Final answer:
A public good is defined as nonexcludable and non-rival, meaning it can't prevent usage by non-payers and one person's use doesn't reduce its availability to others. National defense and radio broadcasts are classic examples of public goods, often requiring government intervention due to the free-rider problem.
Step-by-step explanation:
When determining whether the government is involved in providing a good that is rival or nonrival in consumption and whether it is excludable or nonexcludable, it's important to identify the type of good. A public good has two key characteristics: it is nonexcludable and non-rival.
Non-excludable goods are those where it is not feasible to prevent anyone from using them once they're provided. An example would be national defense; it is impractical to exclude specific individuals from being protected by the nation's defense mechanisms. This type of good is provided for all without direct cost to the users, and their use by one individual does not diminish the ability of another to use it.
Furthermore, non-rival goods do not diminish in availability as more people use them. For example, a radio broadcast is non-rival because one person's listening does not prevent anyone else from listening. Due to these features, such goods typically require government action to ensure they are funded, as they face the free rider problem where individuals may benefit without directly paying for them.