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Teh Gomo Gomo punch.....
lufy body is ruber right?????..
now if he punch someone and strech his arm...isn't that punch suposed to be weak?????...cuz teh skin is made of ruber???..not to mention teh strech process weakenz teh punch in my opinion..
By my understanding he's still made up of bones and flesh though, that can actually bleed and all. All of the cells in his body just happen to be very elastic, rubber like.
And just like a normal human being he can still contract his muscles just fine. Means that he can clench his fists, which are actually as tough as superhuman bones happen to be.
Basically: He's stretching out his arm at superhuman speed, while the clenched superhuman durable fist makes contact.
So yeah, that hurts.
There are many manga notions that don't stick well to real life physics about Gomu Gomu such as G3 becoming so massively powerful despite the arm is literally inflated with air.
It adds up though. High amount of air (which actually has mass) under strong enough air pressure with a pretty tough outer "skin" actually creates an even rather powerful physical body with decent mass.
Luffy's body is still superhumanly durable, even if it's elastic. And while stretched out his tissues keep their size, meaning in contrary to RL rubber, the stretched out parts aren't losing mass and durability, ergo he's not becoming a super soft balloon like being.
Just imagine a tractor tire like wrecking ball is swinging around, you do not want to get hit by that.
Or imagine one of those big gymnastic balls with max air pressure inside, at which you cannot evem push it in anymore, combined with a stone hard exterior.
If you could transform your fist into something like that, while having enough physical strength to be able to swing it around at considerable speed, you could prolly fist fight a smaller bear or some.
Iirc his first G3 attack against Lucci was calculated at around 30+Tons in weight w/o even some sort of overaverage air pressure included.