You can use any coordinate system you like in order to solve a projectile motion problem. to demonstrate the truth of this statement, consider a ball thrown off the top of a building with a velocity v at an angle θ with respect to the horizontal. let the building be 54.0 m tall, the initial horizontal velocity be 8.80 m/s, and the initial vertical velocity be 12.5 m/s. choose your coordinates such that the positive y-axis is upward, and the x-axis is to the right, and the origin is at the point where the ball is released.