Projectile motion has two components: horizontal motion and vertical motion. The two components are independent, to they have no effect on each other. When the two motions are combined, they form a curved path. Some examples are a frog leaping, water sprayed by a sprinkler, a swimmer diving into water, balls being juggled, or an arrow shot by an archer. An example of horizontal motion is when a baseball is being thrown. After the ball leaves the pitcher's hand, the ball's horizontal velocity is constant. The ball's vertical velocity increases because gravity causes it to accelerate downward. The two motions combined to form a curved path. Gravity pulls everything on earth down toward the center of earth. Because objects in projectile motion accelerate downward, you always have to aim above a target if you want to hit it.