Friction does not necessarily increase with an increase in speed. The relationship between friction and speed depends on the type of friction involved. There are two main types of friction: static friction and kinetic (or dynamic) friction.
Static friction refers to the resistance between two surfaces that are not sliding relative to each other. It comes into play when you try to initiate motion between two objects or surfaces in contact. The force of static friction can vary and typically increases as you apply more force to overcome it. However, static friction does not depend on the speed of the objects. Once the motion begins, static friction is no longer relevant.
Kinetic (or dynamic) friction, on the other hand, occurs when two surfaces are sliding against each other. It opposes the relative motion between the objects. The force of kinetic friction is generally considered to be relatively constant for a given pair of surfaces, regardless of the speed at which they are moving. This assumption holds for many common situations, but it is an approximation and may not always hold true in certain cases.
The reason kinetic friction is often considered constant on a graph is that it typically remains relatively consistent within a certain range of speeds. The relationship between kinetic friction and speed becomes more complex at very high speeds or in specialized scenarios involving extreme conditions, such as high temperatures or lubrication. However, for most practical purposes, within a reasonable range of speeds, kinetic friction is assumed to remain constant.
It's important to note that the concept of friction can be more nuanced and dependent on various factors, such as surface roughness, the presence of lubricants, and temperature. Real-world friction phenomena can be quite complex, but the simplified models of static and kinetic friction are commonly used to understand and analyze the behavior of objects in contact.