If ong can change his density at will, not only can he float, not quite flying w/o propulsion, he could take a punch like no other, by the same reasoning above.
F = ma, and it's that acceleration which hurts. Increase the mass and the acceleration goes down.
There's a classic physics demonstration where the lecturer picks a strong-looking student and gives him a big hammer. The lecturer lies down on the lecture bench and puts a heavy block on his chest. I've seen cinder blocks or small sections of railroad track used. The student can give the weight a solid wallop, and the lecturer wont feel much. Partially due to the force being spread out over a larger area, but mostly because the heavy mass doesn't accelerate much, so it shields the professor from the blow.
EDIT: Gratuitous on my part, but another cool demonstration:
I have a cannon ball with eye-loops welded onto opposite sides of it.
I tie a thin string onto each loop, and hang it from one of the strings.
Now.
If I pull down on the free-hanging string with a gradually increasing force, I will break the top string.
If I pull down on the free-hanging string with a sharp tug, I will break the bottom string.
The first case, I'm kinda just increasing the weight of the whole thing, and the upper string supports both the mass and my added force, whereas the bottom string only supports my force.
The second case, I tug quickly, and the acceleration of the mass is slight, so the force doesn't transfer through to the other string before I've torn the bottom string in half, and no longer applying any force.
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