1. The friction between the hand and the pen is used when the pen is rotated by hand.
2. The pencil in the rotating hole inside the wall when there is friction, because, first of all, they have contact, strong interaction, but also have a certain rough surface-friction coefficient is not zero, so effective use of friction
3. And the real cut off the wood of the pencil, because the pen and blade between the powerful role of Blade sharp, strong pressure, you can cut the wood on the pen, and pressure, and not the relationship between friction.