Projectile Motion If i set something moving near the earth it reduces to a 2-D problem we call projectile motion Use a coordinate system with x along the ground, y vertical with respect to the ground (Notice no change in third direction.) Equations of motion reduce to X:△X=Vot a=0 Y: y= yo voyt-g t4/2 y positive upwards Physics 121: Lecture 6, Pg 11Physics 121: Lecture 6, Pg 11 Projectile Motion. If I set something moving near the earth, it reduces to a 2-D problem we call projectile motion. Use a coordinate system with x along the ground, y vertical with respect to the ground. (Notice no change in third direction.) Equations of motion reduce to: X: x = voxt ax = 0 Y: y = yo + voyt – g t2 /2 y positive upwards