Twice the Speed, Four Times the Stop - braking, crumple zones and seatbelts

Every stop is that board’s one-way trip: the tidy motion of a whole car, all moving the same way, turned into disordered heat in the brakes that never turns back into speed.

This lesson’s own board:

The stopping distance itself, drawn for this lesson.

Before you start. You need nothing but the experience of being in a car or on a bike when it stopped hard. One idea from Entropy - why time only points forward helps: ordered things drift into disorder on their own, and never the other way. A joule is the unit energy is counted in; that is the only unit below you might not know, and every number is worked out as it comes up.