This might be an interesting question:
Let's attach a battery into one end of an electric cable. Then we rotate the battery around, with accelerating speed, using 100 Watts of power, while the battery rotates around frictionlessly. (The battery moves in a circle whose radius is the length of the cable)
At the same time we apply 200 Watts of electric power into the cable, this energy travels through the cable into the battery.
We apply 100 Watts of mechanical energy into our end of the cable, and we apply 200 Watts of electric energy into our end of the cable, then the mechanical energy and the electric energy travel into the battery.
The question: What is the speed of the battery after a very long time?
Is this question too easy or too difficult?
Naively I would think that after 1 Giga seconds the battery has 200 Giga Joules of electric energy, and 100 Giga Joules of kinetic energy.
If an object has rest energy of 200 Giga Joules and kinetic energy of 100 Giga Joules, then the object can be said to be a non-relativistic object.