A projectile is any object that is projected by some means and continues in motion by its own inertia. A cannonball shot from a cannon, a stone thrown into the air, or a ball that rolls off the edge of the table are all projectiles. These projectiles follow curved paths called trajectories. When air resistance is neglected (as we will do) the curved paths are parabolic in shape.
It may seem that the paths that the projectiles take will be difficult to analyze. However, if we use vectors to break down the motion into its horizontal and vertical components, the motion of the projectiles becomes relatively simple.
let's examine the horizontal motion of the projectile. Think back
to a ball rolling on a table. If we ignore the frictional
force, then no external net force is acting on the ball.
Therefore there will be no acceleration; the ball will move at a
constant velocity. The same applies to a projectile.
If we ignore air resistance then there will be no net force acting
on the projectile in the horizontal direction. Therefore
velocity of the projectile in the horizontal direction will be constant.
The vertical component of a projectile is acted upon by gravity. It is in free fall and will accelerate at a rate of 9.81 meters per second squared. This will result in an increase in speed, which will in turn result in object falling greater distances in successively equal time periods.
Look at the image below. This is a flash
photograph of two balls released at the same time. Each image of the
balls represents an equal time period of 1/30th of a second.
Both balls are falling freely, but one ball (the ball on the right) was projected horizontally.
Notice that the horizontal displacement of this ball does not
change in equal periods of time. The ball moves an equal distance to right for each successive time period. In the horizontal direction
there is no external force and therefore no acceleration. We
can see that the horizontal velocity of the projected ball is constant.
And in the vertical direction, there is a noticeable acceleration. We can see this because for each successive time period, the
vertical displacement of the ball increases. And comparing
it to the ball dropped freely, we can also see that the change in
vertical displacement is the same for both. Therefore, the vertical velocities
for each will be equal.