Answer the following question given this paragraph: 

Earth’s gravity also affects the acceleration of objects that start out moving horizontally, or parallel to the ground. Look at Figure below . A cannon shoots a cannon ball straight ahead, giving the ball horizontal motion. At the same time, gravity pulls the ball down toward the ground. Both forces acting together cause the ball to move in a curved path. This is called projectile motion .


Q: Which factor causes a cannon ball to move horizontally and then fall on the ground?


A:
projectile motion