either input involves incurring more input costs.

With only one variable input (labor usage) in the short run, each possible quantity of output requires a specific quantity of usage of labor, and the short-run total cost as a function of the output level is this unique quantity of labor times the unit cost of labor. But in the long run, with the quantities of both labor and physical capital able to be chosen, the total cost of producing a particular output level is the result of an optimization problem: