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?
Best Answer
Hopefully this will answer at least some of your questions. If you consider a ball intially at rest on a frictionless surface, if a force is exerted through the centre of mass of the ball it will slide across the surface with no rotation, there will only be translational motion.
If you consider the case where there is friction, if the force is again applied to a stationary ball the frictional force will act in the opposite direction to the force but at the edge of the ball that rests on the ground. This friction applies a torque to the ball which starts the rotation. So static friction is infact necessary for a ball to begin rolling.
The condition for rolling at speed $v$ is that the angular velocity of the ball is given by $$v=\omega r$$ from this the top of the ball will move at speed $2v$ the centre of mass of the ball will move at $v$ and the bottom edge of the ball will instantaneously be at rest. So as the edge touching the ground is stationary it experiences no friction.
So friction is necessary for a ball to start rolling but once the rolling condition has been met the ball experiences no friction.