What I have been able to understand is the only thing that doesn't make it seem like a plothole is transience. Wherever the part of his body is teleported is free from constraints of time and space, therefore whenever he moves, the only parts that are liable to physics laws are the one that are tangible. If his head goes through and he isn't blind from being transported to another dimension is due to the 'instant' factor of it and the fact that this dimension probably acts as a form of 'still' dimension, where nothing has form, and time is not a commodity. So when he goes through it, his head wouldn't notice the transition.
When he moves through in a linear manner, then again, the only part that stays here governs the direction of motion. For example, if his head is attacked first, then he already made a movement with his feet to go north south, up or down or in whichever direction that suits him. The motion is happening, and the intangibility wouldn't effect it because of the worldly constraints that don't apply to that dimension at all. That's what I have been able to understand.
Why is everybody over analyzing the non existent science of the Manga Naruto. Suspended disbelief is the answer to everybody's question as it has been said already....Why he can't Obito still attack while only part of his body is incorporeal? It would make him too Haxx'd and more questions such posed on this thread would be ask'd..Has there ever been a need for a jutsus to be explained scientifically in Naruto before?? Even Comics go around science EX.Bruce's banner transformation into Hulk where does his extra body mass come from?? Another dimension? Ya thinking too much don't hurt Ya brains.
Kakashi was also the one that mentioned how his jutsus worked during their 1st encounter when he was going by Tobi. He mentioned him moving just a parts of his body to another dimension when really it was shown it's his whole body. Later on then learned that their eye technique's were connected Obito and him only possessing half of the true Eye's technique but still closely related as to how he hit him with Naruto's Rasengan in his shoulder.....It's just interesting to think if Obito had both his eye's he would be able to simply Kamui everything W/O touching the person.
Putting aside the whole physics mumbo-jumbo that doesn't mean a thing in a manga, I believe Obito's Kamui function in a pretty simple way:
he activates the jutsu, and every part that is touched gets sent on the other dimension.
The kunai touches him, and thus the part the kunai touches becomes intangible. His hand as well is touching Naruto, and thus becomes intangible, like the chain which is connected to Obito and he used as an intangible/tangible weapon.
Simply put I don't think he can decide what to make solid and what to make intangible, he activates the jutsu to become intangible if touched, and turns it off to attack and/or to absorb.
This seems the only logical reason to me ( apart from PIS ) as to why Obito can't attack while using Kamui
If Tobi had been somebody other than an Uchiha, the origin for Kamui could have been that that is the MS for non-Uchiha with an implanted Sharingan. This would explain why the Uchiha were so xenophobic and closed off. They weren't segregated, they were keeping a lock and key on a dangerously OP power that could ONLY be accessed by outsiders.
The thing with energy and mass is... you don't freakin' DARE dividing by ZERO. 1/0 isn't infinity. mr (mass during move)= m0 (mass when not moving)/ sqrt(1-v^2/c^2). Thus, when v=c 1-v^2/c^2=0. Thus, mr=m0/0. E=mc^2, energy doesn't exist. d=m/v, density doesn't exist. But just add 0.000000000000001 m/s to that speed, you get negative mass, and thus, negative energy. Not infinite, negative. Like it gives you energy, not use. Btw, time will go backwards. And the faster you get, the slower time will flow backwards >.<
be nice, insulting other nations isn't cool :^_^ - UB
And my point was, they wouldn't be black holes, because they wouldn't exist. Or more precisely, they don't travel with a speed of light if they DO exist. Black holes are objects with high gravity, that even light can't escape. Ever heard of neutron star? If its radius is small enough, it becomes black hole. Damn, it's in first class in high school. No wonder why many people in my country go to America or England after getting degree. They have competion there, in America they don't.