With the air density set to 0 kg/m3 (vacuum), what is the relationship between the mass and radius of the ball and its acceleration?

According to Newton's second law of motion, the force of the object applied to accelerate is equal to the mass of the object multiplied by the acceleration. However in reality, air friction are still considered. In this case, if the density of air is 0, hence vacuum and air is negligible, then mass and radius is proportional to the acceleration.