Physics 221
What happens to ∆f/f, the fractional range frequencies over which the resonator oscillates with at least half its maximum amplitude when the damping constant changes from 2 Ns/m to 1 Ns/m?
∆f/f decreases.
The intensity of sound wave A is 100 times that of sound wave B. Relative to wave B the sound level of wave A is +2 dB -2 dB +100 dB +20 dB +10 dB
+20dB
A vibrating source generates a harmonic wave on a string under constant tension. If the power delivered to the string is doubled, by what factor does the amplitude change?
1.41
In the figure, student a has a mass of 95 kg and student b has a mass of 77 kg. They sit in identical office chairs facing each other. Student a places his bare feet on the knees of student b, as shown. Student a then suddenly pushes outward with his feet, causing both chairs to move. During the push, and while the students are still touching one another neither student exerts a force on the other. student a exerts a force on student b, but student b does not exert a force on student a. each student exerts a force on the other, but student b exerts the larger force. each student exerts a force on the other, but student a exerts the larger force. each student exerts the same amount of force on the other.
each student exerts the same amount of force on the other
An object with mass m moves through free space with constant velocity. The magnitude of the force required to maintain this motion is equal to Zero Mg The mass of the object m The force required to stop it
zero
A heat engine extracts usable work by permitting
Heat to flow from a hotter object to a colder object
A string is clamped at both ends and plucked so it vibrates in a standing wave between two extreme positions as shown by the dashed lines below. If the string has length L and the frequency of oscillation of this mode is f, the speed of the waves on that string is (2/3) f L f L (3/2 )f L 3 f L
(2/3) f L
A traveling wave is described by the equation y(x,t) =( 0.01 m) cos(46.5 x + 210 t ), where x is measured in meter and t in seconds. What is the velocity of this wave in units of m/s? Remember that in one dimension the sign of the velocity functions\[ as the direction indicator.
-4.5 m/s
A point source of sound waves emits a disturbance with a power of 50 W = 50 J/s into a surrounding homogeneous medium. What is the intensity of the sound at a distance of 10 m from the source? 5 W/m^2 0.5 W/m^2 0.08 W/m^2 0.04 W/m^2 0.4 W/m^2
.04 W/m^2
What is ∆f/f, the fractional range of frequencies over which the resonator oscillates with at least half its maximum amplitude in the above exercise (damping constant to 2 Ns/m)?
.12
A 0.5 kg mass oscillates about the equilibrium position on a vertical spring with spring constant 10 N/m. Where is its equilibrium position measured from the unstretched spring position (without the hanging mass)? 5 m 0 m 0.49 m 0.05 m 2.04 m
.49m
For a 1 kg mass, and a pendulum length of 1.5 m, what is the ratio of the measured period with a 58 deg amplitude to the the ratio of the measured period with a 2 deg amplitude?
1.008
What is the period (in s) of a simple pendulum of length 0.56 m and mass 0.32 kg near the surface of a planet where the gravitational acceleration is 15.9 m/s2?
1.18s
A ball is thrown directly downward with an initial speed of 8 m/s from a height of 30 m. After what time interval does the ball strike the ground? 3.75 s 1.78 s 1.23 s 2.47 s 6.1 s
1.78s
The temperature in a furnace used to melt Aluminum is 660 oC. What is the average kinetic energy of the air molecules in the furnace? 933 J 9*10^-21 J 660 J 1.93*10^-20 J 1.37*10^-20 J
1.93*10^-20 J
Two glass marbles of radii r and 2r fall through a high-viscosity fluid. What is the ratio of their terminal velocities Vr/V2r?
1/4
A gardener is watering her garden from a hose. With the water pressure full blast holding the hose horizontally she can just reach a distance of 12 m, but needs to water an area up to 15 m away. By what fraction must she reduce the cross-sectional area of the hose, still keeping the hose horizontal, to be able to water this area?
1/5
Find the natural frequency of vibration of the salt molecule NaCl given its effective mass of 13.9 atomic mass units and a spring constant of 100 N/m. ~10^13 Hz ~6.6*10^13 Hz ~4.3*10^27 Hz ~100 Hz ~0.4 Hz ~7 Hz
10 ^ 13
A 1 kg block of aluminum is heated at atmospheric pressure such that its temperature increases from 22 oC to 40 oC. What is the change in the internal energy (in J) of the block of aluminum? The specific heat of aluminum is 9*102 J/(kg K).
16,200 J
A thermodynamic system undergoes a process in which its internal energy decreases by 1,259 J. If at the same time 384 J of thermal energy is transferred to it, how much work (in J) is done by the system? Enter an integer?
1643J
A refrigerator has a coefficient of performance equal to 5.6. If the refrigerator absorbs 100.6 J of thermal energy from a cold reservoir in each cycle, find the work done on the system each cycle (in J).
17.9J
A mass-spring system oscillates with an amplitude of 9.2 cm. If the force constant of the spring of 145 N/m and the mass is 0.69 kg, what is the magnitude of the maximum acceleration of the mass in m/s2?
19.3
A rope has a mass of 0.9 kg and a length of 5.9 m. It is fixed at both ends. If the frequency of the first harmonic on this rope is 1.09 Hz, what is the tension of this rope in N? Enter a number with one digit behind the decimal point.
25.24N
Assume the gravitational acceleration g near the surface of earth in the simulation is 9.81 m/s2. What is g', the gravitational acceleration near the surface of Jupiter, in units of m/s2? Enter a number with two digits behind the decimal point.
25.94
What is the fundamental frequency (in Hz) produced by a pipe of length 1.48 m open at both ends on a day when the speed of sound is 337 m/s?
260.6Hz
How many nodes and antinodes are shown in the standing wave below?
3 nodes and 2 antinodes
A standing wave is established on a 1.2 m long string fixed at both ends. The string vibrates in four segments when driven at 120 Hz. What is the fundamental frequency?
30 Hz
The temperature on a nice summer day is 84 oF. What is the temperature on the absolute (Kelvin) temperature scale? 29 K 357 K 187 K 456 K 302 K
302K
Data: cice = 0.50 cal/(g oC) , cwater = 1.00 cal/(g oC), csteam = 0.48 cal/(g oC), L(ice-water) = 80 cal/g , L(water-steam) = 540 cal/g How much heat is given off if 0.5 kg of steam at 373 K is converted into ice at 273 K? 720 kcal 360 kcal 310 kcal 160 kcal 50 kcal
360
An ideal reversible engine takes in 5 kJ of energy via heat transferred in from a 500 K burner and exhausts heat at 300 K during each closed cycle in the PV plane. The amount of heat exhausted at 300 K each cycle is.. 0 kJ (it's a closed cycle). 3 kJ Not enough information is given to answer this question, 5 kJ
3kJ
A piano tuner detects 3 beats per second when she is tuning a string to a tuning fork of frequency 440 Hz. When she tightens the string, which increases the frequency of the string, the frequency of the beats initially goes down, and as she keeps increasing the frequency of the string, the frequency of the beats starts to increase. What was the original frequency of the string? 437 Hz 440 Hz 443 Hz either 437 Hz or 443 Hz (One cannot tell without more information.)
437
A 2 kg block of aluminum is heated at atmospheric pressure such that its temperature increases from 16 oC to 43 oC. What is the change in the internal energy (in J) of the block of aluminum? The specific heat of aluminum is 9*102 J/(kg K). Enter an integer.
48600J
A stationary source S generates circular outgoing waves on a lake. The wave speed is 5.0 m/s and the crest-to-crest distance is 2.0 m. A person in a motorboat heads directly toward S at 3.0 m/s. To this person, the frequency of these waves is:
4Hz
In the cyclic process on an ideal gas shown in the adjoining PV diagram, what is the net work done by the gas during the cycle?
4PV
A box with a total surface area of 1.2 m2 and a wall thickness of 4 cm is made of an insulating material. A 10 W electric heater inside the box maintains the inside temperature at 15 oC above the outside temperature. What is the thermal conductivity of the insulating material in units of kcal/(s oC m)?cx 54.82 5.3 10^-6 .022 .00053 93.02
5 x 10 ^ -6
The mechanical energy of a mass-spring system is 0.189 J. If the force constant of the spring of 137 N/m and the mass is 0.76 kg, what is the amplitude of the oscillatory motion in cm?
5.25cm
By how much does the entropy of 100 g of water at 100 oC change if the water is very slowly converted into steam at 100 oC? The latent heat of vaporization of water is 540 kcal/kg = 2260 kJ/kg. There is no change since the temperature is constant. 22.6 kJ/K 606 J/K 0.36 J/K 145 J/K
606
Beats are produced when two tuning forks, one of frequency 240 Hz and another of frequency 246 Hz are sounded together. The frequency of the beats is
6Hz
A grandfather clock is running slow. The period of its pendulum is too long. To shorten the period and make the clock run faster, you should.
decrease length of the pendulum
Ethyl alcohol boils at 172 oF. What is Celsius equivalent of this temperature? 451 oC 278 oC 140 oC 78 oC 64 oC
78
An alarm clock makes a sound that has intensity 60 dB at a distance of 1 m. What is the sound intensity of 100 such clocks which are all 1 m away?
80dB
A 0.500 kg mass is attached to a spring of constant 150 N/m. A driving force F(t) = (12.0 N)cos(ωt) is applied to the mass, and the damping coefficient b is 6.00 Ns/m. What is the amplitude of the steady-state motion if ω is equal to half of the natural frequency ω0 of the system?
9.7
A steel bridge is built in several segments, each 20 m long. The gap between segments is 2 cm at 18°C. What is the maximum temperature that the bridge can manage before buckling? α(steel) = 13*10-6 /(°C) Hint: ΔL = α L ΔT).
95 c
We observe the harmonic motion of an object. Its position as a function of time shown in the graph below is given by x = 1* cos(π t), where x is measured in meter and t is measured in seconds. What are the amplitude (A), the period (T), and frequency (f) of the motion?
A; 1 , T = 2s , f = 0.5 Hz
When I bake a sweet potato in the oven, I always put it on a thin sheet of aluminum foil. This is because sometimes the potato exudes a sugary juice that burns and makes a\hard-to-clean-up mess if it drips on the bottom of the oven. When I'm ready to take the potato out of the hot (400o F) oven after an hour of cooking, I find I can pick up the aluminum foil with my bare hands without getting burned. Why?
Because the aluminum foil has a low specific heat and not much mass, so even at a high temperature it doesn't have a lot of thermal energy in it to burn me. f
In any process, the maximum amount of heat that can be converted into mechanical energy
Depends on the intake and exhaust temperatures
In the figure below, two point sources S1 and S2, which are in phase, emit identical sound waves of wavelength 2.0 m. If L1 = 39 m and L2 = 36 m, what type of interference occurs at point P? Beats Destructive interference Constructive interference No interference at all
Destructive interference
In a mixture of hydrogen, oxygen, and nitrogen gases at a given temperature, the molecules having the greatest average speed are those of Oxygen Hydrogen Nitrogen At the same temperature they all have the same speed.
Hydrogen
Pressure vs. volume graphs for a certain gas undergoing five different cyclic processes are shown to the right. During which cycle does the gas do the greatest positive work? I I III IV V
IV
In the figures (a), (b), (c), (d) and (e) below, S is a source of sound of real frequency f and L is a listener. The speeds of the source and the listener are equal (v) in all cases except in (e) where the source is at rest and the listener is moving with speed v (as in the other cases). There is no wind. From the statements below, pick out the wrong statement. In the case shown in figure (a) the apparent frequency as heard by the listener is equal to the real frequency of the source. In the case shown in fig (b) the apparent frequency as heard by the listener is the greatest. In the case shown in fig (c) the apparent frequency as heard by the listener is the least. In the case shown in fig (d) the apparent frequency as heard by the listener is less than the real frequency of the source In the case shown in fig (e) the apparent frequency as heard by the listener is greater than the real frequency of the source.
In the case shown in fig (d) the apparent frequency as heard by the listener is less than the real frequency of the source
A string is clamped at both ends and then plucked so that it vibrates in a standing mode between two extreme positions a and c. Let upward motion correspond to positive velocities. When the string is in position b, the instantaneous velocity of points along the string Is zero, positive, or negative, depending on the position along the string. Is negative everywhere Is positive everywhere Is zero everywhere
Is zero, positive, or negative, depending on the position along the string.
A hypothetical engine operates in a cycle taking in 10,000 J from a hot reservoir and 5000 J from a cold reservoir. In the cycle it performs 15,000 J of work. Such an engine.. -Violates the first law but obeys the second law of thermodynamics. -Violates both the first and second laws of thermodynamics -Obeys both the first and second laws of thermodynamics -Obeys the first law but violates the second law of thermodynamics -Conserves entropy
Obeys the first law but violates the second law of thermodynamics
An object on the end of a spring is oscillating in simple harmonic motion. If the amplitude A of oscillation is doubled, how does this affect the oscillation period T and the object's maximum speed vmax?
T remains the same and vmax doubles
On a humid summer day, perspiration does not cool you off much. Why? -The water vapor in the air is moving too fast to condense on your skin as perspiration. -The air density is extremely low and its pressure is too low to permit water to evaporate. -The air density is almost saturated with water vapor and there is almost no net evaporation. -The air density is extremely low and the air pressure is too high to permit water to evaporate
The air density is almost saturated with water vapor and there is almost no net evaporation.
Suppose you work in the E.R. and a choking patient comes in with their trachea obstructed. The diameter is 1/3 of what it was before the obstruction. You manage to correct the problem and send them on their way safely, explaining that such obstructions are particularly dangerous because
The blocked airway passes only 1/81 the regular amount of air
A gas is in a container with a piston lid and is taken from thermodynamic state, 1, to a new thermo-dynamic state, 2, by several different paths, A, B, and C, in the P-V plane. The internal energy of the gas increases during the process. The internal energy change is largest for path k
The internal energy change is the same for the three paths.
You fill two identical mugs with coffee, but the coffee in one mug is at a higher temperature than that in the other mug. You place the two mugs simultaneously in a microwave oven and turn it on briefly. As a result, you add 1 Joule of thermal energy to each mug. Which mug experiences the larger increase in entropy (if any)?
The mug containing the colder coffee experiences the larger increase in entropy.
Which of the statements below is wrong?
The net force acting on the pendulum bob is F = -mgsinθ. The period increases with amplitude because this force increases faster than harmonic restoring force F = -mgθ.
Which of the statements below is wrong? The net force acting on the pendulum bob is F= -mgsinθ. The period increases with amplitude because this force increases faster than harmonic restoring force F= -mgθ. The period of a simple pendulum noticeably depends on the amplitude for large amplitudes In a zero g environment a pendulum does not execute oscillatory motion. A damped pendulum does not have a constant period. For the same amplitude and the same pendulum length, the period is independent of the mass.
The net force acting on the pendulum bob is F= -mgsinθ. The period increases with amplitude because this force increases faster than harmonic restoring force F= -mgθ.
Blood flows through a section of a horizontal artery that is partially blocked by a deposit along the artery wall. A hemoglobin molecule moves from the narrow region into the wider region. What happens to the pressure acting on the molecule as it moves from the narrow into the wider region?
The pressure increases.
A 200 Hz sound wave has a measured wavelength of 1.675 m. Which of the following statements are true? You can choose more than one statement. The speed of the 200 Hz wave is ~335 m/s The speed of a 400 Hz wave is ~half that of the 200 Hz wave The speed of a 400 Hz wave is ~ twice that of the 200 Hz wave The wavelength of the 400 Hz wave is ~ 80-85 cm
The speed of the 200Hz wave is 335 m/s The wavelength of the 400Hz wave is 80-85cm
In your dorm room you watch the Vols play in an out-of-town game. It is a cliff hangers and the winner will be determined in the last few seconds of play. Everybody is taking their bathroom break right after the game ends. As the toilets flush and hands are washed, the faucet pressure is noticeably reduced, even though the supply pipe to the rooms can carry all the water needed. Why?
The water has to speed up in the supply pipe and therefore the pressure drops in the pipe.
Which of the following statements are true? You can choose more than one statement. a. The volume of the sound is uniform, it does not depend on the vertical position of the head. b. The volume of the sound increases as the head moves vertically from the bottom to the top. c. The volume of the sound decreases as the head moves vertically from the bottom to the top. d. There are two locations, symmetric about the centerline, where no sound can be heard. e. The head moves through 3 locations where the sound waves interfere constructively and two locations where the sound waves interfere destructively.
There are two locations, symmetric about the centerline, where no sound can be heard. The head moves through 3 locations where the sound waves interfere constructively and two locations where the sound waves interfere destructively.
Suppose you are spending your weekend on roller skates trying to get momentum from somewhere. Which of the following will give you momentum in the north direction? Bending forward and back Throwing your keys northward Bending down and throwing your keys straight up All of the above Throwing your keys southward
Throwing your keys southward
Which of the following statements are true? You can choose more than one statement. a. The motion of the speaker produces a slight decrease in the pressure in front of the speaker during the forward motion of the speaker head and a slight increase in pressure during the backward motion of the speaker head. b. When you increase the frequency, the distance between successive regions of high pressure increases. c. To increase the amplitude of a 400 Hz sound wave you must increase the number of times the speaker oscillates back and forth each second. d. To increase the volume of the sound heard by the listener, the speaker must produces larger fluctuations in pressure. e. To produce a lower pitch tone, the speaker must oscillate back and forth fewer times each second.
To increase the volume of the sound heard by the listener, the speaker must produces larger fluctuations in pressure. To produce a lower pitch tone, the speaker must oscillate back and forth fewer times each second.
A sine wave and a square wave cannot have the same Wavelength Loudness Tone quality Frequency Pitch
Tone Quality
You are watching a semi truck bounce up and down on a spring. (Yep it's a toy.) At the topmost point in the truck's path,
Truck velocity is zero but acceleration is downward
Data: cice = 0.50 cal/(g oC) , cwater = 1.00 cal/(g oC), csteam = 0.48 cal/(g oC), L(ice-water) = 80 cal/g , L(water-steam) = 540 cal/g Which requires the most energy? Raise the temperature of 1 gram of liquid water by 50 oC. Melt 1 gram of ice at T = 0 oC. Raise the temperature of 1 gram of ice from 73K to 273K. Vaporize 0.5 gram of water at T = 100 oC.
Vaporize 0.5 gram of water at T = 100 oC.
Can an object's velocity change direction when its acceleration is constant? No, this is not possible because it is always speeding up. No, this is not possible because it is always speeding up or always slowing down, but it can never turn around. Yes, this is possible, and a rock thrown straight up is an example. Yes, this is possible, and a car that starts from rest, speeds up, slows to a stop, and then backs up is an example. Yes, this is possible, but then the net force must change direction.
Yes, this is possible, and a rock thrown straight up is an example.
(A) Can two force vectors of unequal magnitude sum to zero net force? (B) Can three force vectors of equal magnitude sum to zero net force? (a) no, (b) no (a) yes (b) no (a) yes (b) yes (a) no (b) yes
a. no b. yes
The Doppler effect is characteristic of water waves. sound waves. light waves. all of the above choices. none of the above choices.
all of the above
Sound waves
are longitudinal waves
Living cells constitute a low entropy state of matter. Living cells
can exist because they help increase the entropy of the rest of the universe.
When water flows smoothly in a straight hose, its viscosity causes the water to
convert some of its kinetic energy into thermal energy
The total entropy of the universe is always increasing. So, when water freezes, the entropy of the water is increasing. True False
false
A car travels over a dirt road that contains a series of equally spaced bumps (a so-called "washboard" road). While traveling at a given speed the driver experiences a very jarring ride. When the driver drives at a higher speed, however, the ride gets smoother. That is because the car actually leaves the ground at higher speeds. the faster moving car actually crushes the bumps and makes the road smoother. the car's shock absorbers have more friction at higher speeds. at the higher speed more of the up and down motion is converted into forward motion. going faster in the car forces the suspension to oscillate at a frequency higher than its natural frequency.
going faster in the car forces the suspension to oscillate at a frequency higher than its natural frequency.
Do you have a better "high end" (1000 Hz) or "low end" (100 Hz) response with your ears?
high end
A man, with his arms at his sides, is spinning on a light frictionless turntable. When he extends his arms his angular velocity increases. his angular velocity remains the same. his moment of inertia decreases. his angular momentum decreases. his angular momentum remains the same.
his angular momentum remains the same
In any process, the maximum amount of mechanical energy that can be converted into thermal energy
is 100%
You have an empty 20 oz. soda bottle and you blow air over the opening to excite a fundamental standing wave. Now, you slice off the bottom of the bottle (it's plastic) without changing its length very much. You blow over the opening and excite a fundamental standing wave in the bottle with its bottom end open. The frequency of the standing wave in the second case is higher than that in the first case. is lower than that in the first case. is the same as that in the first case. may be higher, lower, or the same as in the first case. It cannot be predicted from the information given. There is no sound is produced in the second case.
is higher than that in the first case
A car's speed changes from 20 mph to 60 mph. By what factor does its kinetic energy change? There is no way to tell It will increase by a factor of 3 It will stay constant It will increase by a factor of 6 It will increase by a factor of 9
it will increase by a factor of 9
A hand moves to produce a periodic wave. You are making a movie. You freeze the movie and get this snapshot. If you advance the movie one frame, the knot at point A would be
lower
The volume of a gaseous sample is increased from V1 to V2 in three different ways. A-->B: isobaric process A-->C: isothermal process A-->D: adiabatic process Then the work done by the gas is
maximum in the isobaric process
In simple harmonic motion the magnitude of the velocity is greatest at the maximum displacement. the period depends on the amplitude. the acceleration is constant. the magnitude of the acceleration is greatest at zero displacement. none of the above.
none of the above
Assume we can change the equilibrium state of a system via two different processes. Assume that the initial and the final state are the same. Which of the quantities ΔU, ΔQ, ΔW, and ΔT must be the same for the two processes?
only ΔU and ΔT
A rock is thrown straight up from the surface of the Earth. Which one of the following statements describes the energy transformation of the rock as it rises? Neglect air resistance. The total energy of the rock increases. The KE increases and the PE decreases. Both the PE and the total energy of the rock increase. The KE decreases and the PE increases. Both the KE and the PE of the rock remain the same.
the KE decreases and the PE increases
Compare the flight of two golf balls given the same impulse by the club. Which ball will travel a larger distance?
the dimpled ball
A bullet is fired horizontally from a rifle on the Moon (no air, g = 1.6 m/s2) with initial speed v0. Assuming a level and endless ground, which statement is false? During the flight, the minimum speed of the bullet is v0. During its entire flight the acceleration is constant. The flight time increases as v0 increases. The bullet moves along a parabolic path.
the flight time increases as v0 increases
The longer the wavelength of a sound wave
the lower its pitch
The maintenance of the life cycle depends on the presence of a source and a sink of energy. the decrease of the entropy of the universe. the fact that organisms higher on the food chain are more organized. the fact that a living organism is a highly probably state. the fact that thermodynamics cannot be applied to the biological systems.
the presence of a source and a sink of energy
Water is flowing smoothly through the pipe shown in the diagram. The pressure is lowest in section A. The pressure is lowest in section B. The pressure is lowest in section C. The pressure is the same everywhere.
the pressure is lowest in section b
Suppose you have an aquarium. You have a solid plastic decoration and a rock of about the same volume but the rock has twice the mass. Both are totally submerged in the water. The buoyant force on the plastic object is about Twice as big as that on the rock One quarter as big as that on the rock The same as that on the rock Four times as big as that on the rock One half as big as that on the rock
the same as that on the rock
A sinusoidal force with a given amplitude is applied to an oscillator. To maintain the largest amplitude oscillation, the frequency of the applied force should be
the same as the natural frequency of the oscillator
Which of the following statements are true? You can choose more than one statement. a. The speed of the 200 Hz wave is ~335 m/s. b. The speed of the 400 Hz wave is ~ half that of the 200 Hz wave. c. The speed of the 400 Hz wave is ~ twice that of the 200 Hz wave. d. The wavelength of the 400 Hz wave is ~ 80 - 85 cm.
the speed of the 200Hz wave is 335m/s the wavelength of the 400Hz wave is 80-85 cm
The liquid level in a glass thermometer rises as the temperature increases because the pressure in the thermometer decreases. the volume expansion coefficient of glass is greater than that of the liquid. the volume expansion coefficient of glass is less than that of the liquid. the volume expansion coefficient of glass is the same as that of the liquid. the pressure in the thermometer increases.
the volume expansion coefficient of glass is less than that of liquid
A water pump generates a constant flow Q, which is causing a turbulent spray (Reynolds number R = 3200) out of this cylindrical nozzle: Which of the following three nozzles could be substituted to achieve a streamline flow (R < 2000)?
tube 1 and 3
Sound does not pass through
vacuum
While a guitar string is vibrating, you gently touch the midpoint of the string to ensure that the string does not vibrate at that point. The lowest-frequency standing wave that could be present on the string
vibrates at twice the fundamental frequency
in a popular classroom demonstration, a cotton ball is placed in the bottom of a strong test tube. A plunger fits inside the tube and it makes an air-tight seal. It is then pushed down very rapidly, and the cotton flashes and burns. This happens because when the air gets compressed its temperature increases. friction between the plunger and the tube ignited the cotton. cotton contains a pressure-sensitive substance that burns upon impact. the air becomes very turbulent and air friction produces heat. it's one of those demos that nobody understands.
when the air gets compressed its temp increases
