Q1.
… it were possible to build a perpetual motion machine, which once started, could continue doing useful work forever, without any fuel or electricity?
Answer
It cannot be built, and the reason is the very theorem this chapter is built on: work done = change in energy. Doing useful work forever would mean handing out energy forever from a store that is never refilled.
Useful work delivered in time t = W
Energy taken from the machine's store = W (nothing else supplies it)
After time t: energy left = E0 – W
As t grows, the store empties and the machine must stop.
Running forever would need E0 = infinite, or energy created from nothing.
Energy taken from the machine's store = W (nothing else supplies it)
After time t: energy left = E0 – W
As t grows, the store empties and the machine must stop.
Running forever would need E0 = infinite, or energy created from nothing.
If such a machine did exist, imagine what would follow:
- No power stations, no dams, no fuel — a small box in every home would run lights, pumps and trains forever, and there would be no fuel bills and no smoke.
- But the same logic would break the rest of physics. Conservation of energy is what lets us predict the speed of a falling ball, the reach of a rocket and the heat from a fuel. If energy could appear from nowhere, none of those calculations would work — and yet they work every day.
Why the idea keeps returning: a well-made wheel or magnet arrangement can spin for a surprisingly long time, which looks like "forever" to a hopeful inventor. Careful measurement always shows a slow, steady loss to friction, air drag and sound. Every proposed design, when tested, has failed at exactly this point.
Did you know? The failure of thousands of such designs is itself scientific evidence. It is one of the strongest experimental supports for the law of conservation of energy — nature has been offered every chance to break it and has never done so.