Only a very few of the Duke Physics Challenges were invented by me, most are problems (or variations of problems) that I have heard or read about. The following are references and links to similar kinds of problems:

Volumes I and II of

The book

At about the same level, the book

An outstanding collection of problems and questions can be found in the book The Flying Circus of Physics, 2nd Edition Jearl Walker (Wiley 2006). This book is particularly good at increasing one's awareness of the world all around and how scientific problems arise from this awareness. Readers of this book should be warned that many of the problems are not physics problems or challenges in the sense that they can be solved with a moderate amount of thinking or experiment. Many of the problems are in fact extremely hard and remain unsolved. Still, nearly every question is interesting and fun to think about. The author has provided references (many of them technical) for the problems.

The following are links to sites with similar kinds of scientific problems and puzzles:

- Boston Area Undergraduate Physics Competition
- Fermi problems.
- Fermi problems of the Science Olympics.
- UMD Fermi problems, require order-of-magnitude estimates to get some insight.

- Physics Challenges for Teachers and Students by Boris Korsunsky. This is a monthly column in The Physics Teacher journal, you can also find past problems with solutions at this webpage.
- Physics Question of the Week (Dept of Physics, U. of Maryland).

- Problems on mechanics in the book
*An Introduction to Mechanics*by Daniel Kleppner and Robert J. Kolenkow; -
Problems on electricity and magnetism, many with solutions, in the
book
*Electricity and Magnetism, Third edition*by Edward Purcell and David Morin;. -
Problems on special relativity in the book
*Spacetime Physics*by Edwin Taylor and John Wheeler; -
Problems in astrophysics in the book
*Astrophysical Concepts, 3rd Edition*by Martin Harwitt. -
Many interesting and elementary applications of physics to
biomechanical problems are given in the delightful book
*Life's Devices*by Steven Vogel.