In short, you can either have a rigid object OR work done by friction, but not both. Instead the wheel turns and there is a new contact point. But just look at a rolling wheel, the frictional force is at the point of contact, but this force doesn't move. Since we are dealing with a rigid object, this force actually doesn't have any displacement (I know that sounds crazy). Finally, there is a frictional force that is parallel to the incline. Remember, the definition of work by a force is: This force also doesn't do any work because the angle between the force and the displacement is 90°. This normal force pushes up on the disk perpendicular to the incline. Since it's part of the system, it doesn't do any work (and we have the gravitational potential energy instead). Why? Because it's actually the gravitational force between the disk and the Earth. There is the gravitational force, but it doesn't do any work. In order to use the work-energy principle, I need to first consider any forces that do work on the system. Let me just pick one at the top of the incline and the other point at the bottom of the incline. Second, I need to pick two points over which to look at the change in energy. For this case, I will choose the system to consist of the disk along with the Earth (that way I can have gravitational potential energy). First I need to declare the system that I will be looking at. In order to use the work-energy principle, I need two things. Without deriving it, I will just say that the moment of inertia for this disk would then be: Suppose the disk has a mass M and a radius R. Now we replace the frictionless block with a disk (actually frictionless disks are hard to come by and thus in a large demand). I skipped some steps, but that problem isn't too complicated. This means that the forces in the x-direction will be: The only force acting in the x-direction is a component of the gravitational force. This means that if I put the x-axis in this direction, the net forces in the x-direction will be mass*acceleration and the net forces in the y-direction will be zero. The block can only accelerate in the direction along the plane.
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