Consider a man standing inside lift which is free from all external influences (gravitational or otherwise). Suddenly someone begins to pull the lift with some acceleration. Now if the man releases a ball he sees it falls onto the floor. Since he sees the lift at rest he concludes that there is a gravitational force acting on the ball downwards. But a man standing outside in an inertial frame sees it is because of the acceleration of that lift which caused the ball to fall. But consider two balls resting on a table inside this accelerated lift the man inside the lift would say that there is gravitational force acting between the balls. How would the man in the inertial frame explain this force?