Net investment, which is the addition to the stock of capital is measured during a period of time, hence is a flow concept. On the other hand, capital which is measured at a point of time is a stock concept. For example, the amount of water which is flowing into the tank from the tap per minute is a flow variable. In the same manner, net investment will flow continuously during a period is . considered as a flow. On the other hand, capital which is fixed at a point of time is like the water at a particular point of time which will be considered as a stock variable.