Question

what is the main difference between long period and short period

Answer 1

We do not define the short run with respect to any calendar period (one year, one month, one week, etc.). and the long run we mean aperiod of time long enough.
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Answer 2

The short period refers to a period of time short enough so that the amounts of at least one or more of the factors of production used by the firm cannot be changed. In a barber shop it takes little time, perhaps a week, to install another chair and find another barber to increase the production of haircuts. For a barber shop, therefore, the short run may be as little as a week.

The long period—that is, in the period long enough for the influence of all factors to be felt—all costs are variable. Moreover, in the long run, a firm can choose its best plant sizes and its lowest-envelope curve. What is or is not a ‘long’ period or ‘short’ period depends upon circumstances such as the introduction of a revolutionary technical change in the method and the costs of manufacture.

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