Tyler Junior CollegePhysics 1405Elementary Physics Section Dos Waves & Sounds
Tyler Junior CollegePhysics 1405Elementary Physics Vibrations & Waves
Important Concepts • Vibration of a Pendulum • Wave Description • Wave Motion • Wave Speed • Transverse Waves • Longitudinal Waves • Interference • Doppler Effect • Bow Waves • Shock Waves
Introducing the Wave • Waves are everywhere! • Sound waves • Visible light waves • Radio waves • Microwaves • Water waves • Sine waves • Cosine waves • Telephone cord waves • Stadium waves • Earthquake waves • Waves on a string • Waves on a slinky
So What’s a Wave, eh? • A repeating & periodic disturbance which moves through a medium. • The medium is the substance or material which carries the wave. • Waves are often symbolized as mathematical functions, i.e., a sine curve.
Waves Carry Energy • The real usefulness of waves is their ability to transfer energy. • Waves can transport energy from one location to another because the particles of the medium interact. • Waves transport energy without transporting matter.
ConcepTest • In order for John to hear Jill, air molecules must move from the lips of Jill to the ears of John. a. True b. False
ConcepTest • In order for a medium to be able to support a wave, the particles in the wave must be • a. frictionless • b. isolated from one another • c. able to interact • d. very light
Categories of Waves • Transverse wave • a wave in which the medium moves perpendicular to the direction the wave moves
Categories of Waves • Longitudinal wave • a wave in which particles of the medium move in a direction parallel to the direction which the wave moves. • Sound is a longitudinal wave.
Categories of Waves • Electromagnetic waves • waves which are capable of transmitting energy through a vacuum • EM waves require NO medium • EM waves are light waves with speed = c. • Mechanical waves • waves which are not capable of transmitting energy through a vacuum • Mechanical waves require a medium
ConcepTest • A transverse wave is transporting energy from east to west. The particles of the medium will move… • East to west only • Both east & west • North to south only • Both north & south
ConcepTest • The sonar device on a fishing boat uses underwater sound to locate fish. Would you expect the sonar to be a longitudinal or a transverse wave? a. transverse b. longitudinal c. both d. neither
Anatomy of a Transverse Wave Crest Trough Amplitude Wavelength
Anatomy of a Longitudinal Wave Compression Rarefaction Wavelength
ConcepTest Which interval represents one full wavelength? a. From A to C c. From C to E b. From B to D d. From C to G
ConcepTest Where can you draw the amplitude? At A or B or C or D?
Frequency • The frequency of a wave is how many crests pass within a time period. AKA, the number of cycles per second. • Units: 1 cps = 1 s-1 = 1 Hertz = 1 Hz
Period • The time of one cycle. • Units: time (s) • Frequency & period are different, yet related.
ConcepTest • A transverse wave has an amplitude of 2 cm and a frequency of 12 Hz, and the distance from a crest to the nearest trough is measured to be 5 cm. What is the period & wavelength of the wave? • A. 0.2 s, 10 cm • B. 0.083 s, 10 cm • C. 0.083 s, 20 cm • D. 0.2 s, 20 cm • E. none of the above
ConcepTest • Assuming everything else remains fixed, as the frequency of a wave increases, the period of the wave _________, and as the wavelength of a wave decreases, the frequency of the wave _________. • A. increase, decreases • B. decreases, increases • C. increases, increases • D. decreases, decreases
ConcepTest • The frequency of rotation of a second hand on a clock is________. a. 1/60 Hz b. 1/12 Hz c. ½ Hz d. 1 Hz e. 60 Hz
Energy Transport & Amplitude • A wave is an energy transport phenomena. • It transports energy without transporting matter. • HOW? • The source of the wave does work on the medium to give it kinetic energy. • This energy then travels through the wave until it reaches its destination. • The kinetic energy then does work on the destination.
Energy Depends on Amplitude • The amount of energy carried by a wave is related to the area under the curve. For two waves of equal wavelength & frequency, …
ConcepTest If the energy of a wave is proportional to the square of the wave amplitude, fill in the chart.
ConcepTest Suppose you create a transverse wave by shaking the end of a rope. To increase the amplitude of the wave you a. also change the frequency b. also change the wavelength c. increase the energy of the wave d. all the above
Speed of a Wave • The speed of a wave is NOT how fast the medium moves, but how fast the energy travels through the medium. • Since, • the speed of a wave can be written as
ConcepTest • What is the speed of a wave that has a frequency of 2 kHz and a wavelength of 2 cm? a. 1 cm/s b. 1000 cm/s c. 4000 cm/s d. 4 cm/s e. 1/4 cm/s
ConcepTest • What is the wavelength of the broadcast signal of the radio station at 102.7 MHz?
Interference of Waves • Two waves can occupy in the same space. • When two waves overlap they “interfere” with each other. • The interference may be either constructive (i.e. waves add) or destructive (i.e., waves subtract). • The “principle of superposition” controls the resultant wave.
The Principle of Superposition • When two waves interfere, the resulting displacement of the medium at any location is the algebraic sum of the displacements of the individual waves at that same location.
ConcepTest Several positions along the medium are labeled with a letter. Categorize each labeled position along the medium as being a position where either constructive or destructive interference occurs.
ConcepTest One wave traveling to the right has an amplitude of 2 cm above the equilibrium position. Another wave traveling to the left has an amplitude of 4 cm below the equilibrium position. What is the amplitude when the two waves interfere? a. 2 cm above equilibrium b. 4 cm below equilibrium c. 6 cm above equilibrium d. 8 cm below equilibrium e. None of the above
Reflection of Waves • Tie one end of a rope to a wall and shake the free end up and down. • What happens? • The wave travels down the rope and is reflected back by the wall. • The reflected wave is inverted.
Standing Waves • Sometimes reflection and interference can produce a standing wave. • A standing wave is a special wave pattern produced when incident and reflected waves interfere.
Anatomy of a Standing Wave • Standing waves have Nodes & Antinodes. • Nodes are the stationary points. • Antinodes are the points of maximum displacement & maximum energy.
Standing Waves in a Pipe • Open at one end • Node at closed end • Antinode at open end • There is a relationship between #N and #A • Open at both ends • Antinode at both open ends • There is a relationship between #N and #A
Doppler Effect • The Doppler Effect is a familiar effect. • Train whistle • Indy race car • Named after Christian Doppler, Austrian scientist, 1803-1853. • The Doppler effect occurs for sound and light. • see physlet
ConcepTest Suppose a train is approaching you as you stand on the loading platform at the railway station. As the train approaches, it slows down. All the while, the engineer is sounding the horn at a constant frequency of 500 Hz. Describe the pitch and the changes in pitch that you hear.
Bow Waves • Bow waves are familiar if you have ever seen a boat zooming through water. • A Bow Waves is a V-shaped disturbance made by an object moving faster than the wave speed. • Bow waves occur on two-dimensional surfaces like water.
Shock Waves • Shock waves are three-dimensional bow waves shaped like a cone. • Shock waves occur when an object travels faster than the speed of sound in a fluid. • Such a super-sonic aircraft will produce a sonic boom.
Sonic Boom • A sonic boom is a loud sound resulting from the incidence of a shock wave. • As the shock wave passes, a person will experience a high pressure then low pressure region which will sound like a BOOM.
ConcepTest What are the differences & similarities between bow waves and shock waves? For a sonic boom to occur, what must occur?
Important Concepts • Origin of Sound • Nature of Sound in Air • Media That Transmit Sound • Speed of Sound in Air • Reflection of Sound • Refraction of Sound • Energy in Sound Waves • Forced Vibrations • Natural Frequency • Resonance • Interference • Beats