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Safety Note: Strobe

  • C2-09: FREE FALL WITH STROBE

    C2-09
    Show the position of a dropped ball at a series of equal time intervals
    Drop the ball with the strobe on at the desired flash rate (about 10-13 flashes per second, or 600-800 per minute, seem to work well). The increasing distance the ball falls between successive strobe flashes is readily apparent.
    C2, FS1, LS1

  • D1-01 STROBOSCOPE AND FAN

    D1-01
    Demonstrates rotational motion using stroboscope
    The motion of a fan can appear to slow down, stop, or "reverse" with the use of the stroboscope, an instrument that emits intense bright light at different frequencies. Questions: What will it look like when the frequency of the stroboscope is faster than the rotating speed of the fan? When it has the same speed as the fan?
    OS6, LS1

  • G3-53 STANDING WAVES IN A STRING

    G3-53
    Demonstrates standing waves on a thin string
    The string is driven by a 60 Hz vibrator with the number of standing wave loops determined by adjusting the tension. Black painted channel is the background for improved visibility.
    OS0, LS1

  • 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

  • N3-08: STROBOSCOPIC COLOR WHEEL

    N3-08
    Demonstrate color mixing.
    Spinning the color wheel on a fast rotator while illuminating it with a bright stroboscope produces a variety of color mixtures. For example, fourfold symmetry would imply that the color is produced by a mixture of four primaries: G+G+B+B = saturated cyan. Others include 2R+G+G+B = R+G+(R+G+B) = Y+W = unsaturated yellow, and B+B+2R+G =B+R+W = unsaturated magenta. The photograph at the right shows the color wheel rotating while being illuminated by the stroboscope blinking at the rotation rate of the wheel. Challenge students to predict the outcome of various combinations
    N3, LS1

    n3-08a

  • O2-21: STROBOSCOPIC CENCO

    O2-21
    Demonstrate persistence of vision using strobosccope.
    A disc contains the letters C E N C O in apparently random order. When the disc is spun rapidly and illuminated by a stroboscope at the right frequency, the word CENCO is clearly formed, accomplishing their advertising goal.
    N3, O2, LS1

    o2-21

  • O2-22: STROBOSCOPE AND FALLING WATER

    O2-22
    Demonstrate how a stroboscope works, and illustrate persistence of vision.
    A water container with a nipple near the bottom is connected to a plastic tube and eye dropper to produce a stream of water which breaks up into a series of water droplets. The water stream is illuminated by a stroboscope and viewed by a TV camera. Adjustment of the strobe frequency can make the water droplets move up or down either slowly or rapidly, or even stand still!
    O2, LS1

    o2-22a

    o2-22b

  • O2-23: STROBOSCOPIC DISCS

    O2-23
    Demonstrate stroboscopic effects.
    Several stroboscopic discs are available with a high-speed rotator and stroboscope. Look them over to find what interests you. Some are similar to the patterns seen on the obsolete stroboscopic piano tuners. Some are motional things like a swinging pendulum, shown in the photograph above. Another is actually the demonstration O2-21: STROBOSCOPIC CENCO, a shameless advertising ploy.
    N3, O2, LS1