Final answer:
A period too short to change plant capacity but long enough for altering its use is the short run. Firms can't change fixed inputs during this time, though other production adjustments are possible.
Step-by-step explanation:
A period of time too brief for a firm to alter its plant capacity yet long enough to permit a change in the degree to which the plant is used is called the short run. In the short run, firms cannot change the usage of fixed inputs, like the plant capacity.
However, it is possible for them to adjust other aspects of production, such as the amount of labor or materials used.
The long run, on the other hand, is a period of time when all costs are variable, meaning the firm has the flexibility to make significant changes to its production processes and capacity.
This period does not have a predefined duration but depends on the circumstances of the specific business. For instance, if a business has a one-year lease on its factory, the long run would be any period longer than a year, since the firm would no longer be bound by the terms of the lease.