Final answer:
Friction decreases an object's kinetic energy by converting it into other forms of energy.
Step-by-step explanation:
Friction affects kinetic energy by converting some of it into other forms, such as thermal energy. When an object is in motion, friction opposes its motion and does negative work on it, reducing the kinetic energy. This is because the force of friction is in the opposite direction of the object's motion.
For example, when you slide a book across a table, the friction between the book and the table converts some of the book's kinetic energy into thermal energy, causing the book to eventually come to a stop.
In summary, friction decreases an object's kinetic energy by converting it into other forms of energy, such as thermal energy.