Friction app for iPhone and iPad
This app gives an interactive simulation of how frictional forces act on a block on an inclined plane. You can see how friction grows to resist gravity as you tilt the plane. It eventually reaches its limit and the block starts to slide. You can experiment with the coefficient of friction to see how this affects the results.
Once the basics have been understood, an advanced mode can be used which allows for sliding or rolling motion depending on the coefficient of friction and the angle of the plane.