The direction of g is towards the center of Earth. Look at Fig. 5.14 in text. When the air resistance F air equals the force of gravity F gravity , the body reaches its terminal speed or velocity. The total force is then zero, and there is no more acceleration, so the speed is constant.
Air resistance tends to be proportional to speed.
N Projectile motion is always a parabola, because there is only a vertical force. The vertical speed changes due to gravity, but the horizontal speed is constant. There exists an attractive interaction between any two material bodies in the universe, and the magnitude of the force on each body is directly proportional to the product of the masses of the bodies and inversely proportional to the square of the distance between their centers. Newton figured out that the same force that causes an apple to fall to the ground from a tree is the same force that keeps the moon in its orbit. He worked out what the relationship must be.