 Module 85, Long Run Expansion of Input Shows. To produce output, firm requires certain number of inputs. When we discuss about the production of output under short run and long run, then there is a difference between the production under short run and long run. When we talk about short run, there are certain factors of production that remain constant and usually to make discussion about the production under short run, we talked about there is only one variable input. We talked about there are two variable inputs. For example, we talked about when weed production is a function of labor for given quantity of all other inputs. We also discussed about weed production is a function of quantity of fertilizer used against the given quantity of other input. We also talked about weed production is a function of labor and capital against the given quantity of other input. The given quantity of other inputs here basically refers to fixed factors of production and against these fixed factor of production, firm has to bear certain cost that is the fixed cost. And the factor of production that a farmer can change that is called as variable factor of production or variable factor of production reference that is the part of the variable cost. And when we talk about the base of fixed cost and variable cost, we talked about there is a total cost and total cost has two components. One is fixed cost and the other one is variable cost. When we talk about fixed cost, this cost does not change with the change in level of output. Also there is increase in level of output but there is no change in fixed cost and this fixed cost is parallel to horizontal axis. But when we talk about variable cost, this cost is equal to zero if farmer is not making any production. Initially, when there is increase in output, variable cost increases at a decreasing rate. But later stage what happens is that variable cost increases at an increasing rate. Then in the same base, we have drawn the average cost, average variable cost and marginal cost curves. And under the short run, all of these curves were shaped that these are U-shaped curves. But when you talk about the long run, what happens is that all factors of production are variable factors of production. In fact, there is no factor of production to which we call as a fixed factor of production. And when there is no fixed factor of production, there is no fixed cost. So all costs under the long run are variable cost. When we talk about expansion path of input usage under long run, there is a difference between the short run and long run. In short run, there are inflexible input expansion that there are certain inputs that does not change under the short run. But what will happen in the long run? Producer, our farmer is very flexible about the input usage. He can use any combination of input to produce the produce given level of output. Now if I summarize all this, the difference that is present between short run and long run that is about the flexibility of input usage. In short run, producer is inflexible about input usage where producer is inflexible about the usage of fixed factors of production. He has to use the given quantity of the fixed factor of production to produce output. But when you talk about the long run, producer is very much flexible about input usage. And the flexibility in the input usage, this we call as producer is flexible, producer is facing a flexible input expansion.