Final answer:
The horizontal velocity of a cannon launched horizontally from the top of a cliff will remain constant at 35 m/s after 1 second, as there is no horizontal acceleration to change its speed.
Step-by-step explanation:
The question pertains to the horizontal motion of a projectile, specifically a cannon launched horizontally from the top of a cliff. According to the principles of physics, specifically Newton's first law of motion, the horizontal velocity of a projectile remains constant if air resistance is negligible. Since the cannon is launched with an initial horizontal velocity of 35 m/s and there is no horizontal acceleration mentioned, its horizontal velocity after 1 second will still be 35 m/s.