The downward sloping nature of the PPC is due to the law of increasing opportunity cost. According to this law, with the fuller utilisation of the given resources, in order to produce an additional unit of one good, some of the resources are to be withdrawn from the production of another good.

Why PPC is downward sloping and concave to origin?

PPC is concave to the origin because of increasing Marginal opportunity cost. This is because inorder to increase the production of one good by 1 unit more and more units of the other good have to be sacrificed since the resources are limited and are not equally efficient in the production of both the goods.

Why is PPF downward?

The production possibilities frontier shows the combinations of goods and services that can be produced efficiently in an economy at a point in time. The production possibilities frontier is downward sloping: producing more of one good requires producing less of others. The production of a good has an opportunity cost.