Physics 106 - How Things Work - Spring, 1998
PART I: MULTIPLE CHOICE QUESTIONS
Please mark the correct answer for each question on the bubble sheet.
Fill in the dot completely with #2 pencil. Part I is worth 67% of the grade
on this examination.
A rubber car tire cannot be melted for recycling because
(A) it contains so much stored elastic potential energy that it spontaneously
bursts into flames shortly after transforming into a liquid.
(B) all black materials, including soot, charcoal, graphite, and car
tires, don't melt.
(C) it is actually one enormous molecule and cannot become a liquid.
(D) the molecules from which it's composed do not form crystals and
only crystalline materials can melt to form liquids.
Answer: (C) it is actually one enormous molecule and cannot become
One way to thicken a sauce that you are cooking is to add starch to
it. The added starch prevents the sauce from flowing easily because starch
(A) are huge chain-like or tree-like structures that entangle one another
and bind the sauce together.
(B) vibrate more strongly than most molecules and absorb energy from
the rest of the sauce. Without its usual thermal energy, the sauce becomes
thicker because it doesn't move easily.
(C) are magnetic and form magnetic domains within the sauce. These domains
inhibit slip and make it difficult for the sauce to deform.
(D) are electrically charged, positive and negative, and attract one
another with electrostatic forces throughout the sauce.
Answer: (A) are huge chain-like or tree-like structures that entangle
one another and bind the sauce together.
For a nuclear weapon to explode normally, the fissionable material in
its core must be assembled very quickly. For technical reasons, a nuclear
bomb developed by a terrorist group would probably not achieve such rapid
assembly. As a result of its slow assembly, such a bomb would
(A) explode powerfully, but at a relatively unpredictable moment. To
shorten the time window over which the explosion could occur, external
nuclear triggers would be needed.
(B) not explode because there would not be enough momentum present to
push the neutrons into the fissionable nuclei of its core.
(C) overheat and push itself apart during the assembly, producing only
a weak explosion.
(D) not explode because there would not be enough energy present to
push the neutrons into the fissionable nuclei of its core.
Answer: (C) overheat and push itself apart during the assembly, producing
only a weak explosion.
A bright beam of light from a laser passes through a tiny pinhole on
its way to a projection screen on the other side of the room. Since the
beam was several millimeters in diameter, the pinhole blocks most of the
beam's light. After passing through this pinhole, the remaining light
(A) converges together rapidly to form an image of the laser on the
(B) forms an extremely narrow beam that illuminates only a tiny spot
on the projection screen-a spot the same diameter as the pinhole.
(C) changes frequency and color because it experiences destructive interference
inside the pinhole.
(D) spreads relatively rapidly so that it forms a rather large spot
on the projection screen.
Answer: (D) spreads relatively rapidly so that it forms a rather
large spot on the projection screen.
You are hosting a romantic candlelight dinner in your room, but you
forgot to buy the candles and all the stores are closed. Your desk lamp
is too white and too bright to provide the right ambience, so you modify
it by inserting a diode between one of the power wires and the bulb. As
the result of this change, the lamp emits a reddish glow that's roughly
half a bright as without the diode. Just for fun, you reverse the diode
so that the end that used to connect to the AC power wire is now connected
to the bulb and vice versa. As the result of this reversal, the lamp
(A) emits a deep reddish glow that is roughly half as bright as before
you reversed the diode and a quarter as bright as without the diode at
(B) no longer glows at all because no current flows through the diode.
(C) still emits the same reddish glow that it emitted before you reversed
(D) emits a brilliant bluish glow that's roughly twice as bright as
without the diode.
Answer: (C) still emits the same reddish glow that it emitted before
you reversed the diode.
A softball weighs about twice as much as a baseball. Suppose that a
softball and a baseball roll off a horizontal table at the same speed and
soon hit the horizontal floor. In that case, the
(A) two balls will land at about the same distance from the edge of
(B) lighter baseball will land about half as far from the edge of the
table as will the heavier softball.
(C) heavier softball will land about half as far from the edge of the
table as will the lighter baseball.
(D) heavier softball will land much, much closer to the edge of the
table than will the lighter baseball.
Answer: (A) two balls will land at about the same distance from the
edge of the table.
While low pressure sodium vapor lamps are the most energy efficient
light sources commonly available, producing a pure orange-yellow light
with a wavelength of 590 nanometers, high pressure sodium vapor lamps produce
a richer spectrum of light that is more pleasing to the eye. One effect
that contributes to this richer spectrum is
(A) interference-in which light traveling to your eye via several paths
within the lamp becomes brighter or darker, depending on how its electric
fields interact with one another.
(B) radiation trapping-in which the 590 nanometer light has trouble
escaping from the lamp.
(C) thermionic emission-in which electrons in the hot electrodes enter
the gas of the discharge.
(D) dispersion-in which the different colors of light travel at different
speeds within the lamp.
Answer: (B) radiation trapping-in which the nanometer light has trouble
escaping from the lamp.
Increasing the diameter of a camera lens, while keeping its focal length
unchanged, does more than just brighten the image on the film so that you
can use a shorter exposure. It also
(A) decreases the depth of focus so that objects at different distances
will no longer all be in focus at once.
(B) increases the size of the image on the film so that you get a close
(C) decreases the size of the image on the film so that you get a wide
(D) increases the depth of focus so that objects at different distances
will all be in focus at once.
Answer: (A) decreases the depth of focus so that objects at different
distances will no longer all be in focus at once.
You are pushing a file cabinet across the floor in a straight line at
a steady speed. Which of the following statements about the forces acting
on the file cabinet is correct?
(A) The magnitude of the force that you're exerting on the file cabinet
must be more than the magnitude of the force that friction is exerting
(B) The magnitude of the force that you're exerting on the file cabinet
must be equal to the magnitude of the force that friction is exerting on
(C) The amount of forward force you are exerting on the file cabinet
must be more than its weight.
(D) If you were to exert twice as much force on the file cabinet, it
would slide across the floor at twice its original speed.
Answer: (B) The magnitude of the force that you're exerting on the
file cabinet must be equal to the magnitude of the force that friction
is exerting on it.
You drop your boom box and one of the two wires that connects the batteries
to the electronics is severed. While you might hope that the device would
continue working at half its normal volume, you find that it doesn't work
at all. That's because both wires are necessary for the boom box to operate.
One wire carries
(A) current to the electronics and the other wire returns that current
to the batteries.
(B) electric current to the electronics while the other wire carries
magnetic poles to the speakers.
(C) electric fields to the electronics and the other wire carries magnetic
fields to the electronics.
(D) current to the amplifier while the other wire carries current to
Answer: (A) current to the electronics and the other wire returns
that current to the batteries.
The energy that is released by a nuclear weapon was put into the weapon's
fissionable core in the process of
(A) heating and melting the fissionable materials when the core is being
(B) assembling each of the core's fissionable nuclei from smaller nuclear
(C) separating the core's fissionable nuclei from the non-fissionable
nuclei with which they are normal found.
(D) raising the core's fissionable materials from deep in the earth
where they were mined.
Answer: (B) assembling each of the core's fissionable nuclei from
smaller nuclear pieces.
In which one of the following situations are you doing (positive) work
on a sack of flour?
(A) When you carry it horizontally at constant velocity.
(B) When you lower it downward at constant velocity from a high shelf
to the floor.
(C) When you drag it horizontally across the floor at constant velocity.
(D) When you hold it motionless above your head.
Answer: (C) When you drag it horizontally across the floor at constant
Many simple forms of stainless steel cannot be hardened by heat treatment,
even though they contain significant amounts of carbon. Regardless of temperature,
these simple stainless steels have an austenite crystal structure, the
same crystal structure that normal steel has when it is very hot. Carbon
doesn't harden austenitic steels because
(A) carbon forms iron carbide or "cementite" in austenitic steels.
(B) carbon forms carbon dioxide in austenitic steels.
(C) carbon is non-magnetic in austenitic steels.
(D) carbon dissolves easily in austenitic steels.
Answer: (D) carbon dissolves easily in austenitic steels.
You are throwing a ball straight up and then catching it as it returns
to your hand. When the ball leaves your hand, its momentum is in the upward
direction but when it returns to your hand, its momentum is in the downward
direction. During its flight above your hand, what happens to the ball's
initial upward momentum?
(A) The upward momentum is converted into kinetic energy.
(B) The upward momentum is transferred to the earth.
(C) The upward momentum is converted into thermal energy.
(D) The upward momentum is converted into gravitational potential energy.
Answer: (B) The upward momentum is transferred to the earth.
A good refracting telescope must use more than one glass lens element
to form a real image because
(A) it takes two lens elements to form a real image: one to converge
the rays and one to stop the convergence right at the focus.
(B) the magnification obtainable with a single element telescope is
so small as to be essentially useless.
(C) dispersion in a single element telescope will cause different colored
lights from the same object to focus at different locations.
(D) a single lens element can only form a virtual image and it takes
a second lens element to turn that virtual image into a real image.
Answer: (C) dispersion in a single element telescope will cause different
colored lights from the same object to focus at different locations.
If you put an egg in a microwave oven, it will become very hot. But
if you put a dehydrated egg (a dried egg containing no water) in the microwave
oven, the egg won't be affected because
(A) the dehydrated egg is too dry to have a temperature. Only foods
containing water or oil can actually become hot.
(B) the dehydrated egg reflects the microwaves while the normal egg
(C) microwaves transfer heat to food principally through its water molecules.
(D) the dehydrated egg is much less dense than a normal egg and doesn't
conduct heat well.
Answer: (C) microwaves transfer heat to food principally through
its water molecules.
The large sugar crystals found in rock candy are essentially clear.
However granulated sugar looks white because
(A) small sugar grains can absorb black light, leaving only white light
to be reflected.
(B) some light is reflected whenever light moves from a sugar granule
to air or vice versa. The randomly oriented granules reflect light in all
(C) small sugar grains are more dense than large sugar crystals, and
they fluoresce white light when exposed to infrared radiation.
(D) small sugar grains are metallic and act as mirrors, scattering light
in all directions.
Answer: (B) some light is reflected whenever light moves from a sugar
granule to air or vice versa. The randomly oriented granules reflect light
in all directions.
A makeup mirror is concave, meaning that it is shaped like a round-bottomed
soup bowl that is being viewed from above the dinner table. When you hold
the makeup mirror a few inches from your face and look at your reflection,
you see an enlarged virtual image of your face. Because of its curved shape,
the mirror is bending the reflected light rays so that they diverge
(A) less rapidly and appear to come from a distant virtual image.
(B) more rapidly and appear to come from a distant virtual image.
(C) less rapidly and appear to come from a nearby virtual image.
(D) more rapidly and appear to come from a nearby virtual image.
Answer: (A) less rapidly and appear to come from a distant virtual
The tubes in fluorescent lamps have a white powder on their inner surfaces.
(A) slows down the white light produced by the gas atoms inside the
tube so that less of that white light reflects as it passes through the
walls of the tube.
(B) conducts electricity from one electrode to the other and emits white
light as current passes through it.
(C) absorbs ultraviolet light and uses the energy from that light to
produce white light.
(D) scatters the white light produced by the gas atoms inside the tube
so that it is both vertically and horizontally polarized and doesn't cause
Answer: (C) absorbs ultraviolet light and uses the energy from that
light to produce white light.
At present, a compact disc can only hold about 1 hour of music. This
limit is due in large part to the fact that
(A) a compact disc can't turn faster than about 5 turns per second without
experiencing a net torque of zero.
(B) light can't be focused to a spot that is significantly smaller than
(C) the ridges in the aluminum layer of the compact disc can't experience
destructive interference unless they have a wavelength that is almost equal
to the distance between them.
(D) a compact disc can't turn slower than about 5 turns per second without
experiencing a net torque of zero.
Answer: (B) light can't be focused to a spot that is significantly
smaller than its wavelength.
Even when the core of a nuclear reactor exceeds critical mass by a small
amount, it doesn't overheat immediately. The fission chain reactions build
(A) the reactor has an enormous thermal mass and it takes many minutes
for its temperature to change significantly, even with nuclear heating.
(B) the half-life of uranium is several minutes so it takes at least
that long for the reactor core to overheat.
(C) it takes seconds or minutes for a neutron released by one fission
to reach another radioactive nucleus and cause it to fission.
(D) some fission neutrons aren't released until seconds or minutes after
the fissions that eventually produce them.
Answer: (D) some fission neutrons aren't released until seconds or
minutes after the fissions that eventually produce them.
Three different lasers are emitting steady beams of light. Each beam
consists of countless copies a single original photon of light. However,
not all colors can be emitted by a laser. Which of the following choices
is possible for these three lasers?
(A) yellow, green, and blue light
(B) green, blue, and white light
(C) blue, white, and yellow light
(D) white, yellow, and green light
Answer: (A) yellow, green, and blue light
The colors that you see when sunlight strikes a cut crystal wineglass
or a cut diamond are a result of the fact that two light waves with
(A) different wavelengths travel at different speeds in most materials.
(B) the same wavelength in air can have different frequencies in a material.
(C) the same frequency in air can have different wavelengths in a material.
(D) the same wavelengths can have different colors.
Answer: (A) different wavelengths travel at different speeds in most
If you "cook" a compact disc briefly in a microwave oven,
(A) it becomes hot enough to soften and sparks leap about its surface.
(B) it vibrates back and forth violently in response to the fluctuating
(C) nothing significant happens because it contains no organic materials.
(D) it becomes permanently magnetized.
Answer: (A) it becomes hot enough to soften and sparks leap about
Cotton fibers are essentially pure cellulose, a glassy, crystalline
polymer formed from sugar molecules. One reason for spraying water on a
cotton shirt just before you iron that shirt is that the water
(A) reflects some of the thermal radiation from the iron so that the
cotton doesn't overheat and cross-link ("vulcanize") during ironing.
(B) absorbs microwaves from the iron so that the fabric becomes hotter
and flattens more completely during ironing.
(C) removes electrons from the cellulose so that it becomes magnetic
and is attracted to the steel bottom of the iron during ironing. This attraction
improves the flattening effects of ironing.
(D) acts as a plasticizer, lubricating the polymer molecules so that
they can rearrange easily during ironing.
Answer: (D) acts as a plasticizer, lubricating the polymer molecules
so that they can rearrange easily during ironing.
Compared to the radio waves used in normal radio broadcasts, the microwaves
used for satellite transmissions have
(A) higher frequencies and shorter wavelengths.
(B) higher frequencies and longer wavelengths.
(C) lower frequencies and shorter wavelengths.
(D) lower frequencies and longer wavelengths.
Answer: (A) higher frequencies and shorter wavelengths.
Electricity produced in a generating plant passes through a large step-up
transformer. This step-up transformer produces the high voltages needed
to send electric power long distances across the countryside. Which of
the following is transferred from the transformer's primary coil to its
secondary coil while the transformer is operating?
(A) Negative electric charges and power.
(B) Positive electric charges, negative electric charges, and power.
(C) Positive electric charges and power.
(D) Power alone.
Answer: (D) Power alone.
The power company uses a step-up transformer to prepare electric power
for delivery to a distant city. The secondary coil of this transformer
has many more turns than its primary coil. As a result, the transformer's
(A) has a much higher current but a much lower voltage than its primary
(B) has a much lower voltage and a much lower current than its primary
(C) has a much higher voltage but a much lower current than its primary
(D) has a much higher voltage and a much higher current than its primary
Answer: (C) has a much higher voltage but a much lower current than
its primary circuit.
Which of the following can cause a stationary charged particle to accelerate?
(A) A stationary wire that is near the charged particle and that contains
a constant electric current.
(B) A stationary, constant electric field.
(C) A constant north pole that is near the charged particle.
(D) A stationary, constant magnetic field.
Answer: (B) A stationary, constant electric field.
(A) creates positive charge.
(B) pumps positive charge from its positive terminal to its negative
(C) pumps positive charge from its negative terminal to its positive
(D) creates negative charge.
Answer: (C) pumps positive charge from its negative terminal to its
The fallout from a nuclear explosion and the waste from a nuclear reactor
contain radioactive isotopes. The atoms of these isotopes differ from stable
atoms in that
(A) their nuclei are unstable but they are chemically indistinguishable
from stable atoms.
(B) their nuclei are unstable and they are chemically unstable.
(C) they are chemically unstable but their nuclei are indistinguishable
from stable atoms.
(D) each atom emits a steady stream of X-ray radiation.
Answer: (A) their nuclei are unstable but they are chemically indistinguishable
from stable atoms.
There is a metal screen on the front window of a microwave oven. This
screen is essential because it
(A) prevents electric charge from accumulating on the window.
(B) prevents the glass from breaking in response to the rapid heating
of its inner surface and the thermal stresses that result.
(C) protects the window from splatters that may occur when food inside
the oven boils.
(D) reflects microwaves and keeps them inside the oven.
Answer: (D) reflects microwaves and keeps them inside the oven.
Two identical spaceships are travelling through deep, empty space and
are not experiencing any forces from their surroundings. Their velocities
are both constant, but one ship is moving twice as fast as the other. Each
ship has a large rocket engine that can be used to push it forward. From
this information, it's clear that
(A) both ships have their engines shut off.
(B) the fast ship's engine is pushing forward twice as hard as the slow
(C) both ships' engines are pushing forward equally hard.
(D) the fast ship's engine is pushing forward four times as hard as
the slow ship's engine.
Answer: (A) both ships have their engines shut off.
A gymnast is bouncing on a trampoline, rising 2 meters above its surface
on every bounce. After the rising gymnast has left the surface of the trampoline
but is still heading upward,
(A) the gymnast is not experiencing any upward forces.
(B) the upward force of the gymnast's momentum carries the gymnast upward.
This upward force is always stronger than the gymnast's downward weight.
(C) the upward force of the gymnast's momentum carries the gymnast upward.
This upward force is always equal in strength to the gymnast's downward
(D) the upward force of the gymnast's momentum carries the gymnast upward.
This upward force is initially stronger than the gymnast's downward weight
but diminishes to zero as the gymnast rises.
Answer: (A) the gymnast is not experiencing any upward forces.
When laser light reflects from a soap bubble, its photons can interfere
with one another. In contrast, when sunlight reflects from a soap bubble,
each photon can interfere only with itself. The reason for this difference
(A) the photons in a laser beam are all part of a single wave, while
those in sunlight are independent waves.
(B) the photons in a laser beam have both electric and magnetic fields,
while those in sunlight have only electric fields.
(C) the photons in a laser beam have both electric and magnetic fields,
while those in sunlight have only magnetic fields.
(D) the photons in a laser beam are both vertically and horizontally
polarized, while those in sunlight are only vertically polarized.
Answer: (A) the photons in a laser beam are all part of a single
wave, while those in sunlight are independent waves.
Most nuclear reactors in the United States are thermal fission reactors,
reactors that slow their fission neutrons by sending those neutrons through
materials known as moderators. Water is a common moderator. When a fast
moving neutron enters water, it usually slows down because it
(A) drags the water molecules with it and experiences water resistance.
(B) pushes and pulls on the electrons in the water molecules and transfers
most of its energy to those electrons.
(C) creates bubbles in the water as it passes and those bubbles carry
away most of its energy and momentum.
(D) collides with the water nuclei and transfers most of its energy
and momentum to those nuclei.
Answer: (D) collides with the water nuclei and transfers most of
its energy and momentum to those nuclei.
You place an aluminum soda can on a hard floor and step on it with your
foot, crushing it flat. The act of crushing the can
(A) decreases the can's total energy.
(B) leaves the can's total energy unchanged.
(C) increases the can's total energy.
(D) has an undeterminable effect on the can's total energy.
Answer: (C) increases the can's total energy.
Vegetable oil and Pyrex glass have almost exactly the same indices of
refraction-the speed of light is the same in both materials. If you submerge
a clear Pyrex dish in vegetable oil,
(A) the Pyrex dish will be virtually invisible because no light will
reflect from its surfaces.
(B) the surfaces of the Pyrex will appear mirror-like-reflecting light
perfectly as the result of total internal reflection.
(C) the Pyrex dish will act as a lens and you will see highly distorted
images of objects beyond the dish.
(D) you will see a pattern of beautiful rainbow colors in the glass
as dispersion separates white light.
Answer: (A) the Pyrex dish will be virtually invisible because no
light will reflect from its surfaces.
The microscopic structure of quartz glass (amorphous quartz) closely
resembles that of
(A) molten quartz (liquid quartz).
(B) crystalline quartz (quartz crystals or quartz sand).
(C) gelatin (unflavored and unsweetened).
Answer: (A) molten quartz (liquid quartz).
An engineer at the company you're working for has just reported finding
an unusual electromagnetic wave. This wave consists only of an electric
field, with no magnetic field accompanying it. You are certain that the
engineer is mistaken because
(A) an electromagnetic wave must have a magnetic field that changes
with time to produce its electric field.
(B) while waves consisting only of electric fields are common and travels
indefinitely through space, they are known as "electric waves," not "electromagnetic
(C) electromagnetic waves always contain both electric charges and magnetic
poles, and magnetic poles are accompanied by magnetic fields.
(D) electromagnetic waves contain moving electric charges and charges
produce magnetic fields when they move.
Answer: (A) an electromagnetic wave must have a magnetic field that
changes with time to produce its electric field.
Some of the X-rays emitted by an X-ray imaging machine are fluorescence
X-rays and are formed when individual atoms in the machine undergo radiative
transitions. While the mercury atoms in a fluorescent lamp also undergo
radiative transitions, those mercury atoms emit ultraviolet light, not
X-rays. The reason for this difference is that
(A) mercury atoms aren't massive enough to emit X-rays, whereas the
atoms in an X-ray machine are.
(B) the electrons in the imaging machine atoms experience much larger
changes in energy during their radiative transitions than the electrons
in the mercury atoms do.
(C) the electrons in the imaging machine atoms experience much smaller
changes in energy during their radiative transitions than the electrons
in the mercury atoms do.
(D) the atoms in an X-ray machine are packed together in a solid and
can amplify each other's waves much more effectively than the gaseous mercury
atoms in a fluorescent lamp can.
Answer: (B) the electrons in the imaging machine atoms experience
much larger changes in energy during their radiative transitions than the
electrons in the mercury atoms do.
If you float an aluminum pie plate on the surface of a pond and move
the north pole of a strong magnet in a clockwise circle just above that
plate, the plate will
(A) be lifted out of the water and will stick to the strong north pole
(B) begin turning clockwise, as though it were being dragged along with
(C) remain stationary.
(D) begin turning counter-clockwise, as though it were being twisted
away from the magnet.
Answer: (B) begin turning clockwise, as though it were being dragged
along with the magnet.
A glass fiber can act as a pipe for light-light that enters the fiber
at its end follows the fiber almost indefinitely without escaping through
the fiber's surface. The light follows the fiber because
(A) the fiber contains many tiny converging lenses that focus the light
back into the fiber over and over again.
(B) electric charges in the glass attract the light and prevent it from
accelerating outward, away from the center of the fiber.
(C) the fiber contains many tiny diverging lenses that focus the light
back into the fiber over and over again.
(D) whenever it tries to leave the glass at a shallow angle, it's perfectly
Answer: (D) whenever it tries to leave the glass at a shallow angle,
it's perfectly reflected.
The rear windows of some cars have holes in them so that motors inside
the cars can operate wiper blades outside the cars. These holes were diamond
drilled at the factory before the glass was tempered. If a local car glass
company were to try to drill a hole in a car window that had already been
(A) the diamond drill would be reflected because the speed of light
is almost the same in tempered glass as it is in diamond.
(B) the diamond drill would break because tempered glass is much harder
than normal glass.
(C) the window would vibrate violently at one of the tones of the "well-tempered"
scale and would probably crack.
(D) the window would tear itself into tiny pieces as soon as the drill
reached the central portion of glass inside the window.
Answer: (D) the window would tear itself into tiny pieces as soon
as the drill reached the central portion of glass inside the window.
You are observing a distant ship with a telescope but you can't quite
read the ship's name. To increase the magnification of the telescope, you
replace the eyepiece with one having a
(A) longer focal length than before.
(B) smaller aperture than before.
(C) larger aperture than before.
(D) shorter focal length than before.
Answer: (D) shorter focal length than before.
You have just put fresh batteries in your flashlight and it's working
nicely. Now suppose that you remove all of the batteries and reinsert them
backward. Despite the fact that their positive and negative terminals are
reversed, the batteries connect properly to one another and to the flashlight,
so that there are no connection problems in the flashlight. As the result
of this battery reversal, the flashlight will
(A) remain lit regardless of the position of its on-off switch.
(B) work in reverse, lighting up when you put its switch in the "off"
position and not lighting up when you put its switch in the "on" position.
(C) work normally, lighting when you switch it "on" and turning off
when you switch it "off."
(D) not light up at all, regardless of the position of its on-off switch.
Answer: (C) work normally, lighting when you switch it "on" and turning
off when you switch it "off."
An expert archer is firing arrows at a target roughly 50 meters away.
Each arrow travels almost horizontally and hits the target at over 100
mph. Once an arrow has left the archer's bow, that arrow experiences
(A) no forward horizontal forces.
(B) a forward horizontal force that gradually diminishes in strength
as the arrow approaches the target and reaches zero at the moment of impact.
(C) a forward horizontal force that gradually diminishes in strength
as the arrow approaches the target but remains reasonably strong even at
the moment of impact.
(D) a forward horizontal force that remains constant all the way to
Answer: (A) no forward horizontal forces.
If you melt quartz (also known as silica or silicon dioxide), it becomes
a thick, gooey liquid. If you cool this liquid quickly, it forms a clear
solid but if you cool it too slowly, the resulting solid is cloudy. The
cloudy solid forms because slow cooling
(A) allows the temperatures to get out of balance, so that the flow
of heat within the material fills it with tiny white filaments.
(B) permits stresses to build up within the liquid so that it experiences
countless microscopic fractures as it hardens.
(C) allows tiny quartz crystals to form and grow within the supercooled
liquid as it hardens.
(D) gives the chemicals time to react and produce tiny white particles
within the colorless solid.
Answer: (C) allows tiny quartz crystals to form and grow within the
supercooled liquid as it hardens.
The X-rays used in medical imaging travel through tissue relatively
easily but are mostly absorbed by bone. That's because
(A) only neutrons can absorb X-rays and the atoms in bone have many
more neutrons than those in tissue.
(B) the atoms in bone are slightly radioactive and the passing X-rays
cause them to undergo fission. Since the atoms in tissue are not radioactive,
they cannot undergo fission.
(C) solids, such as bone, are much better at absorbing X-ray photons
than are liquids, such as those in tissue.
(D) the atoms in bone generally have more tightly bound electrons than
those in tissue and are thus able to absorb higher energy photons.
Answer: (D) the atoms in bone generally have more tightly bound electrons
than those in tissue and are thus able to absorb higher energy photons.
A glass window pane reflects about 4% of the light striking its front
(A) light slows down upon entering the glass and the resulting impedance
mismatch causes some of that light to reflect.
(B) glass is about 16% metallic and this partial conductivity causes
a fraction of the light striking glass to reflect.
(C) glass is about 4% metallic and this partial conductivity causes
a fraction of the light striking glass to reflect.
(D) glass is about 2% metallic and this partial conductivity causes
a fraction of the light striking glass to reflect.
PART II: SHORT ANSWER QUESTIONS
Please give a brief answer in the space provided. Part II is
worth 33% of the grade on this examination.
Answer: (A) light slows down upon entering the glass and the resulting
impedance mismatch causes some of that light to reflect.
Modern audio amplifiers usually contain at least four important electronic
components: resistors, capacitors, diodes, and MOSFETs. Let's take a brief
look at each of these devices.
(A) A resistor behaves like a wire, except that its electric resistance
has been adjusted to a particular value. Like a wire, a resistor wastes
power when current flows through it so that the resistor becomes warm.
If you double the amount of current passing through a particular resistor,
by how much does the power being wasted by that resistor change?
Answer: The power increases by a factor of four (it quadruples).
Why: When double the current passing through a resistor or wire,
the voltage (energy per unit of charge) lost by that current doubles. Since
you are now sending twice as many units of charge through the resistor
or wire each second, and each unit of charge is losing twice as much energy,
the current is losing 2 x 2 or 4 times as much energy per second--four
times as much power.
(B) A capacitor stores energy when it has positive charge on one plate
and negative charge on the other. The voltage difference between those
two plates is equal to the energy that would be released in letting a unit
of positive charge return from the positive plate to the negative plate.
How would this voltage different be affected if you were to move the two
capacitor plates farther apart without changing their charges?
Answer: The voltage difference would increase.
Why: Voltage difference is a measure of the energy that would be
released by a unit of positive charge that flows from the high voltage
side to the low voltage side. If you take a charged capacitor--one with
positive charge on one plate and negative charge on the other plate--and
pull those plates apart, you do work on the charges. After all, the oppositely
charge plates attract one another and you must push them apart as they
move apart. This work ends up as energy in those separated charges. The
energy that would be released when a unit of positive charge flows from
one plate to the other would increase, so the voltage difference between
the plates also increases.
(C) A diode conducts current only in one direction. Most diodes are constructed
out of two different pieces of modified semiconductor: p-type and n-type
semiconductors. If you assembled a diode out of two pieces of n-type semiconductor,
omitting the p-type semiconductor altogether, would the new device conduct
current and, if so, in which direction(s)?
Answer: Yes, it would conduct current in both directions.
Why: The combined system would be all n-type semiconductor. No p-n
junction would form and there would be no "one-way" character to the system.
Since n-type semiconductor is itself an electric conductor, the system
would conductor electricity in either direction.
(D) An MOSFET makes it possible for a small amount of charge on the MOSFET's
gate to turn on or off a substantial current flowing through the rest of
the MOSFET. In a typical n-channel MOSFET, a modest positive charge on
the gate will allow current to flow through the MOSFET from the drain to
the source. What will happen if you instead put a modest negative charge
on the gate of this same MOSFET?
Answer: The MOSFET would not allow current to flow.
Why: Putting negative charge on the gate of the MOSFET would push
more negatively charged electrons out of the nearby p-type segment of the
"n-channel MOSFET". That p-type segment, rather than becoming n-type and
forming the "n-channel" that allows current to flow through the MOSFET,
would become still more p-type. The p-n junctions in the device that prevent
current flow through it would simply become stronger and further inhibit
You have just stepped out of a drenching spring rainstorm. You plug
your hairdryer into the upper socket of a 120 volt electric outlet and
begin to dry your hair. The hairdryer carries a current of 10 amperes,
weighs 10 newtons, and produces air with a temperature of 70° C.
(A) How could you calculate how much electric power the hairdryer is
using (you don't have to do this calculation; just describe how to calculate
Answer: Multiply the current (passing through the hairdryer) times
the voltage (drop across the hairdryer).
Why: The voltage drop across the hairdryer indicates how much energy
each unit of charge deposits in the hairdryer. The current through the
hairdryer indicates how many units of charge flow through the hairdryer
per unit of time. If you multiply these two quantities together, you obtain
the amount of energy deposited in the hairdryer per unit of time, which
is the power delivered to the hairdryer.
(B) As your hair dries out, it gradually becomes stiffer. Your hair is
a polymer that dissolves lots of water. Why does getting your hair wet
make it softer?
Answer: The water (dissolves in your hair and) acts as a plasticizer
(or the water lubricates the hair molecules).
Why: Chemicals that dissolve in plastics and lubricate their molecules
tend to soften those plastics.
(C) You are brushing your hair with a cheap metal comb. The comb gets tangled
in your hair and you pull hard, permanently bending several of the comb's
teeth. What happened to the arrangement of atoms in the teeth as the teeth
Answer: The (planes of) atoms (in the metal crystals) slipped across
Why: In a typical metal, the crystals can deform by sliding one sheet
of atoms across another. This process is known as slip.
(D) You put down the bent metal comb and pick up another comb that's made
of a hard, glassy plastic. This time when you pull on the comb after it
becomes tangled in your hair, several of its teeth break off. Why doesn't
this plastic comb bend the way the metal comb did?
Answer: The atoms and molecules in the glassy plastic aren't mobile
and can't slip across one another, so that the molecules break apart (when
Why: While some plastics can deform permanently, glassy plastics
cannot. They have so little mobility at the atomic and molecular level
that the break before they experience plastic deformation.
Sir Lancelittle headed off to seek his fortune, riding on his noble
horse Fleabag and armed only with the legendary sword Whycaliber. Unfortunately,
Whycaliber was made of pure lead because it was given to him by the Lady
of the Pond and she was into heavy metal.
(A) As Sir Lancelittle sat under a tree that evening, he noticed that
Whycaliber was already bent out of shape. He silently wished that the Lady
of the Pond had been a little less obsessed with purity and had mixed a
few other elements into the lead when she formed the blade. How would adding
other elements to the lead have made the blade stiffer?
Answer: The other elements would have impeded slip (so that sheets
of atoms would not have been able to slide across one another as easily).
Why: The lead sword deforms when its individual crystals experience
slip. By inhibiting slip, alloying elements would harden the metal.
(B) During his travels, Sir Lancelittle was introduced to Plick and Plack,
twin brothers whose swords were made of pure uranium 235. A few weeks after
their meeting, Plick and Plack got into a heated argument over who's turn
it was to empty the garbage. They drew their swords and began to fight
but the moment those two swords struck one another, the brothers disappeared
in a flash, along with much of the surrounding forest. Why were two separated
swords safe, but two very nearby swords unsafe?
Answer: The separated swords are each below the critical mass, but
together they exceed the critical mass.
Why: To experience a chain reaction, fissionable materials must
be assembled in such a way that they use their fission neutrons efficiently.
The separated swords waste too many neutrons to maintain a chain reaction,
but the combined swords give each fission neutron more opportunity to cause
a subsequent fission and thus use the neutrons more efficiently. Evidently,
they exceeded the critcal mass and caused an explosion.
(C) To idle away his free hours, Sir Lancelittle often tossed Whycaliber
into the air and tried to have it slice point first into the soft ground
at his feet. He was only hospitalized twice as the result of this game.
Each time Whycaliber left his hands, it would rise to a peak and then descend.
Neglecting air resistance, in which direction was the net force on Whycaliber
as it rose (after leaving his hands) and as it descended?
Answer: The forces was downward in both cases.
Why: Whether the sword is rising or descending, it is in free fall.
The only force it is experiencing (apart from air resistance) is gravity
and that force is in the downward direction. The sword is thus accelerating
downward throughout its flight.
(D) Astonishingly enough, Whycaliber proved a valuable defensive tool when
Sir Lancelittle was attacked by a band of outlaws armed with a wind-powered
X-ray machine. What aspect of the sword's lead caused it to absorb the
deadly X-rays before they could harm Sir Lancelittle?
Answer: Lead atoms have many tightly-bounce electrons that can absorb
the X-ray photons (via photoemission).
Why: To absorb X-rays effective, an atom must have tightly bound
electrons that can absorb an X-ray photon's energy and end up leaving the
atom slowly. The electrons in small atoms are too easy to remove and therefore
can't absorb all of the X-ray photon's energy easily. A lead atom is so
huge that its tightly bound electrons can easily handle the X-ray photon's
One way to detect an art forgery is to look for chemical elements in
the paint that are not found in other paintings done by the artist. Many
modern paint pigments contain elements that aren't present in old paintings,
so finding one of these elements in a supposedly old painting is a sure
sign that the painting is a fake. A common way to discover these inappropriate
elements is to use a technique called "neutron activation." In this technique,
the painting is exposed to neutrons from a nuclear reactor and the neutrons
stick to the nuclei of some of the painting's atoms.
(A) What causes the neutrons to stick to the nuclei of the atoms?
Answer: The nuclear (or strong) force.
Why: When nucleons touch, they exert strong attractive forces on
one another--the nuclear force.
(B) Once a neutron sticks to the nucleus of an atom, that atom may become
radioactive. Since most nuclei contain neutrons without being radioactive,
why does adding extra neutrons cause some nuclei to become radioactive?
Answer: Isolated (or "lonely") neutrons are unstable and fall apart.
Why: Isolated neutrons fall apart spontaneously with a half-life
of about 17 minutes. Even in a nucleus, an isolated neutron can be unstable
if there are too many other neutrons around. Nuclei with excessive numbers
of neutrons are radioactive because their neutrons fall apart.
(C) These radioactive nuclei often decay by emitting an electron and a
neutrino (actually an anti-neutrino). As the electron and neutrino leave
the nucleus, one of the neutrons in the nucleus becomes a proton. How does
this additional proton affect the atom that contains the nucleus?
Answer: The atom changes into a different element (and its chemical
Why: The number of protons in an atom determines the number of electrons
that atom has (one electron for each proton). By adding a proton to the
nucleus of an atom, you change the number of electrons that atom ends up
with. The atom becomes a different element, with different chemical properties.
(D) The remaining nucleus is often left with extra energy-it's in an excited
state. It soon emits a gamma ray that is characteristic of that nucleus.
By studying the gamma rays coming from the neutron-activated painting,
scientists can determine exactly which elements it contains and whether
or not the distribution of elements is consist with those used by the artist.
Once the neutron-activation test is over, the painting must be stored while
its radioactivity diminishes. While most of its radioactivity vanishes
within days, it remains slightly radioactive for a very long time. If it
loses half of its radioactivity in one day, why doesn't it lose the other
half in one more day?
Answer: It only loses about half its remaining radioactivity
with each passing day.
Why: Radioactive decay is a random process. If a half of a collection
of radioactive nuclei survive a day without decaying, then the clock starts
all over for them and you can expect that only half of the remaining half
will decay on the second day. By the third day, a quarter of the original
nuclei will remain and the clock will again start over for them. Half of
these will survive the third day, and so on.
You are outdoors on a sunny day, playing with a magnifying glass that
is 10 centimeters in diameter, 1.0 centimeter thick, and has a focal length
of 20 centimeters.
(A) You decide to use this glass lens to burn a piece of wood. You hold
it above the wood so that a bright circle of sunlight forms of the wood's
surface. To make the circle's edges sharp and clear, how high above the
wood should you hold the lens?
Answer: 20 centimeters.
Why: The lens has a focal length of 20 centimeters. Since one way
to determine a lens' focal length is to find the distance from the lens
to the real image it forms of a distant object, distant objects will form
real images 20 centimenters from this lens.
(B) How would you have to change the lens to make the sharp circle of sunlight
it forms appear the same size as before, but brighter?
Answer: You would increase the diameter of the lens.
Why: The larger the lens is in diameter, the more light it captures
and works with. Increasing its diameter won't change the location of its
real image, but will increase the brightness of that image.
(C) How would you have to change the lens to make the sharp circle of sunlight
it forms appear larger than before?
Answer: You would increase the focal length of the lens (or, equivalently,
decrease the curvature of the lens).
Why: The farther from the lens the real image forms, the larger
that real image will be. By delaying the focus, a less curved lens--one
with a longer focal length--will have room to create a larger real image.
(D) You've tired of burning wood, so you begin using the lens as a magnifying
glass to inspect a leaf. What's the farthest from the leaf you can hold
the magnifying glass and still see a virtual image of the leaf?
Answer: (Almost) 20 centimeters.
Why: As you move the lens away from the leaf, the virtual image
you see gradually drifts farther away (it's always on the far side of the
lens) and becomes larger. But as the distance from the leaf to the lens
approaches the focal length of the lens, the virtual image reaches becomes
huge and infinitely far away. That huge image is so distorted you can't
really tell what you're looking at, so you do best to stay just a little
closer to the leaf.
Your eye is similar to a camera lens. A camera lens forms a real image
of the scene in front of it on a sheet of film. Your eye forms a real image
of the scene in front of it on your retina.
(A) To form a sharp image of a particular object on the film, the distance
between the camera lens and the film must be carefully adjusted. Suppose
that the camera is focused on an object 3 m (10 feet) away. If the object
moves farther from the camera, which way must the lens move so that a real
image of that object forms on the film-toward the film or away from it?
Answer: The lens must move toward the film.
Why: Light from a more distant object diverges less when it strikes
the lens and focuses more easily. It forms a real image nearer to the lens
and thus the lens must move toward the film.
(B) The lens in your eye can't move toward or away from your retina. Instead,
it changes its curvature. Its middle can either bulge outward to make it
more curved or it can squeeze inward to make it flatter. As the object
you are watching moves away from you, how should the lens of your eye change
so that a real image of the object forms on your retina-should its middle
bulge outward more or squeeze inward more?
Answer: It should squeeze inward more (to make it flatter).
Why: To keep the real image at its same distance from the lens,
you must decrease the focusing ability of the lens in your eye. You do
this by making the lens less curved.
(C) When a person is farsighted, the light rays from a nearby object focus
after the retina and the person's vision is blurry. With the help of eyeglasses,
this person's vision can be corrected so that the person sees a sharp image
of the nearby object. To correct the person's vision of nearby objects,
should those eyeglasses contain converging lenses or diverging lenses?
Answer: The eyeglasses should contain converging lenses.
Why: To focus light from a nearby object into a real image on your
retina, the lens in your eye must become highly curved. If you cannot do
this and can only focus light from more distant objects, you evidently
don't have enough converging strength in your lens. To assist your lens
in converging the rays, you must wear converging lens eyeglasses.
(D) When you select eyeglasses, you have the choice of buying high index
of refraction plastic lenses. A high index of refraction means that the
plastic in the lenses slows light more than normal plastic. These lenses
are 40% thinner than normal plastic lenses because the high index of refraction
plastic bends light more than normal plastic. How does this increased index
of refraction affect the amount of light that these lenses reflect?
Answer: They reflect more light (because the impedance mismatch
between air and the plastic is more severe than normal).
Why: The more light slows down or speeds up in going between air
and plastic, the worse the impedence mismatch and stronger the reflections.
High index plastic lenses reflect more light than low index plastic lenses.