Final answer:
The horizontal velocity of the ball after 1.5 seconds will remain at 20 m/s, which is the same as its initial velocity because no horizontal forces are acting on it.
Step-by-step explanation:
The question pertains to the horizontal motion of a thrown ball in the absence of air resistance. When an object is thrown horizontally, its horizontal velocity remains constant because there are no horizontal forces acting on it (assuming air resistance is negligible). Therefore, the ball's horizontal (x) velocity after 1.5 seconds will be the same as its initial horizontal velocity.
In this case, the ball is thrown with an initial horizontal velocity of 20 m/s. Since there are no horizontal forces such as air resistance affecting the ball's motion, its horizontal velocity will remain at 20 m/s after 1.5 seconds, or at any point during its flight until impact.