replace
c_move (my, vector(40 * (key_w - key_s) * time_step, 0,0), nullvector, IGNORE_ME|IGNORE_PASSABLE);
with
c_move (my, vector(40 * (key_w - key_s) * time_step, 0,0), vector(0,0,-10) , IGNORE_ME|IGNORE_PASSABLE);//global gravity