Question

the arrow flies across the field, two forces act on the arrow while its in the air. Air resistance acts in the opposite direction to the movement and gravity acts downwards. these two forces cannot balance each other, even when they are the same size. why is this???

Answer 1

Because gravity is acting straight down. It does not act side to side. On the other hand, air resistance acts (in this case) only side to side, in the opposite direction of the movement. Even if the two are the same size, they are not acting in the opposite direction of each other, so they do not "balance each other out," i.e., do not stop each other.