This is the boss. No calculator — today $g = 10\ \mathrm{N/kg}$, the round number every engineer keeps for mental work and site sanity checks, and the values are chosen to fit in your head. One runaway truck, one gravel bed, and every answer feeds the next: the energy arriving, the height a bare climb would have needed, the friction the gravel offers per metre, and the length that takes. Write each line down as you go — one early slip rolls all the way to the end of the bed.

The whole chapter is in this one problem. Kinetic energy is what arrives. Potential energy is one way to absorb it. Friction doing negative work is the other, and negative work over a distance is exactly what the work–energy theorem was built to price.

And note what the last line asks for. It does not want another number; it wants a decision — is this bed long enough — the way a real check does. The arithmetic only earns you the right to make it. 5★ under par here is the chapter crown. Clock in.