In my book, static friction means The frictional force that must be overcome to start one surface moving over another. Kinetic friction means the force that opposes the motion of one surface over another. My teacher said that a wheel rolling is a static friction, and it's weaker than sliding friction. She likely said that sliding friction was kinetic friction. I don't understand why rolling is static friction. In my opinion, I think static friction is a friction when an object just starts to move, and kinetic friction is when the object is moving. ANother thing is if rolling is static friction, then it will not be weaker than kinetic but stronger. Hope you can explain to me. thanks.