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# THE MCAT-PHYSICS 4

### WAVES

QuestionAnswer
What does a wave transfer? momentum and energy from one point to another
Can waves transfer matter? NO
Relate v, f, and lambda v = f * lambda
What are the types of mechanical waves? transverse and longitudinal
transverse wave and examples medium displaced perpendicular to wave propagation (light and EM radiation)
longitudinal wave and example medium displaced parallel to wave propagation (sound)
What is the final displacement of a longitudinal wave? 0 --> moves back and forth but still 0
What affects velocity? MEDIUM 1.) forces between molecules 2.) density of particles in medium --> stronger forces, greater restoring force, faster to participate in anther compression wave, faster propagation --> velocity constant for a string
What affects velocity 1.) medium's resistance to change in shape (elasticity) 2.) medium's resistance to change in motion (inertia)
What kind of string moves fastest transverse wave? tense and light
Why does velocity increase with temperature for a gas? Greater the temperature, greater the random velocity, greater the sound wave velocity --> air particles move more and allow better propagation of sound waves
Is density a function of temperature? YES --> volume changes
How does the speed of a sound wave increase / decrease? -increases: higher stiffness (resistance of an elastic body to deformation) and higher pressure -decreases: density
Compare speed of sound in solid, liquid, gas solid > liquid > gas
Does sound wave travel faster in water or air? water --> water is denser but has a greater bulk modulus
What is the velocity of sound waves in a gas limited by? average speed of molecules within that gas --> sound waves move faster through hot gases than cold gases
surface waves "gravity waves" --> gravity is elastic part -mass / density of liquid does not change rate at which surface wave rises and falls -velocity is not changed by density of liquid
Why do waves move faster at higher depths? higher depths have higher pressure
What happens in a dispersive medium for velocity? velocity related to frequency and increases with wavelength
What is power? rate at which wave transfers energy aka intensity
What do frequency and amplitude depend on? wave source
What do density and velocity depend on? medium
What does energy per photon depend on solely? wavelength NEVER amplitude or intensity
Decibals equation B = 10 log (I / Io)
Decibals rule If intensity increases by a factor of 10, decibals increase by addition of 10 decibals
What do constructive and destructive interference have in common? waves have same frequency
What are beats? two waves with slightly different frequencies are superimposed
When a wave transfers from one medium to the next, what happens to wavelength, amplitude, and frequency? smaller amplitude, wavelength changes and the frequency remains the same
What happens when wave reflects off a denser medium? Original and new wave Original wave is inverted 180 degrees and new wave continues along in present orientation with same frequency, smaller amplitude, different wavelength
What happens when wave reflects off a less dense medium? Original and new wave Original upright and new wave also upright with same frequency, smaller amplitude, different wavelength
What is beat frequency? f2 - f1
What is the first harmonic def and aka? -fundamental harmonic -longest wavelength that can be generated
Another name for second harmonic? 1st overtone
What is node? areas with no vibration found at a fixed end
What is antinode? area where amplitude is largest
Discuss wave speed in a uniform medium. Always Constant!!
Relate wavelength, length of string, nodes wavelength = 2L / n
Relate frequency, nodes, velocity, string length f = nv / 2L
Relate period, length, node, velocity T = 2L / nv
What can increase the amplitude and energy of a system? apply vibrating force with frequency that matches natural frequency of a system
Resonance frequencies for open pipe on both ends (1,2,3) L = lambda / 2 ; L = 2lambda / 2 ; L = 3lambda / 2 (count nodes)
Resonance frequencies for closed pipe on both ends (1,2,3) L = lambda / 2; L = 2lambda / 2; L = 3lambda / 2 (count antinodes)
Resonance frequencies for pipe open on one end (1,2,3) L = lambda / 4 ; L = 3lambda / 4; L = 5lambda / 4 (count complete nodes and antinodes)
What can't sound waves travel through? vacuum
Go from 60 dB --> 20 dB, how much does intensity change by? 10,000
General rule of thumb relating distance, intensity, and dB Every time distance from source increases by a factor of 3, intensity drops by a factor of 10, dB decreases by 10
What creates the Doppler Effect? waves are unaffected by speed of source that produces them
Formulas for Doppler Effect? delta f / fs = v / c delta lambda / lambda(s) = v/c -v = relative velocity -c = wave velocity
What do you do with velocities when source and observer are moving in same direction? subtract
What do you do with velocities when source and observer are moving in opposite directions? add
What happens when relative velocity brings source and observer closer? wavelength decreases, frequency increases -add delta f to f(s) -subtract delta wavelength from wavelength (s)
What happens when relative velocity brings source and observer farther? wavelength increases, frequency decreases -subtract delta f from f(s) -add delta wavelength to wavelength (s)
blue shift when source and observer move closer to each other
red shift when source and observe move apart
What happens when objects move in same direction at same speed? no doppler effect
Mechanical waves ONLY transport_____? energy
Within still air, when sound bounces off an imperfect reflector, what remains constant? speed
Light speeds up / slows down when passing into something with higher "n". slows down (both must be solids, liquids, gases)
Sound speeds up / slows down when passing into something with higher "n" speeds up --> speeds up with elastic, stiff bonds
What creates a faster wave? greater elasticity
What type of a wave is an EM wave? transverse --> electric and B-fields are oscillating at right angles
When light goes from air to a medium of higher index, does velocity, wavelength, or frequency stay the same? frequency -velocity slows down and wavelength decreases
When sound goes from less dense to more dense material, does it speed up or slow down? slow down
Within a uniform medium, what always remains constant for a sound wave? what changes? -velocity is ALWAYS constant and frequency
Why do sound waves travel slower in more dense material? When comparing 2 solids, 2 liquids, etc, speed of sound decreases with increasing density
What does speed of sound depend on? -resistance to compressibility of the medium (elasticity) -density
What is wavelength? length to finish one complete cycle
What is frequency? number of oscillations per second
Generally, what remains constant for light vs. sound? frequency vs. speed
What creates rainbows? refraction by water droplets
Do standing wavs propagate? NO
For EM waves, amplitude and intensity increase what? Do they change energy per photon? -overall energy of EM wave -does NOT change energy per photon --> depends on wavelength
What is formula for angular frequency? w = 2*pi*f
When waves are in phase, how many degrees are they apart? 0 or 360 degrees
When waves are out of phase, how many degrees are they apart? 180 degrees
Hooke's Law F = -k * x
PE and KE of a spring PE = 1/2kx^2 KE = 1/2mv^2
At the equilibrium position, describe x, PE, KE, and a. x = 0, PE = 0, KE = max, a = max
At max displacement, describe x, PE, KE, and a x = max, PE = max, KE = 0, a = 0
Period of a pendulum T = 2*pi(square root of L/g)
Period of spring T = 2*pi(square root of m/k)
For ANY wave to go from one medium to another medium, what remains constant? frequency!
Through a prism, does a longer wavelength travel faster or slower? faster --> freq stays constant!
Through a prism, does a shorter wavelength travel faster or slower? slower --> freq stays constant!
When is Doppler effect produced? anytime source moves relative to the receiver
What creates higher pitch? higher frequency
Original Doppler Equation fo = fs [c+/-vo // c+/-vs)
Source and observer both moving toward each other fo = fs (c+vo / c-vs)
Source and observer both moving way from each other fo = fs (c-vo / c-vs)
In dampened harmonic motion, does frequency and period stay the same? yes
In dampened harmonic motion, what decreases? energy diminishes over time
What is a period? time to complete one full cycle (return to original position)
If no force is acting on a spring, then what state is it in? equilibrium
Under dampened harmonic motion, does speed decrease? YES --> lose energy --> lose KE and PE --> KE decrease, velocity decrease
Hooke's Law F = -Kx
In what direction does the force of a spring act in terms of displacement? opposite
PE of a spring 1/2 kx^2 = PE
When is tension greatest in a pendulum? bob points down
What is restoring force for a pendulum? F = -mgsintheta
PE of a pendulum? U pendulum = -mgL (1 - costheta)
As a beat frequency gets smaller, does it become more or less noticeable? more noticeable
With an oscillating particle non-harmonic, where is the speed fastest? equilibrium position
With second harmonic mode oscillation, where is wave speed the fastest? antinode
With second harmonic mode of oscillation, where is speed of string the fastest? antinode
Created by: 507935299

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