cross-product
Explanation
The Joke
A car honks at a pedestrian to move faster. The pedestrian protests that she is walking as fast as she can, but the driver retorts that her absolute speed is not the issue -- what matters is the cross product of her velocity vector with the direction of the curb. Because she is walking at an extremely acute angle relative to the curb, the projection of her velocity onto the direction perpendicular to the road (i.e., the component that actually gets her across the street) is practically zero. The driver even holds up a hand-drawn diagram illustrating the vector math. When the pedestrian asks if the driver carries that diagram everywhere, the driver replies: "Trigonometry is the only thing my wife didn't take when she left."
The Humor
The comedy operates on multiple levels. First, there is the absurdity of someone applying formal vector mathematics to a mundane traffic confrontation -- most people would just yell "hurry up!" but this driver instead invokes trigonometric decomposition of velocity vectors. The driver is technically correct: a person walking nearly parallel to a curb does cross the road extremely slowly regardless of their walking speed, because only the perpendicular component of their motion counts. Second, the punchline about the driver's wife leaving him lands perfectly -- it implies this person's obsessive devotion to trigonometry is precisely the kind of personality trait that drove their spouse away, yet it is also the only thing they have left. The joke that the wife took everything except the trigonometry knowledge is both self-deprecating and darkly funny.
References
- The cross product (and more precisely the relevant concept here, the dot product or scalar projection) are operations from linear algebra and vector calculus used to decompose a velocity into components along different directions. The comic references how only the component of walking velocity perpendicular to the road matters for crossing it.