Final answer:
The resultant force on the human cannonball in the vertical direction during flight is the force of gravity, which is equal to the cannonball's weight calculated as mass times the acceleration due to gravity (9.8 m/s^2).
Step-by-step explanation:
The resultant force on the human cannonball in the vertical direction while flying through the air is primarily due to gravity. When the human cannonball is in the air and is not being pushed by other forces, the only force acting on them is gravity, which pulls them down towards the Earth with a force equal to their mass times the acceleration due to gravity (which is approximately 9.8 m/s2 on the surface of the Earth).
If we ignore air resistance and other potential forces, the resultant force is simply their weight, calculated as mass times the acceleration due to gravity. This force will continue to act on the cannonball until they are acted upon by another force, such as the normal force from the ground upon landing which opposes this vertical motion.