mass/gravity question

Hi, I m working on a vector engine (Vectrex like), and it needs some little physics.

what I have for now is:

; Computing Gravity Vector
gx = Cos(-90 + gravity_dir) * gravity_force * object_mass
gy = Sin(-90 + gravity_dir) * gravity_force * object_mass

; Adding Gravity to the Object's Direction 
dx = dx + gx
dy = dy + gy

I can't think an objet of 50kg falls 50x faster than a 1kg Object.

Any help on the formula?