After the recent chapters, I have seen some people saying that Kid shouldn’t be able to hurt Big Mom just “hitting her with big metal arms” without even using Haki.
I will try to explain how a powerful Devil Fruit together with brute force was able to harm her. First, how does Armament Haki increase damage of an attack?
There are three levels of Armament Haki: Hardening, Emission and Internal Destruction.
When you Harden your attack, you: 1) Increase your defense, making you able to put more force to the attack without being damaged back by its reaction force, and 2) By hardening your body part or weapon, you increase its resistance to deformation, making it hit harder. A wooden rod hurts much less than a steel rod because it is softer, that is, it deforms more on impact, dampening the force.
F = ma, so: Harder attack -> less deformation -> faster deceleration -> greater force -> more damage.
When you use Emission, you make the Haki flow out from you in a burst, causing impact. So it simply adds even more force to your attack.
Internal Destruction is when you use your Emission to destroy something from the inside, causing internal damage and bypassing defenses. So far, only Luffy and Rayleigh are explicitly confirmed to have this level of Armament Haki, but probably other characters can do it as well.
Hardening and Emission increase a lot the force of your attack, but if you have enough resistance and strength, you can still do huge amounts of damage without them.
So, how Kid is doing that much damage to Big Mom by “throwing metal at her”?
First, let’s look at some of his attacks to the Yonkos during this raid.
See, he doesn’t just hit them with metal like a punch, he crushes them! He uses his magnetism to keep his metal constructs together, preventing them from breaking or bending (like Armament Haki hardening), and then crushes them with tons of metal with his superhuman strength. Kid’s attacks aren’t external, like a punch, he hurts their insides by the immense pressure of his attacks, even Queen said it is possible.
The need for Armament Haki Hardening and Emission is bypassed by him hardening his metal with magnetism and by using huge physical strength + tons of metal to have enough force. He doesn’t need his metal to be harder than Big Mom skin because he hurts her insides with the pressure of his crushing attacks.