This is so obvious in this problem, but when you see more complicated formula, you may not know all the assumptions that went into the derivation and quite often you will be using it when you shouldn't.

在这道题中这些都很明显,但是当我们用更复杂的公式时,你可能搞不清楚所有,在推导时的前提,因此很多时候,你都会用错公式

耶鲁公开课 - 基础物理课程节选