Answer:
Initially, the plastic ruler is electrically neutral. So it has no effect on the tiny pieces of paper. When the ruler is rubbed on a dry cloth or hair it gets electrically charged. This charged ruler exerts an electric force on the tiny pieces of paper and attracts them. The piece of paper is attracted by the ruler due to electrostatic force. As the weight of the small piece of paper is less than the electrostatic force due to the plastic ruler, they move towards the ruler.