Final answer:
The horizontal velocity of the ball remains 6.5 m/s after 0.5 seconds since in projectile motion without air resistance, horizontal velocity does not change.
Step-by-step explanation:
The question involves understanding the principles of projectile motion without the influence of air resistance. In such scenario, the horizontal velocity of an object in projectile motion remains constant if air resistance is ignored. So, if a ball is thrown from a housetop with a horizontal velocity of 6.5 m/s, after 0.5 seconds, its horizontal velocity would still be 6.5 m/s.