In the latest series, an experimenter told Eren that titans are very light despite their size. Why, then, does the ground shake when they walk?
It's probably not so much that the titans are very light eventhough they are really large, but that they are light for their size. Meaning that they're still really heavy, but not nearly as heavy as their size would make it seem.
Anime vs Real physics aside, if something really large was light enough not to make the ground shake, at least a little, they're probably not heavy enough to generate the needed friction to run, or if they were to move their body really quickly they'd leave their feet, or if they jumped they'd be in the air for much longer, or if they stomped the ground hard, they'd launch themselves into the air, etc.
All the interactions of the titans make them seem like they weigh enough to interact normally with their surroundings, just that for their size, they actually weigh a lot less than expected.
The whole weight thing is rather unexplained. On the one hand, they said titans are incredibly light, on the other hand, they can dish out destruction on the level you'd expect from a humanoid this size.
Were the weight thing consistent, Titans wouldn't be able to destroy buildings or make the earth shake.
So far, it wasn't entirely explained, it's possible that they weigh enough to cause the ground shake or destroy things, but light enough for the head to be easily kicked around after it was severed.
That’s a question I’ve asked myself a lot. I’ve come to the conclusion that we shouldn’t consider titans as larger human beings as their structure is fundamentally different from ours. It has been proven that when an animal’s size increases by 10 times, the strength increases by 100 times and the weight by 1000 times. For example, I’m about 1 meter 70 tall and roughly 60 kg. If I were to be ten time bigger, I’d be a 17 meters class and have a hundred times my actual strength, but my body would weight 60 tons!
So Titans are light (as revealed by Hange Zoe), but I also think they have abnormally high strength for their size. At the end of the Trost arc, Mikasa said when she saw Eren lifting the boulder that a human that size couldn’t lift it.
Let’s try to calculate (roughly) the weight of the boulder. Eren (15 meter class) is taller than the boulder, and it seemed bigger than the hole that it is supposed to sealed, which was mentioned to be 8 meters high. So I would say it has a diameter of about 10 meters, and so a volume of approximately 525 m3. That makes it more or less 1.4 million kilograms! So of course a human that size couldn’t lift that, unless you can lift 14 tons, plus ten times your own weight with your natural size. So at that point, I understand why the earth was rumbling under Eren’s footsteps.
But I’m getting away from the subject. I’d like you note the fact that after the colossal titan’s appearance in front of the Trost gate, once Eren made him disappear, you can see step marks on the ground, which would lead you to think of an immense weight (I don’t know if you can see it in the manga).
Titans are both abnormally light and strong for their size. The shaking of the ground is just an effect in the anime to emphasize their size and the danger they represent.
Their weight comes from what they are powered by. Thinking about it, they secrete clouds of heat and "Burn out" when too much energy has been expanded (the hybrid humans after using their titan form). And being as impossibly strong, fast, quick motioned as they are (erin lifting a boulder about 50 tons heavier than his titan weights) the best possibility that they run on adrenaline more than blood. Adrenaline is the only reason fighting, sprinting, moving large objects is possible. The titans, abnormal or not, sprint instead of run, crush instead of grab, can THROW another titan, and obliterate both the section of the opponents body and the hand or limb they used. Adrenaline. The titans lack a digest system, which questions what else they have besides bone and muscle. This can give way to a major adrenal system instead of gland, and since adrenaline is activated by the components of blood, thier constant need is both give the adrenaline a power source and cool them down to avoid "burning out", adrenaline works like an acid, and a heat source in desperate times. To explain thier weight is the fact that they lack the organs that weight us down.
The simplest thing I can think of is when Hanji tells Eren about the titans being light, the examples she uses are:
1-A dismembered titan head.
2-A dismembered titan arm.
We also know that titans bodies quickly (or at least semi-quickly) evaporate after being killed. Considering that, we can assume that Henji's data was incorrect due to a large portion of the parts she tested with having evaporated. So the titans may possibly be heavy still, thus, the ground shakes when they move.
The point is: density. Titans are not light, they're just not dense. That is to say, they're light for their size or relative volume.
They still probably have a mass of thousands or tens of thousands of kilograms, but they're much lighter than normal biomechanical physics would dictate for an organism that size.
Typical flesh generally has a density of around 1000kg/m^3, where whatever a titan is made out of might have something closer to 400kg/m^3 or 300/m^3. This does not mean titans are light, it means an equal volume piece of human would be heavier than that equal volume of a titan.
But, titans are huge. An average human male weight around has a mass of around 80kg, so we'll call that a volume of 0.08m^3 (assuming the 1000kg/m^3 density)
Now, a 15-meter titan might have a volume of (lazy guess) 8m^3 Which even with a much lower density like 400kg/m^3 that's 3,200 kilograms, or about 3 and a half US tons.
I probably think that Titans produce alot of energy, and some of that energy goes into the surface, creating a small quake. Objects don't need size to create large forces of energy to create a shake on the ground. You could just drop a 1 ton coin I guess and it will shake the ground. You can even drop a building around 100 meters tall constructed out of aerogel and it will create NO force at all.