AAMC Physics Qpack
Doppler effect
, Δλ/λ = v/c
beta decay
0/-1 e.
1st or 2nd harmonic frequency, which has the shortest period?
2nd harmonic because period is 1/frequency= time. Since 2 nd harmonic is greater than the fundamental, it will have a short period.
Which of the following best describes the movement of an electron after it is ejected from the cathode?
A charged particle ACCELERATES in an electric field. The electron starts with a velocity that increases as it approaches the anode through the vacuum. ** f= Eq , meaning that there is a force involved. Hence there will be acceleration. The direction of the force is toward the cathodes whiles the electron accelerates towards the anode. DO NOT SAY CONSTANT VELOCITY.
pendulum
A device that swings back and forth due to the force of gravity.
Which of the following describes the direction of the magnetic force on an ion moving in an artery past a flowmeter?
A magnetic force acts on a moving charge in a direction that is perpendicular to both the velocity of the charge and the direction of the magnetic field. This is a basic law of the interaction of electric currents and magnetic fields.
Doppler effect
An observed change in the frequency of a wave when the source or observer is moving. Key: Observed cos the frequency is actually the same
What is the magnitude of the detected sound frequency shift from 170 Hz during the projectile flight described in the passage?
As the object moves up and slows down, the frequency shift is negative and falls to zero at the peak of the object's flight; as the object falls, the shift becomes increasingly positive.
isotopes
Atoms of the same element that have different numbers of neutrons. They have same atomic number
A block of weight W is pulled across a rough floor by a rope that exerts a force T on the block. The frictional force between the floor and the block is F. Which of the following expressions equals the frictional force F when the block moves with a constant speed? T cos O or TsinO
Because the object moves at a constant speed, the net force on the object is zero. In the horizontal direction, the sum of the forces must be zero. This will occur if the horizontal component of T, T cos θ, has the same magnitude as the frictional force F *. ie. the normal(cosO) is predominant since the friction virtually has no effect and it's moving at a constant speed. Constant speed = no net force = no NET acceleration, Therefore Fx must equal Ftx. Resolve T in x and Y. Tx = Tcostheta, Ty = Tsintheta. The answer is D. **think of you driving at 60mph, if you want to keep that speed, you don't need to accelerate more(net), you keep it the same.
A receiver is in a jet flying alongside another jet that is emitting 2.0 x 106 Hz radio waves. If the jets fly at 268 m/s, what is the change in frequency detected at the receiver?
Because there is no relative motion between the jets, there is no frequency shift. 0 hz
Capacitance
C=Q/V
When switch S is closed to the left, charge begins to accumulate on the capacitor. Charge cannot accumulate indefinitely because:
Charges cannot build up indefinitely in a capacitor. The two plates of capacitors collect opposite signs. As more charges arrive, it is harder to fill the plates as more charges arrive until an equilibrium is reached. Successive charges brought to the plate are repelled by charges collected earlier **key: the two plates collect opposite signs the rest is clearly stated.
Which of the following occurs when electrons are ejected from the cathode towards the anode?
Current exists when electrons can flow through the circuit. Current is literally charge traveling through a set distance(C/s) So the travel of electrons from the cathode to the anode allows for a continuous/unbroken flow of charge. If there was no movement of electrons between the electrodes, then the current would stop (similar to temporarily disconnecting the battery). ie. this enables current to flow through a circuit.
The converging lens in Figure 1 is removed and the diverging lens is placed in position B, as shown in the figure below. Which of the following best describes the light rays from the diverging lens in this configuration?
Each light ray will refract at each surface of the lens and will increasingly diverge from the others because of the shape of each surface, so only diverging rays can result. **I choose parallel thinking that they will continue in the same path but it is not true. DIVERGING RAYS ONLY AFTER REFRACTION
In order to determine the relative speed of approach of a sound source by Doppler measurements, three of the following items of data are necessary. Which one is NOT required?
Given that v is the speed of sound in the medium, the Doppler equation for a source that is approaching (receding from) an observer can be written as: fo=fs(v+vo/v+vs). Therefore you dont need distance.
what is the unit of voltage?
J/C
At a given temperature, the resistance of a wire to direct current depends only on the: voltage or resistivity?
Ohm's law is Voltage is a function of current and resistance. However that doesn't mean that resistance is determined by voltage and current. Resistance is an independent variable in Ohm's law. The way to determine resistivity of a wire is by using the resistance equals resistivity times length over area.
Sound of a known frequency, wavelength, intensity, and speed travels through air and bounces off an imperfect reflector which is moving toward the source. Which of the following properties of the sound remains the same before and after reflection?
Speed of sound depends on the medium it is in. Hence the answer is speed that remains constant. Intensity = P/A (in W/m^2) and is proportional to amplitude^2 . Also remember A of a sphere is 4pir^2. Intensity could change (attenuation) As a side note frequency would not change in a different medium, but could change with an imperfect reflector. As a side note: Wavelength and speed WOULD change in a different medium, and would also change with an imperfect reflector
For a given laser-cavity mode, the standing-wave pattern within a laser cavity can be viewed as a superposition of two traveling waves. Their amplitudes and directions are described by which of the following?
Standing waves are created from waves with equal amplitudes but moving in opposite direction
Period and Frequency
T=1/f. Think of period as a complete circle on the graph, not half. It's either the two small ones(up and down)or the big whole in the middle.
A small negatively charged particle is placed near a fixed positively charged particle (Q). Which of the following describes the motion of the negatively charged particle?
The Coulomb force between the negative and positive charges is attractive. That force accelerates the negatively-charged particle toward the positively charged particle. **anywhere you can think about force(+ and-), then think of acceleration.
Why are the percentages of the change in frequency and wavelength much greater when sound waves are used instead of radio waves in these experiments?
The Doppler equation for frequency is Δf/f = -v/c for a given relative velocity between source and detector. Thus, the frequency shift Δf depends inversely on the speed of the wave in the medium in which it propagates, c. The velocity of sound is much smaller than that of electromagnetic radiation, so for the same relative velocity, the frequency and wavelength shifts are much greater for sound than for radio waves. ***Sounds travels slower than any EM.
Which action requires a larger absolute value of work: lifting the weight from A to B with constant speed, or lowering the weight from B to A with the same constant speed?
The absolute value of the work done is mgΔh where Δh is positively defined, and because none of these values changes in magnitude when the mass goes up or down, these actions involve the same amount of work. **Work = mgh. Same work going down or up!
After a block began to slide, how did its speed vary with time? (Note: Assume that the tension and kinetic friction forces on the block were constant in magnitude.)
The coefficient of kinetic friction is always lower than that of static friction. Therefore there is a net accelerating force on the block once it starts to slide. A constant force on a mass produces a constant acceleration (Newton's second law). Thus, the velocity of the block increases linearly with time. ** i was thinking that 1/2mV^2 but i did not pay attention to word CONSTANT. VERY IMPORTANT.
Which of the following describes the direction of motion of alpha, beta, and gamma rays in the presence of an external magnetic field?
The examinee must decide how alpha particles, beta particles, and gamma rays will be influenced when they move in a magnetic field. Magnetic fields exert a force on moving electric charge. The magnitude of the force is directly proportional to the electric charge and particles of opposite sign experience forces of opposite direction. Gamma rays possess zero charge, and thus experience no force from a magnetic field and will travel in a straight line. Alpha and beta particles possess charges of opposite sign, and thus will experience forces in opposite directions- so their trajectories will bend oppositely. **B=qvBsino, look at the q
What best describes changes that occur as the electron sea moves from position A to position B in Figure 1?
The figure for this question shows three stages of the movement of the electron sea around the protons. A maximum displacement, an equilibrium (all electrons and protons in maximum contact = thermodynamic happy place), and a minimum displacement - should make you think of a spring oscillation. We know that since energy is neither created or destroyed, it can only be converted. As the "spring" moves from a maximum displacement (electrons farthest away from protons, which is a position high in potential energy) to equilibrium, this potential energy is converted to kinetic energy during that movement such that at the max vs at the equilibrium, 1/2kx^2 = 1/2mv^2. **Max displacement or further away, think of PE. **I quickly choose the opposite because once I saw movement, I choose kinetic energy but that I was wrong way to think. First see if there is a displacement, if yes, then PE is existing first.
An astronaut on Earth notes that in her soft drink an ice cube floats with 9/10 of its volume submerged. If she were instead in a lunar module parked on the Moon where the gravitation force is 1/6 that of Earth, the ice in the same soft drink would float:
The floating ice cube implies that its weight is balanced by the buoyant force on it Wice = mg = rfluid*Vsubmerged*g Note that both the weight and the buoyant force are proportional to g, making the numerical value of g irrelevant to the volume of the ice cube that is submerged. Thus, B is the best answer.It will still have 9/10 submerged. I was thinking that lower gravity(based on just buoyant force) means less will be submerged but it lead me astray. It is balanced by two forces.
constructive interference
The interference that occurs when two waves combine to make a wave with a larger amplitude
destructive interference
The interference that occurs when two waves combine to make a wave with a smaller amplitude. It is 180 out of phase
If the magnitude of a positive charge is tripled, what is the ratio of the original value of the electric field at a point to the new value of the electric field at that same point?
The magnitude of the electric field E of a point charge is given by E=kq/r If q is tripled, E also will be tripled. Thus, B is the best answer.
Vacuum Phototube
The next set of questions will depend on this
Increasing the frequency of each photon that is directed at the cathode will:
The only effect the photon frequency has on the ejected electron is on its kinetic energy. Photon energy equals cathode work function plus electron kinetic energy. The number of electrons ejected (the current) depends on the number of incident photons. ie. you have to see electron as current in some way due to the previous experience.
Kinetic energy and photoelectric effect
The photoelectric effect causes electrons to be ejected from a metal plate when photons of light are absorbed by the metal. The energy of a photon is given by the equation E = hf, where h = 6.6 x 10-34J·s (Planck's constant), and f is the frequency of the photon. To free an electron, the energy of a photon must be GREATER THAN(NOT EQUAL) than a quantity called the work function of the metal. The ejected electron will have a kinetic energy equal to the photon's energy minus the work function.
What is the approximate number of wavelengths of light that can travel in 1 direction within a retroreflecting bead that has a diameter of 5 × 10-5 m? (Note: The speed of light = 3 × 108 m/s, and its frequency is approximately 1015Hz.)
The wavelength λ of the light may be found from the relationship for wave speed v = λf, thus, λ= v/f = (3.0 × 108 m/s)/( 1015 Hz) = 3.0 × 10-7 m. The number of waves that will fit along a diameter is found by dividing this wavelength into the diameter D = 5.0 × 10-5 m, so the number of waves is D/λ =(5.0 × 10-5 m)/( 3.0 × 10-7 m) = 1.7 × 102. **diameter threw me off
Assume that the density of Ball 1 is 8.0 × 102 kg/m3. Ignoring the atmospheric pressure, what fraction of Ball 1 is above the surface of the water? 1000kg/m^3
There are two formulas you could use to solve this problem. The easier route is Fraction Submerged = Object density/Fluid density. Density is less than that of water. d(ball)/d(water) = V(displaced)/V(ball) =D1/D2 = V2/V1 = 8*10^2/1*10^3 = V2/V1 = 8/10 = 4/5 IN WATER and hence 1/5 OUT OF WATER . Final answer 1/5. ** I didn't know the in water is the real formula and out of water is 1-that answer.
1st harmonic (fundamental)
This the fundamental. Do not forget
Visible light travels more slowly through an optically dense medium than through a vacuum. A possible explanation for this could be that the light:
Visible light travels more slow in optically dense media because it is absorbed and reemitted by the ATOMIC structure of the optically dense medium. It's not due to nucleus. Think of electrons only cos thats what is excited.
electric field
a field of force surrounding a charged particle. V=ED Unit of an electric field N/m. The unit is based on another derivation
The intensity level of Sound B is 20 dB greater than the intensity level of Sound A. How many times greater is the intensity level of Sound B than the intensity level of Sound A?
always drop the 10, remember that 10 log 10 =1. Therefore the racket components must be in 10^something. answer=100 10log(i/io)=20. dropping 10 in front, you must find 10^2 to make it 20 at the end.
right hand rule
hand on plane with forefingers pointing B and thumb pointing qv, F will come out of palm
What physical quantity is represented by the area of quadrilateral OABC in Figure 2?
it said area not slope. Hence 1/2b*h = distance
oscillation
the act or state of swinging back and forth with a steady, uninterrupted rhythm. Therefore a spring is a pendulum
2nd harmonic frequency
two times the resonant frequency/ fundamental frequency.
a spring pendulum is
undergoes oscillations. Therefore it's PE= 1/2 kx^2, where x is displacement. Please note, you also use the same formula for something that moves back and forth. eg. if electrons move back and forth, it's PE can be calculated by this.
cross sectional area of artery
xr^2
