Reply To: How to define gravity by vector components and why acceleration vector is -F.

[mohan.urs]

Hey,
Check this from help

However if you want to apply it in a certain direction - you can always define a coordinate system where on of the axes is parallel to your vector and use that to define the gravity load. 

And regarding the acceleration direction it is intended to be like that to represent the physics properly.
For more information on this check out these help links which explain them with examples
Standard Earth Gravity
Acceleration

Regards,
Mohan Urs