The purpose of this paper is to develop a simple dynamic model to analyze researchers' interest in answering questions for an online question and answer (Q&A) knowledge market. With use of a System Dynamics model, an online knowledge market can be modeled as a marketplace where consumers ask and researchers answer questions to make knowledge transactions. In this market, consumers price their questions to acquire answers, and a company maintains the online knowledge market by determining the optimal price allocation to researchers. This paper focuses on the design of a simple model based on Zhang & Jasimuddin's equation that represents the dynamics of researchers' interest in answering questions in a specific kind of online knowledge market. The model shows this behavior of researchers. Furthermore, implementation of this model results in generating new and useful insights about the policy option analysis for developing new strategies in same knowledge markets.