2 ms·

Reframe the system to the moving water. The fish is then swimming upstream. All frames of reference are valid. So yes, perpetual motion.

• plutonorm 5y ago
  what about sailing up wind?
• PeterisP 5y ago
  Reframe the system to the moving water and put a watermill or a hydroelectric power plant in there in there - perpetual motion and extracting energy. A flowing river is not a closed system.