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Originally Posted by Ich
I don't quite understand that logic. What laws of physics forbid this? I'd expect that the less weight the head has, the less impact it has on the trajectory when you throw the shaft. If you'd have a heavy head and a light shaft, or hollow one like you suggested, then that would really **** up things. Unless you'd use it like in hammer throwing. BTW, from what I know, the soldiers had leather straps or something like that to propel their pila. They didn't throw them like a spear.
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Afaik it's about air friction and force/acceleration.
When you throw something air friction exerts a force on it that's proportional to the speed and size it has.
But the acceleration (in that case the break action) is force through mass as F=m*a.
So a smaller mass is slowed down more quickly by air friction.
Now imagine throwing two objects of the same size but with different mass tied together.
The light object will be soon slower than the heavy one, which means that the heavy one will move to the front pulling the light one with it.
It's a bit similar with the javelin, if the front end is lighter than the back end the javelin will start to turn in flight messing up tracetory.
That doesn't mean that the tip has to be more heavy than the shaft but heavy enough that the centre of mass is closer to the front than to the back end.
I'm quite sure that applies to athletic javelins as well, at least all arrows I've seen are constructed that way.