# Projectile Motion - Explanation

 Projectile motion is motion under the influence of gravity. If we stand at the edge of the roof of the Science Building and throw a ball up at an angle, it moves up and then down vertically while it moves horizontally. This is projectile motion. To better understand this projectile motion, let's move back and then look at it through the eyes of two different and special observers. What is the motion seen by a far-distant observer on the ground? This observer is far enough away she has lost depth perception but can clearly see the ball rise and fall. She observes free fall, just as if the ball were thrown straight up. This is vertical motion with constant acceleration. What motion is seen by an observer overhead? This overhead observer is high enough that he has lost depth perception but can clearly see the ball move horizontally. He observes horizontal motion with constant velocity. Projectile motion, then, is a combination of vertical motion with constant acceleration (free fall that we have already discussed) and horizontal motion with constant velocity.
 We will throw the ball and stop the motion every second and look at the velocity. We throw the ball so it moves up with an initial vertical velocity of viy = 20 m/s and so it moves horizontally with an initial horizontal velocity of vix = 15 m/s. (We could also describe this as having an initial velocity of vi = 25 m/s at an angle of 53° from the horizontal).

We throw the ball so it moves up with an initial vertical velocity of viy = 20 m/s and so it moves horizontally with an initial horizontal velocity of vix = 15 m/s.

Now we will look at its velocity every second:

Look at the horizontal components; look at the vx's. This is horizontal motion with constant velocity.

Look at the vertical components; look at the vy's. This is common, ordinary free fall; this is vertical velocity with constant acceleration.

We will throw the ball one more time. This time, we will again stop the motion every second. This time we will look more closely at the position.

As before, we throw the ball so it moves up with an initial vertical velocity of viy = 20 m/s and with an initial horizontal velocity of vix = 15 m/s. (We could also describe this as having an initial velocity of vi = 25 m/s at an angle of 53° from the horizontal).

Look at the horizontal positions; look at the x's. This is horizontal motion with constant velocity (vx = 15 m/s).

Look at the vertical positions; look at the y's. This is common, ordinary free fall; this is vertical velocity with constant acceleration (a = - g = - 10 m/s2).