The marginal product of capital is downward sloping due to the law of diminishing marginal returns. This law states that as more and more units of capital are added to a fixed amount of labor, the marginal product of capital (i.e. the additional output generated by each additional unit of capital) will eventually decrease. This is because the additional units of capital become less and less productive as they are added, as there may be constraints on how much labor can effectively utilize the capital or how much physical space is available to accommodate the capital. Therefore, the marginal product of capital curve slopes downward to reflect the diminishing marginal returns that occur as additional units of capital are added.
