Two things will accelerate towards each other when there is a pair of forces pulling them together. This is because according to Newton's second law of motion, the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the mass of the object. In other words, the more force that is applied to an object, the greater its acceleration will be; and the more massive an object is, the less its acceleration will be. When two objects are pulled together by a force, the force is applied to both objects, and so both objects will accelerate towards each other.