Newton's Law of motion.

So, to swing the mass of the pocket watch, the energy is coming from the mainspring.

I suspect external forces are at play.

As far as the watch is concerned, this is lost energy.

I don't follow. Isn't whatever is lost while swinging one way regained on the return swing?

You need to have your house fitted with a giant tourbillon.

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But considering the OP question seriously, no I don't think that the watch will run slow. The energy of the spring is still the same and so is the mass and inertia of the watch.

I need to think about this, it's cool though.

I believe this was first observed by Huygens and is the foundation for all the resonance synching watches which make an attempt to use that in order to improve timekeeping.

If you fix the watch to something I don't think you will do much as you won't prevent "communication" between the moving parts of the movement and the rest of the world. Indeed, what I would expect to happen is that if you attach a "weight" or "blockade" to the watch then the whole contraption will start swinging, albeit at an amplitude you couldn't measure without SciFi age equipment in case of something big, but energy also depends on mass.

That said, keep us posted because I am curious and hope to be wrong!

Every movement requires energy

I’m going to consider time while I drink my coffee on this glorious Sunday.

Do I have too much time on my hands.

Yes.

So the assumption is that these watches are unleashing excess power? Inefficient, but a better option than not providing enough. Doesn't the escapement control the release rate?

I'm not sure what the practical point of any of this is, but I wouldn't bother putting in a weaker main spring. This seems like a lot of Gedanken hand wringing.

I wonder whether that lost energy would make them run slow or rather reduce the power reserve, or run time.

The rate is controlled by the palette fork and escapement releasing wound spring potential energy. The energy to rock the case is not available to release elsewhere so seems to me it might just wind down sooner while retaining its precision. 🤷🏼‍♂️

A wise man has spoken.

IMO watches have a few things in common with firearms. Both are machines/Tech. Some are built to tighter specs & you pay for that. Some you pay more for nostalgia or hype. In both cases accuracy and precision can be affected by operator error. Take care of the machine , use it wisely & always practice best habits. I hear very often "Rolex doesn't keep good time" "Why buy a Patek? It keeps shitty time and my uncle took his water skiing and now it's busted." These statements all equal operator error.

Post Script: I really couldn't resist posting a few pics of my son over the years. He's a watch enthusiast and a fair shot

OK. I've tried to be technical about this.

I have not cleaned or lubricated this while in my possession.

I've removed the caseback, so that my timegrapher can hear better.

Inconclusive results.

I don't know the lift angle of this watch. When sitting in the display case, the beat error has halved, compared to being clamped and the amplitude has considerably. In the display case the daily error after being left for several minutes, varies too much to reach a conclusion. Maybe being clamped while crown up, the timegrapher can hear the movement better than when the display case is just sat on top?

If someone with a cleaned, lubricated movement wants to pursue this further🙄😁

There is no practical point😂 Just a need to know. 😊

Could it be that beat error is causing this? If the balance is swinging one way more than the other (which yours is with 7.9ms of beat error) would this be enough to unsettle the whole watch case. Not sure just a thought!

Unfortunately the post is fixed, and I'm not that far into the hobby to consider resetting the spring length. 🤪😮

I was just curious about where the energy came from to swing the watch, never intended to strip and lubricate the movement. Although the beat error correction, will stick in the back of my head. 😊 So a strip down at some time will be inevitable. 😁 Thanks for the video. 😎

No while in isolation the energy in an oscillating body is conserved in the real world the mechaniemergy will be lost to air resistance. So if this is caused by the energy in the mainspring I’m not sure if it will slow the watch down but will most certainly reduce its power reserve