If a firm can instantaneously and costlessly adjust its capital stock, then,as shown by Jorgenson (1963), its decision about how much capital to use is essentially a static decision in which the marginal product of capital is equated to the user cost of capital.
The firm's investment decision becomes an interesting dynamic problem, in which anticipations about the future economic environment affect current investment, when frictions prevent instantaneous and costless adjustment of the capital stock.
The investment literature of the last three decades has focused on two types of frictions —adjustment costs and irreversibility.