6.5:
Frictional Force
Whenever an object is in motion along a surface, the surface exerts contact forces on it, namely a perpendicular normal force and a parallel friction force.
The frictional force always acts in the direction opposite to the relative motion between the two surfaces. The coefficient of friction, μ, is the ratio between the frictional and the normal force.
A puck slides more easily on an ice floor than on a concrete floor due to the difference in coefficient of friction values. The larger the coefficient of friction value, the higher is the frictional force and vice-versa.
Mathematically, the magnitude of frictional force is represented as a product of the coefficient of friction and the normal force.
Thus, the frictional force is less for a smaller normal force as the area of contact between the surfaces is smaller. If normal force increases, the contact area also increases, and so does the frictional force.
6.5:
Frictional Force
When a body is in motion, it encounters resistance because the body interacts with its surroundings. This resistance is known as friction, a common yet complex force whose behavior is still not completely understood. Friction opposes relative motion between systems in contact, but also allows us to move. Friction arises in part due to the roughness of surfaces in contact. For one object to move along a surface, it must rise to where the peaks of the surface can skip along the bottom of the object. Thus, a force is required that just sets the object in motion. Much of the friction experienced is due to the attractive forces between molecules of the two objects, so even perfectly smooth surfaces are not frictionless. In fact, perfectly smooth, clean surfaces of similar materials tend to adhere, forming a bond called a “cold weld”. Frictional forces, such as 
, always oppose the motion or attempted motion between objects in contact.
This text is adapted from Openstax, University Physics Volume 1, Section 6.2: Friction.