This model illustrates the motion of a projectile ? the green ball, which leaves the ground moving at 10m/s in a direction 53? above the horizontal. A description of projectile motion involves the analysis of two independent types of motion: horizontal, described by the equation x - x0 = v0xt + (1/2) axt2, and vertical, described by the equation y - y0 = v0yt + (1/2)ayt2. The projectile moves with zero acceleration in the x-direction (ax = 0) and gravitational acceleration in the y-direction (ay = -9,81 m/s2). Therefore the blue ball, which moves without acceleration along the x-axis, and the red ball, which is initially moving upward with gravitational acceleration, arrives at position x and y, accordingly, at the same later time.