When you're shooting an arrow, you may have noticed that the arrow doesn't travel in a straight line. Instead it curves, and if you shoot your arrow straight without a target, it will eventually hit the ground. The force on the arrow is a common one that people deal with on a day to day basis. That force is gravity.
Forces on the Arrow
Once the arrow leaves your bow string, it has several forces acting on it. One is projectile motion. That is the motion the bow string imparts when the arrow is shot. The second is drag. Drag comes from air resistance the arrow encounters as it cuts through the air. Although arrows are very aerodynamic, there is always some air resistance as long as there is air. There is also gravity -- the same force which holds us to the Earth and keeps the Earth revolving around our Sun. This force will act on your arrow the moment it leaves the bow string.
How Gravity Works
Anything with mass has gravity. That means the Earth, you, the bow, and even the arrow have some amount of gravitimetric force. Granted, you don't have as much attraction as a planet or star, but the gravity your mass generates attracts the planet just as the mass in your body is attracted to the Earth. Thankfully gravity isn't a very strong force until you're dealing with huge amounts of mass; otherwise, your arrow would never fly, you'd never be able to walk, and in fact, if the force of gravity was stronger due to an increase in mass, you'd be squished into the Earth, rather than standing.
Gravity has an acceleration constant of 9.8 meters per second squared or roughly 32.2 feet per second squared. Acceleration is defined as the change in speed (velocity) over time. This becomes crucial when considering an arrows trajectory.
How Gravity Interacts with an Arrow
As said earlier, 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.
What This Means for the Archer
So, if you see a target, whether practicing or hunting, the moment you shoot the arrow, the arrow is going to start dropping. The farther out your target is, the greater the drop is going to be because the arrow has been in flight longer. Archers compensate by aiming higher than the actual target they're shooting at. What you need to first determine is the place where your arrow is zeroed on the target. Let's say for argument sake, it's 20 yards. Once you move up to 30 yards, you may see a drop in the arrows you shoot. (For the discussion, let's say 3 inches.) You will have to compensate for that drop by adjusting your sights for that range so that you'll shoot three inches higher when you use those sights. You'll need to make adjustments for each of your sights so you will hit when you line up the right sights.
By understanding how gravity works, you can become a more accurate archer either for competition or hunting.
Archery Equipment - Archery Supplies - Archery Gear - Archery Accessories
Leader Bowhunting supplies and bowhunting gear for 35+ years