Equipotential lines significance and application What is analogous to equipotential lines in real life? What are its applications?
 A: Suppose you're hiking on a somewhat conical mountain. The contour lines on the topographical map of the mountain are (gravitational) equipotential lines. Their significance in real life is that if you move along a contour line (meaning that you're not climbing up the mountain, just moving along at the same height), you don't do any work against the gravitational field. To rephrase -- equipotential lines are paths along which you can move "for free", without doing any work.
Of course, you must struggle in your hike against friction, and hence you do work even if you resolve to stick to an equipotential line, but you work against friction here, not against the gravitational field.
A: In real-life we have equipotential surfaces, since real-life is three dimensional. The most of famous equipotential surface is called "The Geoid", of which there are many, e.g.: https://cddis.gsfc.nasa.gov/926/egm96/egm96.html
The application are numerous, many stemming from its central role defining Mean Sea Level. It's also indispensable for any application requiring detailed understand of Earth's gravitational field (gravity anomaly and vertical deflection, though those are not equipotential surfaces).
