I think this would be the environment. That's my guess
The umbra is the innermost and dark part of the shadow where like it’s created by any light source and is completely blocked by the occluding body. It’s a Latin term hope this helps man!
When you drop a ball, it is dropped DOWNWARDS so the number is represented by a negative number. If you toss the ball up it will be a positive number.
To answer your question:
When an object is dropped downwards there will be a negative number since the object is falling downwards.
