This is the boss. No calculator — the masses are clean tonnes, the speeds whole metres per second, and every line is built to be done in your head. One collision, two files, and every answer feeds the next: each car's momentum, then the total, then the coupled speed, then the impulse on one car, then the force its structure carried. Write each line down as you go — a slip on line one rides all the way to the verdict.

Two house rules for this paper. The contact time is quoted in milliseconds, and $\mathrm{kg \cdot m/s}$ divided by milliseconds lands directly in kilonewtons — a shortcut worth trusting once you have seen why. And for the single g-force line, take $g = 10\ \mathrm{m/s^2}$, the round number every physicist keeps for mental work. The last question ends on a judgement rather than a number, which is how real investigations end. 5★ under par here is the chapter crown. Clock in.