How Heavy Should an Arrow Be? Flight Arrows, Crossbows, and the Physics Behind Them

How long or thick or heavy should an arrow be? Why are quarrels and bolts short, and is this good? Longbows and crossbows.

Even in twelve minutes, I only touch on this, and as you'll see from the awkwardness of the edit, I forget to say near the start that I was talking mainly about flight arrows - those designed for great range. I spent a few hours trying to find all the great and clear pictures I had of barrelled arrows, in vain - sorry about that.

Several people in the comments have said that I am using the terms 'inertia' and 'momentum' in a confusing manner. People are generally familiar with the fact hat heavy things do not want to go anywhere, and that when they are moving they are difficult to stop, and I used the terms to describe these two things. Both inertia and momentum vary with mass, so what I said was not wrong, but it is also true that some physicists will use the term 'inertia' also to describe the reluctance of a moving body to being slowed down. I apologise to any physics teachers out there who want to get the complete definition of 'inertia' into the heads of their pupils.