Balgus said:
Longbows have a longer range (i don't know the mechanics) so the arrows are launched at a greater initial angle than a shortbow. Because of this, their downward angle is greater, providing more gravitational momentum.
Whereas a shortbow's flight path has to be more level, and so all the force comes from the bow itself, the longbow is usually launched at a 30 degree angle and the momentum of it from falling out of the sky is what does a lot of the damage.
Uh, better revisit high school physics. A projectile (whether fired on an arcing path or a flat path) only gains energy from gravity when fired at a target below the level of the launch mechanism (measured relative to a horizontal plane normal to the gravity vector).
For simplicity, we'll talk about an arrow fired in a vacuum (things slow down faster in atmosphere, but the essential physics is the same). When fired, the arrow's velocity can be resolved into two components: a horizontal component and a vertical one. The horizontal component remains constant until the arrow hits the target (aside from drag slowing, but we're talking about a vacuum here).
The vertical component is what you're talking about. For an arrow fired perfectly horizontally, the vertical velocity vector always points downward (as a result of gravitational acceleration), which is why the arrow descends (ignoring lift effects that actually cause most projectiles to rise slightly during flight) until it hits the ground. Two arrows fired from the same height hit the ground with the same vertical velocity, regardless of the distance they travel due to horizontal velocity.
For an arrow fired at an upward arc, some of it's velocity is on a horizontal component, and some is on a vertical component (upward vertical vector). Gravity has the effect of gradually reducing the upward velocity (as a result of the downward gravitational acceleration vector) until zero velocity is reached, at which time the arrow's velocity vector turns to point downward, and the arrow accelerates downward. Because gravitational acceleration is constant over the distances we're dealing with, and because no other force acts on the arrow, at the point the arrow reaches the altitude it was fired from, it has exactly the same downward velocity (magnitude) that it had when fired upward.
In other words, when firing on an arc at a target on the same level, the arrow has the same velocity (and hence energy) at the target as it had at launch -- all gravity has done is changed the direction of the velocity vectors. Air resistance has the net effect of slowing the arrow and hence sapping energy, so in atmosphere the arrow fired on a long arc actually has slightly less energy than one fired in a straight line, because drag has a longer time to act on the arrow.
Which arrow does a better job at penetrating armor (fired flat or in an arc, given teh same energy) depends on the geometry of the armor, mechanics of the materials, and relation of the arrow's velocity vector to armor thickness. For a vertical plate, an arrow arriving at an angle is actually less likely to penetrate, because it has a greater thickness of material to penetrate (which is why the armor on most modeern tanks is sloped).
So why fire in an arc? To get more range. Any projectile fired level from a human being's height hits the ground in exactly the same time. By firing upward, you increase the time for the horizontal component of velocity to act, increasing your range -- which is maximum at a firing angle of 45 degrees.
The longbow's mechanical superiority isn't due to its firing arc. It's due to the mechanics of the bow, specifically the strength of the draw and length of draw, as others have said. Longbows tended to have much stronger draws than shortbows (though composite and compound bow types can be made stronger). All the strength of the draw does -- assuming the archer is strong enough to make full use of it -- is determine the maximum force that is available to act on the arrow.
The other critical factor is the length of the draw. The longer a force has to act on a projectile, the more energy is imparted to the projectile. For two bows of a given strength and othewise identical properties, the bow with the longer draw will impart greater energy to its arrow, and hence greater potential range and penetrating power.
This is a simplification, of course, as there are still other factors -- losses in bending of the arrow material itself, the energy discharge curve of the bow and bowstring -- serve to create even more differences between bows. The latter factor can potentially have a greater effect than draw length, which is why modern compound bows can outperform longbows even with shorter draws, as they have more efficient energy discharge curves.
All of which is long-winded way of saying "Yep, it makes sense for longbows to do 1d8 and shortbows to do 1d6, and for mighty compound bows to be able to add more damage."
