1. No it does not as the rock will accelerate downwards (because of gravity) but we can also see that at the first second the rock fell 16 feet compared to the 2 to 3 seconds gap where it fell 80 feet (144-64).
2. All the way down to the ground. Each time gap is getting bigger and bigger (it's accelerating) and the gap between 5 and 6 s should be bigger than 4 to 5 and it is not as the rock cannot fall through the ground but only hit the ground
you may have to change the english a bit and english is not my first language
Pythagorean Theorem. One of the most common Pythagorean triples is 3, 4, 5 with 3 and 4 the legs and 5 the hypoenuse. That will make 9, 12, 15 anotther trible because you are tripling the side lengths. 12-9=3 so this triangle works.
