Objections to Mechanical Perpetual Motion

The First Law of Thermodynamics — a "mantra"?

The first objection to perpetual motion raised by an orthodox scientist will probably be "It can't work because it will violate the First Law of Thermodynamics." Let's cast a critical eye over this law.

First, a point almost too minor to mention: The First Law is not the first law of thermodynamics. That honour belongs to the Zeroth Law of Thermodynamics.

The First Law of Thermodynamics can be written as the expression

             Q = (Uf - Ui) + W

or sometimes in differential form

            dQ = dU + dW

where Q is the heat absorbed in a thermodynamic system
            Uf is the internal energy function of the system in its final state
            Ui is the internal energy function of the system in its initial state
            W is the work done by the system

So, it is obvious both from its title and from its expression, that the First Law of Thermodynamics is applicable to heat engines, i.e. to devices whose operating principles depend on heat being absorbed or delivered. The same is true of the Second Law of Thermodynamics, which a perpetual motion machine is also sometimes said to violate.

If anyone wishes to extrapolate the First Law and/or the Second Law to apply to devices that do not depend on heat transfer, surely they must explain, in detail, why we should take any notice of them? Otherwise, for such devices, these laws risk being regarded as just impressively-titled but meaningless mantras.


The weight-driven perpetual motion machine.

Temporarily assuming the rôle of an orthodox scientist, here I'll set out as well as I can, what I understand to be the main objections raised by modern science to a weight-driven perpetual motion machine.

I'm aware that some critics might be unhappy with the way I've worded these objections. So I'm happy to be corrected — if there are any objections to my wording of these objections!

As far as I can see, there are only three basic possibilities for the operating principle of a weight-driven perpetual motion machine. These could act either alone, or in combination.

Gravity

i) Gravitational mass is what matters, i.e. the machine works by the action of the Earth's gravity on its weights.

Objection:— The Earth's gravitational field g is conservative, so if we obtain energy of mgh by allowing a mass m to fall through a height h within that field, we must always expend no less than mgh to return it to its starting point (or to any other point at the same height). We must return the mass to its starting point in order for the machine to continue to operate. Whatever path the mass follows within the field, while it is falling or rising, makes no difference to the energy obtained or expended over the total fall or rise.

Inertial propulsion

ii) Inertial mass is what matters, e.g. the machine is a wheel containing a number of "inertial propulsion" devices arrayed around its rim to give a "Catherine wheel" effect.

Objection:— An inertial propulsion device cannot work. If it did, it would have to disobey Newton's third law of motion, one of the most fundamental and never-broken laws in physics.

Energy from Earth

iii) The machine extracts energy from the rotating Earth.

Objection (Quoting directly from a Nobel Laureate in physics):—

"If your process is to extract energy from the Earth's rotation then the Earth will slow down to conserve energy, and conservation of angular momentum would appear to forbid this". The law of conservation of angular momentum is another of the most fundamental and never-broken laws in physics.

Frankly, unless I'm overlooking some subtlety, this last objection seems to be a fairly weak one. If we can postulate a device that transfers energy from the Earth to itself, then why can it not also transfer angular momentum from the Earth to itself, in such a way that both energy and angular momentum remain conserved overall? I may have more to say about this later, but I don't want to get too far ahead of what is intended to be a more or less chronological blog.