This is the boss. No calculator — tonight $g = 10\ \mathrm{m/s^2}$, the round number every physicist keeps for mental work, and every value is chosen to fit in your head. One car, one long ride, and every answer feeds the next: the energy banked at the top, the speed it becomes in the valley, the squeeze that swallows it, the force at full compression — and then the only thing the operator actually cares about, which is whether the buffer holds.

Write each line down as you go. One early slip rolls all the way to the bottom of the hill, and the finish will trace exactly where the cascade started. The last stage is a verdict, not a number: compare what you computed against what the buffer is rated for, and commit.

Everything from the chapter rides in it. 5★ under par here is the chapter crown. Clock in.