Final answer:
A 100 mi/h pitch and a 10 mi/h pitch both hit the ground at the same time when thrown horizontally from the same height because gravity acts on them equally and independently of their horizontal velocities.
Step-by-step explanation:
The reason a 100 mi/h pitch thrown horizontally by a major league player will hit the ground in the same amount of time as a 10 mi/h pitch thrown horizontally by a child from the same height is that the vertical motion of both pitches is solely due to gravity, which is constant and acts equally on both balls.
The horizontal motion has no effect on the time it takes for an object to fall to the ground because gravity acts only in the vertical direction. This principle of physics means that all objects in freefall near the Earth's surface accelerate downwards at the same rate (approximately 9.8 m/s2), regardless of their horizontal velocity.