This has nothing to do with science, however your thoughts on words trigger a question I always wanted to ask. Succinctly, why use mendacious when lying is what we mean?
Thanks for asking! I will gladly answer any word questions I can, science and math related or not. The answer to your question is actually very straightforward: mendacious is an adjective and lying is the present participle of the verb to lie. Contextually mendacious will be modifying a noun-a mendacious student or a mendacious lover-while lying as a present participle can be used like an adjective or like a verb. For an adjectival example: Jack is a lying sack of... When it acts like a verb: Jim was lying. There is a little bit more to it than that, but you wanted it succinctly…
You are right in thinking that they are both mostly synonymous.
I welcome all word questions and will answer them as best I can!
ecard by kidsneedscience.