Here's a start, any object falling from the sky has its weight multiplied due to the gravitational push towards earth. So for instance if the arrow was shot into the air and landed - it would have landed at the gravitational push of about 32.2 ft per second (or 9.8 m/s) for each second that the arrow falls until it reaches its terminal velocity. For e.g. the arrow is going down in 10 seconds, 9.8 x 10 = 98. So the arrow hits at a speed of 98 m/s. In short = The higher the arrow is shot into the air the more faster it is going down. So going back to your kevlar being strong enough to withstand arrows - it won't stand a chance as the arrow is going down at a fast speed (you add the speed and weight coming down towards the target together), there is a likelihood that it will penetrate the kevlar especially if the target is sitting or standing still. So yes, in this sense a simple arrow can be more deadlier than bullets and can penetrate kevlar. And if you still don't believe me then why not try an arrow made out of titanium steel and see
