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Lesson Idea Work Up - D&C 41-44

In my earlier post, I talked about how I'd started brainstorming for possible science connections which I could use to teach the next Come Follow Me Primary lesson, on D&C 41-44. I've now decided to opt for the idea around trumpets. I noticed that the word "trump" came up twice during this section:

D&C 42: 6 And ye shall go forth in the power of my Spirit, preaching my gospel, two by two, in my name, lifting up your voices as with the sound of a trump, declaring my word like unto angels of God.

D&C 43: 18 For the day cometh that the Lord shall utter his voice out of heaven; the heavens shall shake and the earth shall tremble, and the trump of God shall sound both long and loud, and shall say to the sleeping nations: Ye saints arise and live; ye sinners stay and sleep until I shall call again.

The way that brass instruments like trumpets create sound is really interesting. (Disclaimer - this is my best attempt at providing a simple explanation, but this is not my area of expertise so I may have described some key details incorrectly. However I hope this provides enough of a picture to help with planning the lesson.) 

I think we'll go for a musical theme this time. There's some really interesting science behind how trumpet valves work, which could tie into the lesson theme. Image is from Unsplash, the site for freely usable images, courtesy of Mathias P.R. Reding,


First, let's think about a basic horn instrument (of which the trumpet is a complex example). Below, I've drawn a quick sketch of what happens. (1) The musician blows through the mouthpiece, and as they do so, they vibrate their lips. (2) This causes the air inside the instrument to vibrate, and sound waves are generated which propagate through the instrument. (3) The wave reflects back at the end of the instrument, setting up a standing wave. (4) Some of the waves escape from the end, creating the sound that we eventually hear.

Quick sketch of what happens when we blow into a simple horn instrument.
 
If the musician blows into the instrument the right way, they can create standing sound waves with a specific frequency that creates a pleasant sound. This is known as the resonant frequency, where the energy in the waves build up and become very strong. (This is because when the frequency of the sound wave matches the natural frequency of the instrument, the instrument can absorb large amounts of energy from these sound waves, instead of losing it to friction.) Different instruments with different shapes, lengths and materials have different resonant frequencies, and this results in the variation in sounds that we get. Sounds produced at the resonant frequency will become loud when they emerge from the instrument, due to their higher energy.

Longer instruments can produce a different (lower pitched) sound, since the resonant frequency reduces when you have a longer column of air to vibrate. However it becomes impractical to switch to a different instrument when you want a lower pitch sound. Trumpets address this through the use of valves which allow the musician to change the length of the air column, since pressing down on a trumpet valve causes the air to go through a longer series of pipes. 

Experiments for the Lesson

From the perspective of the lesson, here are some aspects that we could use:

  • The scriptures talk about "lifting up your voices as with the sound of a trump". We could talk about how the power of the the gospel is akin to providing a vessel through which our voices get magnified, just like a trumpet. The spirit could be thought of as a means in which the energy in our voices gets amplified, just like the resonance effect.
  • The scriptures also say "the trump of God shall sound both long and loud". The trumpet allows us to change the length of the air column through which the sound is generated. Heavenly Father's trumpet-like voice can adjust its length to be able to reach different people, who may be receptive to different sounds.   
We could illustrate these ideas as follows:
  • We could get the children to build their own instruments. There are lots of nice tutorials online for kid-friendly trumpets, but an easy one is to take a plastic drinks bottle, cut off the end, squash the open end, and then practice blowing through the mouthpiece with vibrating lips. 
    • Get the children to practice blowing through these bottles until they can hear an amplified horn-like sound.
    • Once they know how to do it, get them to keep blowing but with and without the bottle instrument, and listen to how much louder the sound is when they blow through the bottle. This demonstrates the usefulness of the vessel for conducting our voices.
    • If you can have enough kids with different bottles, you could have fun with a little impromptu orchestra! Different lengths or shapes of bottles should give different sounds, which helps to illustrate the idea behind instruments like trumpets which let you control the sound.
    • If you don't have plastic bottles, then a variation on this idea is to use milk bottles filled with different volumes of water (example YouTube video). The main physics principles are similar here.
  • To demonstrate the sounding waves that get established within the instrument, we could use a slinky coil. Here's an example YouTube video showing this off, or you could have the kids do it themselves if they have a slinky to hand. 
  • To demonstrate the resonance effect, you could use this wine glass experiment (example YouTube video), where you rub a finger along the rim of a glass that's partially filled with water. This is really nice for demonstrating the effects of matching the input vibration frequency with the natural frequency of the object (the glass in this case, or the instrument) to generate a dramatically amplified sound.

This still needs to go through the all-important "does-it-link-to-the-primary-lesson-topic test" (a.k.a. wife test) on whether it adequately teaches the key scriptural principles (in this case, the meaning of the two scriptural verses at the top of this article), but once it's done so, I'll put up a final lesson plan. Any thoughts or suggestions would be greatly welcome!


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