Friction is the resistance to motion that occurs when two surfaces interact. This force acts parallel to the surfaces in contact, always opposing the direction of motion or the potential for motion. The central question is whether the force needed to start an object moving differs from the force needed to keep it moving.
Static Friction: The Force of Rest
Static friction is the force that prevents an object from beginning to move when an external force is applied. This force automatically adjusts its magnitude to be exactly equal and opposite to the applied force, keeping the object stationary. For example, if you push lightly on a heavy box that remains still, the static friction force pushes back with the exact same strength.
The force of static friction is not fixed but increases up to a specific maximum threshold. Once the external force exceeds this maximum resistance, the object will begin to slide. This maximum point, known as limiting friction, represents the greatest resistance the surfaces can offer before motion begins. This force is related to the coefficient of static friction, a value dependent on the materials in contact.
Sliding Friction: Resistance During Motion
Once the maximum static resistance is overcome and the object is in motion, the friction transitions to sliding friction, also known as kinetic friction. This force constantly opposes the relative movement between the two sliding surfaces. Sliding friction slows down an object and must be continuously countered to maintain a constant speed.
Unlike static friction, the force of sliding friction is a constant value for a given pair of surfaces. This force does not change significantly based on the object’s speed. The resistance encountered during this phase is lower than the initial resistance required to start the motion.
The Reason for the Difference
Static friction is consistently stronger than sliding friction due to the microscopic nature of surface contact. Even smooth surfaces are covered in tiny peaks and valleys called asperities. When two objects are at rest, these microscopic irregularities settle and interlock with the valleys of the opposing surface.
This close contact allows for the formation of temporary atomic bonds, sometimes called “cold welding,” between the molecules of the two surfaces. Overcoming static friction requires enough energy to break both the mechanical interlocking of the asperities and these stronger adhesive bonds. The coefficient of static friction (mu_s), which quantifies this resistance, is greater than the coefficient of kinetic friction (mu_k).
Once the object is moving, the surfaces constantly bounce over each other. The time available for asperities to interlock deeply or for strong atomic bonds to form is greatly reduced. The force required to maintain motion, the sliding friction, only needs to break weaker, rapidly forming bonds.
Observing Friction in Action
The difference in strength between the two types of friction is evident in many everyday situations. Pushing a heavy piece of furniture across a room requires a burst of force to start moving it. Once it is sliding, a smaller, sustained force is sufficient to keep it going, illustrating the transition from high static friction to lower sliding friction.
In automotive engineering, this principle is fundamental to anti-lock braking systems (ABS). ABS prevents wheels from locking up and skidding, which would cause sliding friction. The system modulates the brakes to keep the wheels rotating slightly, maintaining the maximum possible static friction. This static grip provides greater stopping power and allows a car to accelerate, turn, and brake effectively.