I was actually going to write the same thing in the series thread, but realised that speed as such does not require any energy input. Only acceleration/deceleration requires energy. Momentum is static (conservation of momentum). So, however an object gains its speed, the speed is ultimately a physical attribute that has got nothing to do with magic anymore and thus is nothing Touma's power can remove. You could say the acceleration is separated from the deceleration in that sense.Originally Posted by Munsu
However, I still can't accept any fancy explanations how he can negate energy just like that. I simply can't see such a difference between matter and energy as the manga author apparently can. Both are physical properties of this world. For magic (or ESP functions) to have any effect on a human, it should somehow affect this reality, either by creating matter or energy interaction (the magic is converted into energy, like heat or it changes the state of matter directly). Otherwise magic shouldn't have any effect whatsoever on Touma or anybody or anything else. So, somewhere along the way Touma only negates the magic converted into energy but fails to do anything about the magic affecting matter.