Question

every contact and field force exerted by the external world on a system has a specific, identifiable cause, called the

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Explanation:

In physics, when considering forces exerted on a system from the external world (whether contact forces like normal force, frictional force or field forces like gravitational force, electrostatic force), the entity or object that causes that force is defined as the agent. For example, when a book is on a table, the table is the agent exerting the normal force on the book, and the Earth is the agent exerting the gravitational force on the book.

Answer:

agent