Illustration Sliding friction force acts on a stone

A square stone is pushed upwards on an inclined wooden surface. The gravitational force $$m\,g$$ acts towards the earth and pushes the stone onto the wooden surface. The atoms of the wood surface prevent the stone from falling through the wood by electrical repulsion. These atoms exert a force on the stone that points perpendicular to the wood surface. This force is called the normal force $$F_{\text N}$$.
Because the surfaces of the stone and the wooden plane are abrasive, there is a sliding friction force $$F_{\text R}$$ (also called kinetic friction force) that acts against the motion of the stone. In the experiment, one finds that this sliding friction force is proportional to the normal force: $$F_{\text R} ~\sim~ F_{\text N}$$. This means: If you double the normal force (e.g. by increasing the mass of the stone), the frictional force $$F_{\text R}$$ doubles, too.