negligible
Explanation
The Joke
Two people stand near what appears to be a massive space elevator structure. One person, alarmed, shouts: "Wait... wait... you just assumed the cable mass was negligible?" The other calmly responds: "It's a tradition." Someone else yells: "Run! Run!" The caption reads: "It was a mistake to let physicists engineer the space elevator."
The Humor
In physics education, it's standard practice to "assume negligible mass" for ropes, cables, and pulleys in textbook problems. This simplification makes calculations tractable for students but is obviously absurd when applied to real engineering — especially for a space elevator, whose cable would extend tens of thousands of kilometers into space and whose mass would be one of the most critical engineering challenges. The joke is that physicists, trained to reflexively ignore cable mass, carried this classroom habit into an actual engineering project with catastrophic results. The word "tradition" makes it even funnier: the physicist isn't defending the assumption on technical grounds but treating it as an inherited cultural practice that can't be questioned.
Broader Context
SMBC has an extensive catalog of jokes about the gap between theoretical physics and practical engineering. Weinersmith, who collaborates with scientists regularly, loves the absurd simplifications that are standard in physics pedagogy — frictionless surfaces, spherical cows, massless ropes — and frequently imagines what would happen if these assumptions escaped the classroom and entered the real world. This comic is a particularly clean execution of that premise, with the space elevator providing a context where "negligible cable mass" is maximally dangerous.