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We know that in the Mortal Engines series, some cities are mining cities, doing exactly what it sounds like: mining the materials that their city needs for fuel, and so forth, instead of cannibalizing other cities. This makes sense: cities have enough energy to move themselves, so they should have more than enough 1 to accomplish the task of mining.

Why don't they all do this, though? It seems like mining is more lucrative and safer than chasing after and fighting a handful of small cities for materials, which could also lead to material loss during the combat.

1Specifically, the kinetic friction force is just the gravitational force multiplied by the coefficient of static friction (because treads). The work is this force times distance. The static friction, let's say, is similar to rubber on concrete (0.5). Thus, roughly speaking, moving the mass of a city a certain distance (say 100 km) takes as much energy as would be needed to lift an equivalent mass half the distance (50 km). In reality, excavation of useless material will cause the energy needs to be much greater, but this doesn't matter much (if 2% of excavated material is usable, bringing a full city's mass of it up from 1 km underground is on the same order as moving the city 100 km over land). There will be extraction and reworking costs, but there will also be such costs with consuming smaller cities.

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    Cows and wolves – Valorum Dec 10 '18 at 8:58
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    I don't have the books handy for any direct quotes (hence comment not answer), but essentially the cities became mobile when due to widespread seismic upset meant staying still was too dangerous. The moving cities become almost a cultural thing, where the idea of settling down is seen as a sign of weakness and barbarism, as being the biggest, meanest city is a sign of prestige – Psycrow Dec 10 '18 at 11:24
  • @Valorum - That's a good analogy: wolves rarely successfully attack adult cattle. – Adamant Dec 10 '18 at 12:11
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    @Adamant - Sure, but when they make a kill, the wolf is sitting pretty for weeks without having to spend all day chewing cud – Valorum Dec 10 '18 at 18:08
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    @Valorum - The energy ratio doesn't make sense in the context of the number of prey cities. The ratio of wolves to prey is much smaller, and their energy requirements to hunt relative to herbivores or even photosynthesizers much more balanced. – Adamant Dec 10 '18 at 18:13
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The books are set in the dying days of the traction cities when the old hierarchy and ecology is breaking down. One of the first things we learn in Mortal Engines is that London has been in hiding for many years because it's afraid of becoming prey to one of the larger continental cities because the mining towns have become so scarce. When the traction cities were new there were many, many mining towns in the Hunting Grounds. In such an extremely prey rich environment the cities were better off taking multiple small towns with full resource bunkers every week than doing their own mining. As a result the cities either never had, or just didn't keep up, the equipment they would need to extract resources for themselves, they have to hunt to survive.

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    Quotes referencing the rarity of prey and of the stripped out condition of mining opportunities would improve this answer. – Harabeck Dec 11 '18 at 23:17

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