Simplify the following text for a Grade 7 native English-speaking person.
Original: Kinetic ( or dynamic ) friction occurs when two objects are moving relative to each other and rub together ( like a sled on the ground ) . The coefficient of kinetic friction is typically denoted as Î 1\/4 k , and is usually less than the coefficient of static friction for the same materials .
Simplified: Two kinds of friction are static and kinetic .