Follow

PDD Jan 2016

  • C3-12 PENCIL AND PLYWOOD

    C3-12
    Dramatically demonstrate inertia

    A pencil is accelerated to almost the speed of sound by blasting it through a four-foot tube using a carbon dioxide fire extinguisher. The pencil will readily impale itself through a piece of 3/8" plywood. With a little bit of luck the pencil point will be virtually intact, although sometimes you need to re-sharpen it after the demonstration.

    CAUTION: Be sure that the hose fitting is securely attached to the tube and that the plastic shield is in place before firing. The shield should be latched in place, with no debris blocking its edge from meeting the baseplate

    Engagement Suggestions
    • • Before using, encourage your students to predict what will happen to the pencil.
    • • For advanced students, discuss the energy involved in the problem and where the kinetic energy of the pencil went after the collision.
        • Background

        This demonstration can be presented in multiple ways. It has been offered classically as an illustration of the principle of inertia – the pencil is in motion at a high velocity, and continues in motion despite the intervening wood until arrested by a greater force. Alternatively, consider the high velocity and high momentum of the pencil. The abrupt deceleration at the plywood means a high impulse. The pointed pencil has a very small cross-sectional area, resulting in force applied over a small area leading to a high momentary pressure.

        Linked below is a slow-motion video of the collision, shot at 600 frames per second. A fun class activity could be to use the video to measure the motion of the pencil and estimate its momentum and kinetic energy, based on what you see in the video and by measuring typical lengths and masses for wooden pencils.

    FS1

  • C5-14 ROCKET TRIKE

    C5-14
    Demonstrate Newton's third law of motion

    Pressing the fire extinguisher handle expels carbon dioxide out a nozzle straight behind the tricycle, causing forward thrust of the tricycle. Be sure the exhaust is not oriented to hit the audience or anything else likely to be adversely affected but a sudden blast of cold air.
    Background
    This is a dramatic illustration of Newton's Third Law of Motion: the principle of action and reaction. The mass of gas being ejected out of the back of the tricycle at a very high velocity imparts an equal and opposite force to the tricycle, which thus moves forward. The tricycle is much more massive, so it does not move as quickly, but the acceleration is still very real - be careful not to run into the wall!
    FS1

  • G1-35: MASS ON SPRING - EFFICIENT MODEL

    G1-35
    Illustrate the motion of a mass on a spring.
    Just lift mass and release to start oscillations. This one is relatively efficient, so its vibrations last a long time.
    FS2

  • G3-28 SUSPENDED SLINKY

    G3-28
    Shows longitudinal and transverse traveling waves & standing waves
    Transverse or longitudinal pulses can be created by appropriate motion of your hand at one end of the SLINKY. Using your hand you can also create transverse standing waves and discuss the overtone series. Gently vibrating one end of the spring (either by hand or using the motor) at the appropriate frequency creates longitudinal standing waves.
    FS1

  • G3-43 WHIP

    G3-43
    Illustrates transverse wave motion.
    A wave started down the whip increases its velocity as the whip decreases in diameter toward the tip. By the time the wave reaches the tip of the whip, the velocity of the whip motion can become greater than the speed of sound in air. The "cracking" of a whip is believed by many physicists to be a result of the sonic boom thus created.

    Please consider carefully how to appropriately present this device in class if used.
    G3

  • H1-01 BELL IN VACUUM

    H1-01
    Demonstrates sound wave requirement for a medium

    An alarm-style electric bell is mounted inside a large glass bell jar, with external switches to control both the bell and the pump. This enables the instructor to compare the propagation of sound and light.

    Start the bell, then pump the air out of the jar. Air pressure in the jar is read by the large gauge. As the air is removed, the sound intensity decreases, ultimately to nearly zero. Turn off the vacuum pump when the jar is evacuated and crack the valve open, allowing air to re-enter the jar. As the pressure increases the sound of the bell comes back, but without the noise of the pump.

    Engagement Suggestion
    • Consider asking the students to make predictions before each step - how will removing the air change what they hear? What they see? What will happen as the air returns?
    • Compare this to videos the see of people working in the vacuum of space, in real life and in the movies. What do you see and hear in real life? How is this presented in fiction, and why?
    Background
    There are subtleties to this effect. The pump is not creating a true vacuum within the chamber. The vast majority of the air has been removed, reducing the environment’s ability to transmit sound; but the other (perhaps more important) effect in play is the difference in density between the interior of the chamber and the glass and the external atmosphere; this creates a major change in impedance, causing what little sound can be transmitted within the chamber to reflect back. Also, off course, the bell is not floating in free space, and some vibrations can always be transmitted through the supports and wires.

    For small groups, also consider H1-04, a more portable version of this demonstration.
    FS1

  • H1-31: SOUND LEVEL METER

    H1-31
    Demonstrate use of a sound level meter.
    Several loud sources can provided upon request, including musical instruments, noisy laboratory apparatus, and a portable audiotape machine with earphones. You can also invite students to bring up their own devices to test. It is surprisingly easy to get over 100dB in earphones. The sound level meter can viewed by a TV camera and displayed on the main screen.

  • H2-33: SPEAKER AND EXPONENTIAL HORN

    H2-33
    Demonstrate the effect of an exponential horn enclosure.
    A small loudspeaker is held up behind the opening of an exponential horn. The sound becomes much louder, especially in the bass. A horn enclosure has the effect of taking an extended source such as a loudspeaker and creating the best impedance match with the outside world, providing the most coherent plane wave. Compare this to H2-32, which uses the same speaker with a flat baffle. Invite students to speculate about what the effects the different shapes have.
    H2, OS5

  • H3-12: ROARING TUBE - 4 FT

    H3-12
    Demonstrate standing sound waves in air excited by convection currents.
    A switch is held closed, activating a nichrome wire coil in a vertical glass tube, leading to a very loud roar at about 130 Hz, the fundamental frequency of a four-foot air tube. This is the classic Rijke tube demonstration with an electrical heater replacing a gas burner and screen as the source of the convection currents.

    Consider combing this with H3-13, and invite students to make predictions about the differences in pitch and volume.
    FS1

  • H3-14 TWIRL-A-TUNE

    H3-14
    Demonstrates standing wave resonances in an open tube
    This popular toy is available in many stores and students may have seen it before, but this is an opportunity for them to explore how it works. To produce resonant frequencies of the tube, hold the tube by one end, keeping that end free for flow of air, and swing it around your head. Increasing the speed of the rotation raises the harmonic produced. Up to seven harmonics can be produced, illustrating the notes of the overtone series. The fundamental can only be produced by blowing gently into one end. SUGGESTIONS: Read Invited talk : Sounds Like Fun, presented by Paul Doherty of the Exploratorium at the 2004 meeting of the AAPT at Sacramento, CA, discussing how the twirl-a-tune works.
    H3

  • H3-17 FLAME TUBE

    H3-17
    Demonstrates standing waves in a tube
    A loudspeaker in one end of a four-inch diameter galvanized iron tube creates standing waves in propane gas in the tube. The gas emerges out of a series of small holes in the top of the tube, forming a long line of flames when lit. Any sound resonant with the length of the tube can create standing waves in the gas which are readily visible as a pattern in the height of the flames. Both rhythm and frequency response can be seen nicely in music. An oscillator and a cassette deck are provided with the demonstration to be used as simple sources for the loudspeaker. Or, a voice or other music or audio can introduced using a microphone and amplifier or external input jacks, available upon request.
    FS1

  • H3-61 BEAKER BREAKER

    H3-61
    Breaks a glass beaker with sound

    An audio oscillator and 100 Watt power amplifier are used to drive a heavy-duty horn driver which is mounted in the back of the plastic beaker cavity with the sound emerging through a hole, which can be seen in the photograph. The beaker is positioned on a foam pedestal in front of the speaker hole. A microphone is mounted at 90 degrees from the position of the speaker.

    The beaker is marked with its primary resonant frequency, found in advance using digital spectrum analysis of a recording of the beaker ringing after being tapped. Most beakers have two possible resonant modes 45 degrees apart, due to the weight of the spout; the most effective technique is to drive the resonance with the spout facing directly away from the speaker. Set the frequency of the oscillator as shown on the beaker, with an amplitude of around 140mVpp. The oscilloscope will show two waveforms, the input signal and the signal picked up by the microphone. You may need to adjust the frequency slightly to account for changes in temperature or age since the beaker was tested; slowly shift the frequency by tenths or hundredths of a Hertz to find the amplitude peak (do not try to tune by watching for a displacement in the phase relationship, as there is a time delay between the signals introduced by the hardware). This done, set the strobe around 3000 cycles per minute, and adjust it until you can see the sides of the beaker flexing.

    This can be used to show the resonance of the beaker. You can also, optionally, shatter it, by increasing the input voltage at resonance. Be careful not to exceed 1Vpp.

    After the resonant frequency is found and the amplitude turned up, the oscillation of the beaker can be caused to exceed its elastic limit and thus to shatter. See the video links below to view a slow-motion video of the beaker at the moment it breaks.

    Engagement Suggestion
    • Show the students that there are two different resonant frequencies, and challenge them to develop theories of why this is.
    • Consider using this in conjunction with H3-62 to illustrate the effects of the beaker's spout in a more obvious (and quieter) manner.
    Background
    This process of driven resonance potentially leading to mechanical failure can be related to many engineering problems. This is an excellent opportunity to discuss how physics applies to real-world problems, like the Tacoma Narrows Bridge collapse.
    Also, be sure to explore our directory of oscillations and waves simulations to show other examples of complex mechanical oscillations.
    FS1, LS2, SU5

  • H3-71 STROKED ALUMINUM ROD

    H3-71
    Illustrates longitudinal standing waves in an aluminum rod.
    Apply powdered violin rosin to your fingers or wear a rosined glove and stroke the aluminum rod firmly while holding it at a nodal point. Holding it in the center produces the fundamental, holding at 1/4 of the way from one end produces the second harmonic, holding at 1/6 of the way from one end produces the third harmonic, etc. The rod is about 6 ft long, and the speed of sound in aluminum is about 16,700 ft/sec, so the frequency of the fundamental is about 1400 Hz. The sound is very loud and lasts a long time; the Q for this system is around 100,000!
    Alternatively, request an (optional) mallet to use with the rod. Use the mallet to strike the rod on one end; by holding the rod at a node or antinode, all or some modes can be excited or damped.

  • I3-18: VACUUM BAZOOKA

    I3-18
    Illustrate one effect of atmospheric pressure and force.
    A tennis ball is positioned near one end of an evacuated tube. When the plate sealing that end of the tube is rapidly knocked off, air at atmospheric pressure enters the tube. The ball is propelled by the force arising from the atmospheric pressure of air to create a bazooka effect along with a loud noise.
    I3, I0

    i3-18a

  • I4-14: CHANGE OF STATE WITH BANG

    I4-14
    Demonstrate that the volume of a gas is much greater than the volume of the same amount of liquid.
    Fill the small flask with liquid nitrogen and place the balloon over the top. As the liquid nitrogen turns to gas its volume increases, ultimately bursting the balloon. This is a change of state with a bang, hee, hee, har, har.
    I4, I0

  • I4-31 ICE BOMB

    I4-31
    Demonstrates forces created by freezing water
    A pipe elbow with end caps is filled with water, sealed by tightening the ends, and dropped into a metal container of liquid nitrogen. Within about one minute the water freezes, expanding sufficiently to break the cast iron with a loud crack and a big cloud of vapor.
    I0, I4, SU5, OS6

  • J4-51: THEREMIN

    J4-51
    Demonstrate the theremin
    A theremin is a musical instrument, invented in the early twentieth century by Russian scientist Dr. Theremin, which uses capacitance to change the pitch and the loudness of the sound. It was popular in dance bands in the first half of the twentieth century, and even used by The Beach Boys in the 1960s. By moving your hands up and down over the triangular capacitor plates on the top of the box, the frequency and loudness of the sound can be varied to produce a musical tune. Perhaps one of the most elegant examples of theremin music is the Rachmaninoff "Vocalise" performed by Clara Rockmore, the most well-known theremin artist ever, with Nadia Reisenberg on the piano. This music is on a CD, The Art of the Theremin, which will be found in our library of CDs in the "MUSIC" section of the demonstration storage.
    J4, ME3

  • K2-62 CAN SMASHER - ELECTROMAGNETIC

    K2-62
    Blasts a soda can into two pieces using electromagnetism

    A 400 microfarad capacitor is charged to 3000 volts (1.8 kilojoules) and discharged through a three-turn coil into which an aluminum soft drink can has been positioned. With the circular windows open, the two pieces of the can will be blasted over thirty feet to the sides of the large lecture hall. Charging the capacitor to less voltage results in a can with a "waist."

    This device can be explained in two distinct ways:
    (1) The rapidly rising current creates a rapidly rising magnetic field along the axis of the coil, which in turn induces an electric field going in circles inside the coil. The induced electric field causes an electron current in the can which experiences a vxB force in the magnetic field of the coil, causing the can to break into two pieces which are blown to the opposite sides of the lecture hall.
    (2) A type of "theta pinch" phenomenon. More information on this is available from Wikipedia. Another way to understand this is that the induced current around the can is opposite to the current in the primary coil, since it is opposing the change in flux. These concentric opposite currents repel each other, so the can is pinched and torn apart and ejected out the sides.

    This is an UNFORGETTABLE DEMONSTRATION. A must when you cover electromagnetism.

    This video, from the Video Encyclopedia of Physics Demonstrations, shows the operation of the can crusher with an animation illustrating (1) the electron current in the coil, (2) the vector magnetic field that it creates, (3) the induced electric field within the coil created as the coil current rapidly rises, (4) the electron current circling in the can created by that induced electric field, (5) and the vxB force on the electrons moving around the can.

    Following a description of the crusher electronic components, the animation is displayed. The animation may be stopped so that the directions can be studied in detail for the five (5) quantities listed above. Using the left hand rule (for electrons) the directions can be verified; note that according to Lenz's law the direction of the electron current induced in the can must be in the opposite direction to the electron current in the coil.

    Note that the magnetic field at either end of the coil possesses both an axial and a radial component; the electron current in the can is entirely azimuthal. Using the left hand rule to determine the direction of the cross product of the electron velocity and the magnetic field, it can be seen that the axial component of the magnetic field leads to an inward force, crushing the can, while the radial field component leads to an axial force, away from the plane of the coil at both ends of the can, causing the two parts of the can to move rapidly away from the coil. (In the large lecture hall the two parts of the can will be blown to the sides of the lecture hall.)

    The web site http://hibp.ecse.rpi.edu/Can_Crusher/home.html contains a drawing and animation showing how the RPI electromagnetic can crusher works.

    FS1