A mass m is attached to a spring with stiffness k and a damper with coefficient c. However, the mass is not constant. The mass is a small bucket of sand that leaks at a constant rate of α kg/s. The bucket starts with mass m0 at t=0 and is displaced from equilibrium and released. Assuming the leak is slow enough that the damper and spring coefficients remain constant relative to the changing mass, derive the equation of motion and solve for x(t) for the underdamped case.
She had found them in the most unlikely of places: not the official library repository, which only held the last three years, but in the discarded “free bin” outside the Mathematics Department’s old staff room. A retiring professor had purged his office, and someone had tossed a whole archive. To anyone else, it was recycling. To Elena, it was the Rosetta Stone.
The exam hall was a vast, echoing space of terror. 300 students, 300 blue books, 300 rapidly beating hearts. Professor Finch sat at the front, polishing his glasses with a handkerchief, a faint smile playing on his lips. wtw 238 past papers
She spread the papers on a secluded carrel, the kind with high wooden walls that felt like a confessional booth. The first paper, from 2015, looked almost gentle. "Solve the following first-order linear ODE: dy/dx + 2y = e^x." She smiled. She could do that in her sleep.
He was silent for a long moment. Then he nodded, once, slowly. "That," he said, "is the difference between a student who solves equations and an engineer who solves problems. The past papers aren't a key to my exam, Miss...?" A mass m is attached to a spring
"Every paper since 2015," Elena replied. "You repeat themes every four years. 2024 was a mechanical systems year. The leaky bucket was the logical culmination."
Elena opened the exam booklet.
Somewhere, a future student would find it, stay up until 3:00 AM, and see the pattern. And the cycle would continue.
But Elena didn't mind. She had graduated. And she had left the folder of past papers in the free bin outside the staff room, wrapped in a clean plastic sleeve, with a new label: The bucket starts with mass m0 at t=0