Q.Distinguish between stock and flow. Between net investment and capital which is a stock and which is a flow? Compare net investment and capital with flow of water into a tank.

Stock vs. Flow

A stock is a quantity measured at a point in time — it has no time dimension. A flow is a quantity measured per unit of time — it has a time dimension (e.g., per year, per month).

Net Investment and Capital

Capital is a stock — it is the total value of physical assets (machinery, buildings, etc.) existing at a given moment.

Net investment is a flow — it is the addition made to the capital stock during a period of time (usually one year).

where $K_t$ is capital stock at end of period $t$, and $I_{net,t}$ is net investment during period $t$.

Water Tank Analogy

Think of a tank with water:

• The amount of water in the tank at any instant is the stock — this corresponds to capital.
• The rate at which water flows into the tank (litres per minute) is the flow — this corresponds to net investment.

If the inflow rate is positive, the water level (stock) rises. If inflow stops, the level stays constant. If there is an outflow (depreciation), the net change in stock equals inflow minus outflow — exactly how net investment changes the capital stock.

The Concept: Stock Versus Flow

In economics, the distinction between a stock and a flow is fundamental. A stock is a quantity measured at a specific point in time — it tells you how much exists right now. A flow is a rate measured over a period of time — it tells you how much is being added or subtracted per unit of time.

Think of a bathtub. The amount of water in the tub at 3:00 PM is a stock. The rate at which water is coming in from the tap (litres per minute) is a flow. The stock changes because of flows: if the inflow is greater than the outflow, the stock rises; if outflow exceeds inflow, the stock falls.

Capital and Net Investment

Capital refers to the stock of productive assets — machinery, buildings, tools, infrastructure — that an economy owns at a given moment. It is a stock. You can say, "India's capital stock at the end of 2023 was ₹X lakh crore." That is a point-in-time snapshot.

Net investment is the addition to the capital stock over a period (say, a year). It is a flow. Specifically:

Gross investment is the total spending on new capital goods. Depreciation is the wear and tear (or obsolescence) of existing capital. Net investment, therefore, is the change in the capital stock.

The Water Tank Analogy

Imagine a water tank. The water level in the tank at any instant is a stock — it corresponds to capital. The rate at which water flows into the tank (say, from a pipe) is a flow — it corresponds to net investment.

• If the inflow rate is 10 litres per hour, and the tank initially has 100 litres, then after one hour the stock becomes 110 litres. The flow (10 litres/hour) has increased the stock.
• If the inflow stops (net investment = 0), the stock remains constant (assuming no leakage).
• If there is a leak (depreciation), the net inflow (inflow minus leakage) is like net investment. If leakage exceeds inflow, the stock falls.

Thus:

• Capital = water level in the tank (stock)
• Net investment = rate of change of water level (flow)

Conclusion