Fundamentally, what does a friction force do for a rolling object? I am very confused by the way my textbook explains it. In my textbook , it says:
A wheel rolling on a horizontal flat surface at a constant velocity experiences no friction force. Why?
On the same surface, there is an acceleration of the wheel pointing to the right (probably caused by a force), so the ball is angularly accelerating in the clockwise direction. In this case, a friction force appears, and it is also pointing to the right. How come? What does the friction do for this wheel?
On an inclined plane, a ball freely rolls down the surface. The direction of friction is up the ramp, which confuses me because in the previous example the friction force was in the direction of the wheel's acceleration.
And there is a difference if a wheel is freely rolling and if there is a torque acting on the ball's center of mass. WHY???
What does friction do in these cases? How does it cause an object to roll?