Ballistics is the study of projectile motion. A projectile is an object that has been launched, shot, hurled, thrown, or projected by any other means and that then travels on its own along a ballistic path. For instance, a baseball player throwing a ball from center field to the infield usually throws the ball in a slightly upward direction. The ball's path travels along an arc from the outfield to the infield. Mathematically, the arclike path taken by the ball is known as a parabola.
Ballistics has long been a subject of interest to scientists because bullets, cannon shells, arrows, and other weapons travel in ballistic paths. Military leaders have always valued the information that scientists were able to provide them concerning the proper way in which to aim their guns and bows in combat.
Consider a bullet fired from a rifle that is held parallel to (in the same direction as but never touching) the ground. The path taken by that bullet is affected by two forces. The first force is the velocity given to the bullet by the force of the rifle. (Velocity is the rate at which an object moves in a specified direction; it is measured in meters per second.) That force tends to make the bullet move in a straight line, out of the mouth of the rifle and parallel to the ground. If there were no air present, there would be nothing to slow the motion of the bullet and it would keep traveling with its original velocity.
A second force also operates on the bullet: the force of gravity. As the bullet travels away from the gun, it is pulled downward by Earth's gravitational field. Instead of traveling in a straight line, then, it travels in a curved path towards Earth's surface. That curved path, typical of projectile motion, is a parabola.
The exact shape of the bullet's path is determined by two factors: the mass of the bullet and the velocity with which it travels. The heavier the bullet is, the stronger Earth's gravitational field will pull on it. And the faster the bullet leaves the rifle, the greater its tendency to travel in a straight line away from the gun.
Finding the path for any kind of projectile is an easy problem in physics. If one knows the mass of the object and the velocity with which it is projected, then its pathway can be calculated by well-known formulas.
The practical importance of this calculation is obvious. If a naval ship fires a rocket at an enemy vessel, the path of the rocket must be known. Otherwise the rocket may travel beyond the enemy ship or fall into the water before reaching it. A rocket scientist has to know the path of a space probe launched to Mars if the probe is to land exactly on target rather than sailing on past its intended destination.
Air resistance is another important factor affecting projectile motion. As a rifle bullet travels through the air, it tends to slow down because of friction between the bullet and the air through which it passes. The amount of friction, in turn, is influenced by a number of factors. Among these factors is the shape of the bullet. Most bullets (and other kinds of projectiles) have pointed front ends—a feature that reduces air resistance. A bullet with a blunt front end would experience a great deal of air resistance and would slow down rapidly. Rotation affects air resistance as well. A good quarterback always tries to place a spin on a football. This helps the football to travel through the air more smoothly than it would without the spin.