I have a friend who isn't properly visualizing energy loss in a moving car.
His idea: Cars that can run forever without being recharged - - > while the engine rotates the front wheels the move the car forward, there is a device at the back wheels using the back wheels' rotation to generate and store power. The engine draws this restored power to move the car forward and the cycle continues. Now, this is a "perfect" scenario where there's zero energy loss through friction, heat, sound, etc.
I first told him that something similar was already being done: regenerative braking. He said his idea was to generate energy **while* the car was moving.
My counter to why that was rubbish: I reminded him the amount of energy in a given system remains constant. Nothing created or destroyed, just transferred form one form to another. Thus he could not "recover" energy that was being spent to move the car forward. He moved on to elaborate that there would be two engines, one rotating the front wheels with an attached dynamo to recover energy, and another rotating the back wheels. I tried to explain he had added another energy consumer, with no way to recover this extra energy being spent...and that's how it would go. You add an extra engine to overcome the dynamo's resistance and move the car, you are spending more than you are recovering, and will eventually have to charge up. To help him visualize, I made a sketch, trying to be more realistic: (please ignore my use of battery capacity, it's only there for the concept, not to be realistic)
I also mentioned to him that this scenario was even ignoring the other loads of problems and dangers that present themselves with having wheels rotating at different speeds. Plus the wear on the slower rotating ties which are effectively being dragged against the road. For some never-to-be-discovered reason, he simply could not understand that to get the slower (front) tires to NOT be a problem, you have to increase the front engine power output for to match the wheel rotation speed of the rear wheels. Again, only recovering a fraction of that energy (where did the rest of it go? He can't understand and I can't figure out how to explain it to him). To counter my logic, he proceeded to mention that trucks don't have an engine attached to all their wheels. Hence, some energy can be captured from those "free-spinning" wheels. Oh my God. Newton's first law of motion applies here: A car with the engine rotating the wheels towards a given direction goes nowhere if say, a dynamo is offering equivalent or greater resistance in the opposite direction to capture that energy (again, assuming a "perfect" scenario with no energy loss). I tried to elaborate that to move the vehicle, you first exert a force (engine power output) great enough to move the vehicle from its static position (resistance? gravity? I don't understand physics enough to know what to call this), and anything like a dynamo (friction, resistance) serves to make that more difficult, so you can't simultaneously "recover" energy you are spending. I proceeded to explain that regenerative braking worked because the generated energy was being recovered from the moving vehicle's momentum through offering resistance, hence why the car slows down and why you can't sustain-ably use it while the car is in motion.
This guy is super stubborn to being corrected once a very dim bulb lights up in his mind and he believes it a genius idea and can't be dis-proven. I am horrendous at teaching constructively, and would appreciate someone breaking down why his idea is a victim of basic laws of physics.
I even stated if that were possible, it would already have popular applications in industry like electric vehicles.