宏观经济学(曼昆)第3章 国民收入2.ppt

* * The MPC was defined in chapter 3 and used in various chapters since. * * * * Pick a point on the consumption function; that point represents a particular combination of consumption and income. Now draw a ray from the origin to that point. The slope of that ray equals the average propensity to consume at that point. (Why? The slope equals the rise over the run. The rise from zero to that point equals the value of C at that point. The run from zero to that point equals the value of Y at that point. Hence, the rise over the run equals C/Y, or the APC.) At higher values of Y, the APC (or the slope of the ray from the origin) is smaller. This is what Keynes conjectured: at higher values of income, people spend a smaller fraction of their income. * * * * Notes: Interest cost--If firms borrow in the loanable funds market (from chapter 3) to finance their purchases of capital, then they incur interest. But even if firms use their own funds, they incur an opportunity cost equal to the interest they could have earned had they purchased Pk worth of bonds instead of spending Pk to buy a piece of capital. Depreciation cost-- Remind students that ? is the depreciation rate, the percentage of capital that wears out each period. If the firm starts the period with $1000 worth of capital and the depreciation rate = 0.03, then at the end of the period, the value of the firm’s capital equals (1-0.03)?$1000 = $970. * If the price of capital, Pk, falls during the period, then firm incurs a capital loss, which increases its cost of capital. In this example, the price of capital rises, so the “capital loss” is negative. (Or, there’s a capital gain which we subtract from the cost, because the increase in the price of new capital reduces the cost of capital.) * The assumption in the first line says that the price of capital rises as fast as the general price level. The real cost of capital equals the nominal cost divided by the price level, just as the real wage e