transfer energy without transfer of matter

A vibration is back and forth movement A single disturbance or vibration is a wave pulse If the source repeatedly vibrates it produces periodic motion

Demonstration: What happens when a tuning fork is placed in water? Sound is produced when matter vibrates Demonstration: What happens when a tuning fork is placed in water? Since sound is produced when matter vibrates, sound is a form of mechanical energy.

Crest midpoint trough Crest: High point in the wave Trough: Low point in the wave --------Midpoint of wave where the wave is in equilibrium Amplitude Distance from the midpoint to the crest or trough The higher the amplitude the stronger the wave

Wavelength is: the distance from one point on the wave to the identical point on the next wave. Crest to Crest, or trough to trough

How many wavelengths or wave cycles pass in one second.  Frequency is: How frequently a vibration occurs. How many wavelengths or wave cycles pass in one second. If 2 wavelengths (or vibrations, or wave cycles) pass a point in one second, then the frequency is: 2 vibrations/ second. (Or 2 cycles/ sec)

Hertz is: a unit of frequency (abbreviated Hz) It was named after   It was named after Heinrich Hertz who demonstrated radio waves in 1886. One vibration per second is one Hertz.   2 vibrations (or waves, or cycles) per second is ________Hertz 2

Longer vibrations, so lower sound DEMONSTRATIONS: Hacksaw blade: (hold blade against table edge. Change length hanging over and vibrate it. Describe the frequency of vibration and pitch (highness or lowness of the sound) when the hacksaw blade is long. Longer vibrations, so lower sound Describe the frequency of vibration and pitch when the hacksaw blade is shorter. Higher vibrations so higher pitch

What is the relationship between the amount of water Containers with water. (pop bottles to blow across or crystal glass to rub finger around)  What is the relationship between the amount of water in each container and the pitch? Less water vibrating means higher pitch. More water vibrating is lower pitch The longer the length of vibrating air above the water, the Lower the pitch when blowing into a bottle

If a wave's frequency is known, then you can calculate the period of time it takes for one wave to pass.   If the frequency is 2 Hz, then the time it takes for 1 vibration is ½ second . Explain the relationship between Period and Frequency: Frequency = 1 Period = 1 Period (T) Frequency It is an inverse relationship If freq = 5 Hz then Period = 1/5 or .2 seconds

The Sears Building in Chicago sways back and forth at a vibration frequency of 0.1 Hz. What is its Period of vibration? T = 1 / f 1 / 0.1 cycles/sec = 10 seconds/cycle It takes 10 seconds for it to sway back and forth one time

Speed of a wave depends on the medium through which the wave travels. WAVE SPEED Speed of a wave depends on the medium through which the wave travels. Speed depends on: Density of materials how many molecules there are to bump into each other Elasticity how well it changes shape and then springs back. More elastic, the faster the waves travel. Temperature higher the temperature, the faster the molecules move Sound travels about 330 m/s - 350 m/s depending on the temperature of the air. It travels 4 times faster in water, and 15 times faster in steel.

λ Greek letter Lambda = wavelength   Wave Speed = frequency x wavelength λ Greek letter Lambda = wavelength Wave Speed Freq. λ

Sound waves at a concert reach your ears at the Frequency (Hz) Wavelength (m) Wave Speed (m/s) 160 2.13 340 264 1.29 340 396 0.86 340 528 0.64 340 Sound waves at a concert reach your ears at the same time. Lower notes have long waves, high notes (higher frequency) have shorter wave lengths, but they all have the same wave speed.

If a water wave vibrates up and down 2 times each second and the distance between wave crests is 1.5 meters, what is the frequency of the wave? What is its wavelength? What is its speed? F = 2 Hz Wavelength = 1.5 m Speed = 2 cycles x 1.5 m = 3 m/sec Sec Cycle

What is the wavelength of a 340 Hz sound wave when the speed of sound in air is 340 m/s? Wavelength = speed = 340 m/sec = 1 meter/cycle Frequency 340 cycles/sec 1 miles = 1609 meters. If the speed of sound is 340 m/s, how long does it take for thunder to travel 1 mile? T = distance/speed 1609 m = 4.7 seconds 340 m/sec So about 5 seconds/mile

TRANSVERSE WAVES Motion of the wave is at right angles to the direction of travel Examples: waves on the surface of liquids, Radio waves, light waves slinky demo

LONGITUDINAL WAVES motion of waves is in the same direction

Examples: sound waves SMOKE RINGS

AS A REVIEW OF THE WAVE TYPES Transverse rarefaction Longitudinal compression

NATURAL FREQUENCY: When you drop a wrench and baseball bat on the floor, you hear 2 different sounds. They vibrate differently. When any object composed of an elastic material is disturbed it vibrates at its own special set of frequencies. This natural frequency depends on factors such as: elasticity and the shape of the object RESONANCE: When the vibration of one object causes another object to: vibrate at its natural frequency A dramatic increase in Amplitude occurs.

Swinging on a swing pump in rhythm with natural frequency of the Examples of Resonance Swinging on a swing pump in rhythm with natural frequency of the swing, go higher and higher Tuning fork can cause another to vibrate Tacoma Narrows Bridge destroyed by a 40 mph wind (video) Wine glass shattering

Constructive Interference: when one wave crest INTERFERENCE OF WAVES Constructive Interference: when one wave crest overlaps another crest and they build together Animation courtesy of Dr. Dan Russell, Kettering University

Destructive Interference: when one wave crest   Destructive Interference: when one wave crest overlaps a trough and they cancel each other out. USES noise canceling earmuffs for pilots, jack hammers Animation courtesy of Dr. Dan Russell, Kettering University

SHOW MOIRE PATTERNS

Standing Waves: when wave is reflected back exactly opposite to the original wave A = anti-node B = node Animation courtesy of Dr. Dan Russell, Kettering University

Beats: periodic variation in loudness of sounds caused when 2 slightly different frequencies are sounded together demo Animation courtesy of Dr. Dan Russell, Kettering University If one tuning fork vibrates at 264 Hertz, and the other at 262 Hertz, then they are “in step” 2 times each second. A beat frequency of 2 hz is heard.

THE DOPPLER EFFECT: The Doppler effect is a change in frequency of a wave due to the motion of the source or receiver. Example: change in pitch of a car engine, horn or siren as it passes you As a sound wave approaches you, the pitch is higher than normal because the waves Come faster, at a higher frequency   After it passes, the sound waves are farther apart so the sound is Lower

Doppler Effect occurs for visible light too.   Short lights waves are Blue and Long wavelengths are Red Astronomers can measure whether stars and galaxies are moving toward us or away from us by looking at the light shift.  Blue Shift means - the light is coming faster – stars are moving forward Red Shift means- the star is going away They can calculate the speed of a spinning star. There is a blue shift on the side spinning toward us and a red shift on the side spinning away.

Police use the Doppler effect of radar to determine the speed of a car. A computer in the radar gun compares the frequency of the radar emitted by the antenna with the frequency of the reflected waves.

OSCILLOSCOPE: A device that is used to measure electrical signals as waves. It shows the strength of the wave (amplitude) and the period of time between each wave. You can then calculate the frequency of the waves.