Ah Shucks!
thanks, nice to know I'm not alone in my thoughts. I always think that before getting tied up in forgotten 4th form physics, it's a good idea to look at the practicalities of the situation and how they relate to other instances (eg nautical?). Otherwise, I can feel 'Houghton and Carruthers) being pulled from the bookshelf and dusted off.
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Another day in paradise