Professor Popkiss cannot earn these profits on the long-run if he stays in perfect competition because of the free entry and exit which shift the industry supply curve. Indeed, new firms that will enter the industry will increase the supply in the industry, which will then make the market price fall. Consequently, as the price will fall, the industry supply will move down, the output will decrease which will then decrease the profit maximising output. In other words, he could only maintain supernormal profit in this perfectly competitive market on the short run, before new entrants have eroded their profits down to a normal level. Therefore, if the professor wishes to earn these profits on the long run, he cannot stay in a perfectly competitive