If the current interest rate is high, the opportunity cost of current consumption is high because the money spent to finance current consumption could have been saved. So the forgone interest from savings is the opportunity cost of current consumption. A high interest rate then implies a high opportunity cost while a low interest rate implies a low opportunity cost of current consumption. So there is an inverse relationship between interest rate and current consumption. Since most capital goods are purchased with borrowed funds, interest rate is the main cost of investment.
Capital goods have long lives - they yield returns for many years.
In deciding how many units of capital to buy, a firms compares the marginal benefit of an addition unit of capital with the marginal cost of capital. It is the interest rate. An inverse relationship exists between the quantity of capital used and its marginal product. As the interest rate R increases, the MC of capital increases and fewer investment opportunities become profitable. Net Exports Interest rates affect net exports via the exchange rate.