1. The problem asks why the force of friction (CF) is equal to the coefficient of friction (k) multiplied by the normal force (BC). 2. The formula for frictional force is given by: $$CF = k \times BC$$ where $CF$ is the frictional force, $k$ is the coefficient of friction, and $BC$ is the normal force (the force perpendicular to the surfaces in contact). 3. This formula comes from the physical principle that frictional force is proportional to the normal force. The coefficient of friction $k$ is a constant that depends on the materials in contact. 4. The normal force $BC$ represents how strongly the two surfaces are pressed together. The greater the normal force, the greater the frictional force. 5. Therefore, multiplying the coefficient of friction $k$ by the normal force $BC$ gives the frictional force $CF$: $$CF = k \times BC$$ 6. This relationship is fundamental in physics and engineering to calculate frictional forces in various systems.