For any projectile launched horizontally, the projectile is given an initial horizontal velocity.
The initial vertical velocity is of course equal to 0.
For, example, let's say that you launch a cannonball horizontally as shown in the picture below.
Once the cannonbal is in the air, there are two things happening with the cannonball at the same time.
The cannonball will be affected by the horizontal speed and the vertical speed. First, let us talk about the horizontal speed.
Notice that there is no horizontal acceleration because there is no horizontal force acting on the cannonball . With no horizontal acceleration, the cannonball will move horizontally with constant velocity.
Below, we show constant velocity by using vectors that have the same size.
Second, as the cannonball move horizontally, gravity will force the cannonball to fall vertically as well. Therefore, the cannonball will experience free fall. We saw in the lesson about free fall that objects in free fall accelerate. This means that the speed is not constant, but keep on increasing. Since the speed is increasing, the object will cover a greater distance in each successive equal time interval. Below, we show a speed that is increasing
We can combine the vertical and horizontal movement/speed of the cannonball to see what we end up with. See below the horizontal speed, the vertical speed, and the resultant speed in red. The ball move along the green curve which is a parabola. The vectors shown in red (resultant speeds) are tangent to the green curve although it does not look perfect.
The path traced above in green is the path that a projectile will follow when the following conditions are met.
Next, we learn about the path and speed of a projectile launched at an angle.