A production function is a function that shows different combinations of inputs and technology that is associated with a given level of output. A change in output is promotional to changes in technology applied in production process, however, due to diminishing returns; growth of output is usually less than proportionate change in each input.
Marginal product of capital is the change in output resulting from a change in units of capital employed in the production process. Average Product of Capital is the ratio between output in a production process and the capital employed in that process, average product differs with marginal product in that average product includes effects of previous employed capital, whereas marginal product only takes into account only the added units of capital. Considering the assumption of diminishing returns, such that returns from subsequent units of capital are less than that of the previous unit, according to our model this makes marginal product to be less than average product.