Projectile Motion By Professor Walter Lewin Projectile motion is a fascinating example of two independent motions happening simultaneously: horizontal and vertical. When you throw an object, like a ball, horizontally, it moves forward at a constant speed because no horizontal forces act on it (ignoring air resistance). Meanwhile, gravity pulls the ball downward, causing it to accelerate vertically at 9.8 m/s². Despite being separate, these motions combine to create a curved path known as a parab