What does friction do to an object in motion?

Friction is a force that opposes the relative motion of two surfaces in contact. When an object is in motion, friction acts in the opposite direction to its movement. This opposition means that friction will reduce the object's speed over time.

For example, when you slide a book across a table, the friction between the book and the table surface will slow the book down until it eventually stops. Thus, the role of friction is crucial in slowing down moving objects, which is why the correct response highlights that friction slows an object down. Understanding this force is essential in various real-life applications, such as in braking systems of vehicles, where friction is intentionally used to reduce speed safely.