What Is Static Friction?
Static friction is the resistance to movement when two non-moving solid objects are in contact with each other. Static friction matches applied force and prevents motion. When the applied force overcomes static friction, the object starts moving, and at this point, static friction ceases and kinetic friction comes into play.
Static friction is what keeps an object stationary when it is placed on a surface. It is, for example, what keeps a wooden block from sliding down a small incline. Static friction varies based on the force applied to the object, but has a maximum value. This is calculated by multiplying the coefficient of static friction by the normal force (the force applied perpendicular to the object).
The coefficient of static friction itself varies depending on the pair of materials in contact and is measured based on the angle of inclination at which motion just starts to occur. In general, rough objects tend to have higher coefficients of static friction. Lubrication often lowers the coefficient of static friction of a pair of materials. For example, The coefficient of static friction between dry concrete and rubber is 1.0 but decreases to 0.30 when wet, resulting in a more slippery surface.
Rolling friction occurs when one object rolls across the other. However, since both objects are stationary relative to each other at the point of contact, this is classified under static friction.