When to use ideal system and when not to.

[rraidermd]

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So i missed a question on my practice test only on the basis that I applied ideal instead of non ideal when it came to the conversion of PE to KE. The question only asked the relationship between PE and KE for a cyclist that is riding down a hill without pedaling. Am I supposed to always take into account nonconservative forces? I swear I heard somewhere from EK for instance that you are always suppose to assume ideal and conservative forces unless explicitly stated so. Any thoughts?

 

[Cawolf]

I would assume drag and friction forces are at play due to a "real life" scenario.