Heavy armor can protect from arrows but are not impervious to it, plus most armors are designed to protect the front and the back mainly leaving spots here and there since the knight needs to move or just look around (a lucky arrow on the visor place even when closed might penetrate and kill the knight). A knight could tank quite a lot arrows but they dont need to kill the Knight just impede their movement or just kill the horse which was far less armored and often resulted on the knight dieing by being trampled or by the fall anyways. Even staggering some knights might be enough to crash against the near knights since they used formations. On that sense the Knights became the medieval version of tanks without infantery support (they would die if they get close due to having far worse armor or even none) and were picked apart easily.
Gravity starts interacting with the arrow once it leaves your bow string. That means that your arrow starts falling when you and your bow aren't holding it up. Now, there is a certain amount of air resistance or drag on the arrow, which causes a small amount of resistance to gravity, but given that the arrow's shape when dropped doesn't build up much air resistance, for all intents and purposes, it accelerates at 32.2 feet per second squared. This means that if you shoot an arrow and drop one from the same height at the same time, they will hit the ground simultaneously. (Assuming no other force interacts with those arrows.) The longer the arrow flies through the air, the faster the speed is when it hits the ground. So, the flight path of an arrow is in the shape of an arc with the highest point being when gravity overcomes the vertical propellent force imparted to the arrow. In layperson's terms, if you shoot level, the moment the arrow leaves the bow string, it loses height.
To add an extra complication, heavier arrows, that is, arrows with more mass, will move more slowly than a lighter arrow when shot with the same bow. As a result. the heavier the arrow, the faster the arrow will fall. Now that arrow will penetrate the target deeper than the lighter arrow, given Force = Mass x Acceleration. Add more mass or more acceleration, and your arrow will penetrate further, as a general rule. Thats why volley fire was used against heavy targets: to increase the penetration and avoid the most armored areas like the chest, same way that against modern tanks you dont atack its front if possible because the armor is the thickest there unless you have enough firepower to simply not care