Watch this lecture about long-run aggregate supply. The short-run aggregate supply curve slopes upward due to the lag between product prices and resource prices that makes it profitable for firms to increase output when the price level rises. The long-run aggregate supply curve is vertical when a country is at full employment. The long-run aggregate supply curve is vertical because, in the long run, resource prices adjust to changes at the price level, which leaves no incentive for firms to change their output. In the long run, prices and wages have no effect on the aggregate supply curve.