<grumble> From HNQ
physics.stackexchange.com/q/487180/123208 "I decided not to accept a single answer because different answers provided me with different ways to understand this. Thanks to all!" What?! I'm sure Bob D doesn't care about the measly 15 points, especially after a couple of days of hitting the rep cap, but it's the principle of the thing... Does the OP think he'd have scored all those points without Bob D's excellent answer? </grumble>