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Insights Direct Echo-Based Measurement of the Speed of Sound - Comments

  1. Jun 28, 2018 #1
  2. jcsd
  3. Jun 28, 2018 #2

    Dale

    Staff: Mentor

    That is fun! Not often that you get to set off fireworks for science
     
  4. Jun 30, 2018 #3
    Yep. I'm actually going to use "Chemistry of Pyrotechnics" to put together a few labs for next year (supposing the local school is pleased enough to let me coordinate a few labs for them again.)

    The challenge with these deals is not making them fun. That's a given. The challenge is connecting the "Gee Whiz" part of it to some interesting science in a way that tests a hypothesis reasonably within the learning objectives and in the Goldilocks zone (not too hard, not too easy, just right.)

    It's easy to pretend one is doing science when all the students remember is the "Gee Whiz" and no one remembers the learning objectives.
     
  5. Jun 30, 2018 #4

    Dale

    Staff: Mentor

    I think that is a succinct summary of the problem with pop-sci presentations. It is good that you are focusing on more than just the fun, but including both fun and learning objectives.
     
  6. Jul 2, 2018 #5
    In a paper coming out this fall in TPT, colleagues and I identified three challenges in the typical introductory physics lab design:

    1) simple experiments connected with learning objectives
    2) experiments sufficiently accurate for comparisons between theory and measurements without gaps when students ascribe discrepancies to confounding factors (imperfect simplifying assumptions, measurement uncertainties, and “human error”), and
    3) experiments capturing student attention to ensure due diligence in execution and analysis.

    So that can be summarized in three goals: 1) learning objectives 2) accuracy (I like 1%) and 3) Gee Whiz factor. I like the firecracker echo experiment, because it has all three (which is rare) plus a 4th that is often a constraint 4) Cheap.

    I've been working a lot this past year with a number of resource-constrained schools: home schools, private schools, foreign schools, and public schools in underfunded districts. Some times it feels like it comes down to:
    A) What interesting things can you do with a microphone as an accurate timer?
    B) What interesting kinematics can you catch with an available video camera and analyze in Tracker? (Or otherwise use the camera as a timer to 1/30 sec)
    C) What "virtual" labs can you do by downloading historically important or other interesting data (Boyle, Kepler, etc.)?

    I've got mixed feelings about calling an analysis activity a real "laboratory" if someone else did the experiment and collected the data. But these can have a hypothesis, a quantitative test of the hypothesis, data analysis, and a traditional lab report. I wouldn't want a lab program to rely too heavily on these, but better than skipping labs completely due to resource constraints.
     
  7. Jul 3, 2018 #6

    sophiecentaur

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    A very cheap way that has good accuracy / consistency is to stand a distance from a large wall and use a hammer to hit a metal object. That is obvs so far. The clever bit is to strike the metal exactly when you hear the echo, and repeat. You repeat until you are accurately in sync with the echo pulses. Then you measure the time for 10, 20 or more echos. The accuracy gets better and better with more pulses.
    Classic integration method to average out errors. ms timing accuracy is possible with enough pulses.
     
  8. Jul 4, 2018 #7

    sophiecentaur

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    I really worries me that students seem to confuse simulation with reality all the time. It's the Startrek effect. They ask why their simulation is not giving the answers they expect. It's GIGO without having any way of chasing the fault in the model. A simulation is so much cheaper than hardware and you don't need lab space nor need to tidy up for the next class. You can see why 'the system' likes to encourage it.
     
  9. Jul 4, 2018 #8
    I consider downloading real data acquired from a third party as a different (better) class of lab than computer simulations. For example, last year, I had a physical science class download and analyze both Brahe's original data and modern data for testing Kepler's third law. Later, (for a different lab), I had them download available orbital data for earth satellites to test Kepler's third law in that system. I had a physics class analyze Robert Boyle's original data (from his historical publication) to test Boyle's law.

    In my view, these labs are not as good as real, hands on experiments where students acquire the data themselves. But they do more accurately represent the scientific method by comparing predictions from proposed models (usually the hypothesis) against _real_ experimental or observational data. There are many historical cases where science really works this way - a model is validated against data acquired by a different party.

    In contrast, testing a predictive model or hypothesis against a simulation is not a version of the scientific method that I think we should be teaching in introductory labs. That's not how the scientific method really works, and using simulations for labs runs a significant risk of confusing students about the scientific method itself.
     
  10. Jul 7, 2018 #9
    I encountered an equivalent phenomenon several years ago while walking on a local college campus. I passed between a blank wall of a building and a pulsating garden sprinkler. My left ear heard the sprinkler, which produced psst sound as it spurted about four times a second. My right ear heard the echo off of the building. I was able to position myself so I heard both sounds simultaneously. I saw that I was hearing the direct sound of the nth spurt and the echo if the (n-1)th spurt. Given the period of the sprinkler spurts and the distance from the sprinkler to the wall I could get the speed of sound.

    If I could get my students access to that setup, I'd ask them to predict where the sound and echo are heard simultaneously, and design the experiment to test the prediction.
     
    Last edited: Jul 7, 2018
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