Sled

a sled has no engine. everything it does, it buys with height.

draw a track in pencil and a sled runs it: it can never climb back above the line it started on

A sled carries no engine, so every bit of speed it has anywhere on a track was bought by falling. That makes its speed at any point a measure of one thing only: how far below its starting height it currently is. Draw a hump that pokes above the line the sled set off from and it cannot get over it, no matter how steep the run-up, because the run-up cannot give back more than it took. The ruled line across the sheet is that starting height, and once you have watched a sled stall against it a couple of times you start reading any slope as a speed before anything has moved.

it can only spend the height it has fallen, so the ruled line is a ceiling: a hump above it is one you drew too high.

Concepts: energy, motion, conservation.

An interactive you play with one thumb, on Wundr.