Longitudinal Waves. Waves Waves transfer energy without transferring matter A pulse is a single disturbance A periodic pulse creates a wave

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1 Review: 1. A ball dropped from rest falls freely until it hits the ground with a speed of 20 meters per second. The time during which the ball is in free fall is approximately 2. In a flashlight, a battery provides a total of 3.0 volts to a bulb. If the flashlight bulb has an operating resistance of 5.0 ohms, the current through the bulb is Waves Waves transfer energy without transferring matter A pulse is a single disturbance A periodic pulse creates a wave Two groups of waves 1. Mechanical waves sound, ocean waves, etc. 2. Electromagnetic waves light, radio, microwaves, x rays, etc. Waves can be transverse or longitudinal Transverse Waves The vibration or motion of a particle is perpendicular to the motion of the wave Examples of transverse waves: Ocean waves, Electromagnetic (EM) waves, strings Longitudinal Waves The vibration or motion of the particle is parallel to the motion of the wave Examples of longitudinal waves: Sound Longitudinal Waves Compression Rarefaction 1

2 Characteristics of periodic waves Period Speed of a wave Frequency Amplitude Wavelength 1. What is the period of a 60. hertz electromagnetic wave traveling at meters per second? 2. A wave completes 5 cycles in 2 seconds over a dista nce of 3m. a) What is the wavelength? b) What is the frequency? c) What is the period? d) What is the speed of the wave? 3. What is the frequency of a wave with a wavelength of 500 meters if it travels at a velocity of 25 meters per second? What is its period? Page 5 review questions What is the magnitude of the centripetal force on a 2 x 10 3 kg car as it goes around a 25 meter curve with an acceleration of 5 m/s 2? 7. A 0.5 kg ball is thrown vertically upward with an initial kinetic energy of 25 joules. Approximately how high will the ball rise? 2

3 Simple Harmonic Motion Masses go to and fro Oscillation a periodic variation from one state to another Simple harmonic oscillator an object that repeats a period motion Potential energy Kinetic energy Total Energy Question: A pendulum completes 10 swings in 15 seconds. What is the period (T) of the pendulum in seconds? What is the frequency (f) of the pendulum in Hz? Question: A mass on a spring oscillates at 3.2 Hz. What is the period of the system (in seconds)? How long will it take the system to complete 120 oscillations? Pulses Wave Building Blocks A pulse is a single vibratory disturbance that transfers energy but NOT mass. Amplitude is the height of a pulse above equilibrium shows ENERGY A medium is a continuum of particles of a single type ex: Water, air, metal rod, etc. The medium stores then releases energy as pulses travel through it. Pulse Periodic Wave When pulses change mediums, they are Interference Pulses that encounter one another in a medium interfere Pulse amplitudes are added together using the principle of superposition Two types of interference: Constructive Interference Occurs when amplitude of two pulses is in the same direction Energy of the two pulses is added together to produce a higher amplitude 3

4 Destructive Interference Occurs when pulse amplitudes are in opposite directions ENERGY of the two pulses partially or completely cancel out to produce a lower amplitude. 8. If 10 coulombs of charge are transferred through an electric circuit in 5.0 seconds, then the current in the circuit is 9. In driving down a hill a car loses 500 joules of potential energy but gains 1500 joules of kinetic energy. If the work done to overcome friction was 200 joules, determine the total work done by the engine to go down the hill. Sound Waves A speaker pushes air forward and backward. The soun wave travels until it reaches your ear. What type of wave is the sound wave? Through what medium is the sound travelling? What would happen to a sound wave in a vacuum? The amplitude of the sound wave is also the: Are the air molecules that bounce off the speaker the same ones that reach your ear? Sound travels through air at 331 m/s Sound travels faster through denser mediums: Speed of sound in water Speed of sound in steel Humans can hear sound from 20 Hz to 20kHz. Frequency = Which strings will have a longer wavelength? Which will have a higher pitch? v = f λ What happens to the wavelength when you make a trombone longer? What happens to the pitch? 4

5 1. At STP, sound with a wavelength of 1.66 meters has frequency of: 4. If the speed of a wave doubles, its wavelength will be: 2. Middle C has a frequency of 262 Hz. What is the wavelength of middle C in air? 5. What is the period of a sound wave with frequency 600 Hz? 3. What is the wavelength of middle C in water? 6. If the period of a sound wave is doubled, its frequency will be: 7. A sound wave with period 5.5 x 10 2 s is travelling through the air. What is the wavelength of the sound wave? 9. You see lightning, and 5 seconds later you hear thu How far away from you did the lightning strike? 10. Mr. Vigneaux yells Echo across the grand canyo 2.5 seconds later, he hears his echo. What is the width the canyon where he is yelling? 8. How much time does it take light from a flash camer to reach a subject 6.0 meters across a room? Page 11 review questions When 800 joules of work ar done on an object, its potential energy increases by 300 joules and its kinetic energy increases by 400 joule. What was the work done against friction? 22. In a seris circuit containing two lamps, the battery supplies a potential difference of 1.5 volts. If the current in the circuit is 0.1 amperes, at what rate does the circuit use energy? 5

6 Wave Interference Waves are superimposed like pulses Resonance Resonance occurs when a medium is vibrated near its Constructive interference occurs to an extreme degree causing vibrations with huge amplitude If the vibration has enough energy and the correct frequency, the medium may be damaged or destroyed! Examples of resonance: When you push somebody on a swing, you push with the same frequency as the swing to get maximum amplitude Color is due to incoming light matching the natural frequency of electrons orbiting the proton in an atom Instruments create a standing wave at a natural frequency to create a note. Some instruments allow you to change the length of the instrument to change the frequency and allow more notes! 6

7 23. What is the approximate electrostatic force between two protons separated by a distance of 1.0 x 10 6 m? Wave Fronts A wave front is made up of all points on a wave that are vibrating in phase (in unison) Wave fronts are separated by one wavelength. 24. What is the total resistance of the circuit? Rays can be used to represent the direction that a wave is travelling in. Rays are drawn perpendicular to wave fronts. The Doppler Effect The apparent change in the of a wave when the source and observer are moving relative to one another When the source and observer are approaching each other, the observed frequency is than the actual frequency of the wave. When the source and observer are moving away from each other, the observed frequency is than the actual frequency of the wave. Note that this change in frequency is only apparent. The actual frequency of the wave depends only on the For light the Doppler shift is observed as a (higher apparent frequency) or (lower apparent frequency All waves that reach a boundary can be reflected, refracted (transmitted), absorbed, and/or scattered. Reflection A wave that reaches a boundary can be reflected Light mirror Sound echo Law of reflection: (NOTE: All angles are measured from the normal to the surface) 7

8 Refraction (transmission) When a wave enters a new medium, there is a change in angle of the wave due to the change in speed. Medium is the material through which a wave is travelling (air, glass, water, etc.) Fast medium to slow medium Wave bends Slow medium to fast medium Wave bends Wave incident along the normal change in but no change in Change in speed of the wave results in a change in. The of the wave is unchanged! (Depends only on the source creating the wave) Diffraction The of a wave to travel around corners, obstacles, or openings in its path How much a wave diffracts depends on how the wavelength compares to the size of the opening (smaller opening, more diffraction) Huygen s Principle: Every point on the wave acts as a new point source 8