The derivation of key formulae is something that features every year in a few leaving cert questions - which I'm inclined to think is a good thing. By knowing where even a few formulae come from, students can see that all formulae come from somewhere: that they're not the random inventions of scientists from the past, handed down without question through the subsequent generations.
At the same time, they can be a bit time-consuming in class, and at times it can be frustrating for everybody involved. The most committed learn them quickly and gain nothing by seeing them repeated many times, and the least committed don't learn them - and therefore also gain nothing by seeing them repeated many times. this makes it hard to really give the group in the middle what they need: time to learn and time to think things thorough. So this year, I ran through the derivations on a recording. Students can work through it at their own pace and repeat it as often - or as seldom as they wish.
The recording is available here for anybody who wants to use it...
And the related PowerPoint is here...
(Please let me know if you spot any errors)