Why things fall over on the Moon

[Gfamily]

A twitter thread
https://x.com/DrPhiltill/status/1761531341978427402

[IvanV]

Any chance someone could make that visible to non-twits, please?

I'm guessing that lower gravity means that the energy well that allows something to remain in unstable equilibrium is much shallower. So it needs a smaller impulse to get something out of that energy well and into a condition of falling into a more stable equilibrium.

For example, a rod stood on its end is in unstable equilibrium, because fallen over is a more stable equilibrium. But it stays in that unstable equilibrium, absent impulses to knock it over, because to transition from standing on its end to falling, the centre of mass initially has to rise a small amount, to get it past the tipping point.

Clearly on earth, there are impulses from winds, etc, which are absent on the moon. The reduced presence of impulses to knock things over might potentially allow things to remain in unstable equilibrium for a long time on the moon, despite the shallow energy well. But maybe we are talking about astronauts clumsily knocking things over, rather than the ability of a rock to stay standing in an unstable equilibrium. Or maybe there are natural impulses on the moon to knock things over, that, in combination with the lower energy well, means that rocks on their end do fall over more easily than on the earth. For example, you'd have heat-cold effects much larger on the moon because day-night temperature differences are much larger in the absence of an atmosphere and much longer day-length. That's the bit I'm not sure about.

[philip]

https://threadreaderapp.com/thread/1761 ... 27402.html
When moving sideways at any given speed the kinetic energy is the same irrespective of gravity. Tipping over involves having enough energy to overcome a gravitational restoring force that does depend on gravity. So in reduced gravity objects tip over at lower speeds.

[IvanV]

https://threadreaderapp.com/thread/1761 ... 27402.html
When moving sideways at any given speed the kinetic energy is the same irrespective of gravity. Tipping over involves having enough energy to overcome a gravitational restoring force that does depend on gravity. So in reduced gravity objects tip over at lower speeds.

Thanks.

So related mainly to human actions tipping things over. Not a discussion of whether random rocks in unstable equilibrium can remain like that for a long time. But I suppose there is a strong implication that rocks moving in a landslide would be much less likely to come to rest in a condition of unstable equilibrium on the moon than on the earth. That might explain why we don't see many rocks in unstable equilibrium lying around on the moon, despite the lack of weather to tip them over.

[Gfamily]

https://threadreaderapp.com/thread/1761 ... 27402.html
When moving sideways at any given speed the kinetic energy is the same irrespective of gravity. Tipping over involves having enough energy to overcome a gravitational restoring force that does depend on gravity. So in reduced gravity objects tip over at lower speeds.

Thanks.

So related mainly to human actions tipping things over. Not a discussion of whether random rocks in unstable equilibrium can remain like that for a long time. But I suppose there is a strong implication that rocks moving in a landslide would be much less likely to come to rest in a condition of unstable equilibrium on the moon than on the earth. That might explain why we don't see many rocks in unstable equilibrium lying around on the moon, despite the lack of weather to tip them over.

The latest "Awesome Astronomy" Podcast has an interview with Dr Phil Metzger (originator of the tweet) and it ends with a discussion of the rather catastrophic excavation under the launchpad during the very first all-up 'Starship' launch in April last year.
The rocket thrust broke through the concrete under the launch pad, and entered the water rich sandy sub-soil where the heat caused the same effects as a volcanic eruption, throwing rocks out at up to 90 metres/second.
The relevance to this is that many of the same physical conditions may be found towards the southern pole of the Moon, which is where there are plans for landings as part of the Artemis programme, and the return to Earth launch is likely to have a similar effect (given the relatively high water content of the polar lunar soil).

[Sciolus]

For example, a rod stood on its end is in unstable equilibrium, because fallen over is a more stable equilibrium. But it stays in that unstable equilibrium, absent impulses to knock it over, because to transition from standing on its end to falling, the centre of mass initially has to rise a small amount, to get it past the tipping point.

A pedant writes: the word for that (where a system is stable but only very locally) is metastable, rather than unstable.

[shpalman]

Any chance someone could make that visible to non-twits, please?

I'm guessing that lower gravity means that the energy well that allows something to remain in unstable equilibrium is much shallower. So it needs a smaller impulse to get something out of that energy well and into a condition of falling into a more stable equilibrium.

For example, a rod stood on its end is in unstable equilibrium, because fallen over is a more stable equilibrium.

If it has any kind of energy well it's a stable equilibrium, it may just require some activation energy to be able to access a stable equilibrium at a lower energy.

A rod with a flat end can stand on a flat smooth horizontal surface "indefinitely" but a rod with a pointy end can't. That latter is an unstable equilibrium.