Question

Captain American throws his shield straight down off a roof with a speed of 50 m/s . What is the acceleration of the shield halfway down to the ground?

Answer 1

IF this is happening on Earth, then the acceleration of gravity wherever the captain and the roof are is about 9.8 m/s² downward.

IF his shield is not affected by air resistance, and it's free to behave under the influence of gravity alone, then its acceleration is 9.8 m/s² downward, from the moment it leaves his hand until the moment it hits a building, a tree, a car, a bus, an evildoer, or the ground.  

Whatever speed it has when it leaves his hand, in whatever direction, makes no difference.