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THE MCAT-PHYSICS 4
WAVES
| Question | Answer |
|---|---|
| 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 |
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