Why was the physics book so sad?

This joke gets its giggle from a clever bit of wordplay, specifically a pun. The humor hinges on the double meaning of the word "problems." In the academic world, especially in a subject like physics, "problems" refer to the exercises and questions found at the end of chapters, designed for students to solve. But in everyday language, "problems" are difficulties, worries, or sources of sadness and stress.

Anyone who has ever cracked open a physics textbook can likely empathize with the sheer volume of "problems" contained within. Physics is a rigorous subject that demands a lot of practice, and those pages are often packed with challenging equations and scenarios, which can certainly feel overwhelming to the student trying to master them. The idea of the book itself feeling the weight of all those tasks is a funny, relatable twist.

The joke playfully anthropomorphizes the inanimate physics book, attributing human emotions like sadness to it because of its content. It's a lighthearted jab at the sometimes daunting nature of academic subjects, finding humor in the unexpected application of a very human feeling to an object that's just doing its job.