# Frictional Force and Normal Contact Force

## Understanding Frictional Force

• Frictional force is the force that acts against the relative motion of two surfaces in contact with each other. It always acts parallel to the surface of contact and against the direction of motion.
• Friction is not a fundamental force but arises due to electromagnetic interactions between the microstructures of two contacting surfaces.
• The amount of frictional force depends on two things: the type of surfaces in contact (material properties) and how hard they’re being pushed together.
• There are two main types of friction: static friction and kinetic friction. Static friction prevents motion, while kinetic friction opposes ongoing motion.
• Static friction resists the initiation of relative motion and its magnitude adjusts to match the applied force up to a maximum value.
• Kinetic friction - sometimes called “sliding” or “dynamic” friction - operates once relative motion has started. It remains nearly constant regardless of the speed of movement.
• Both static and kinetic friction can be calculated using the respective coefficients of friction (usually denoted by the Greek letter μ). The greater the coefficient of friction, the greater the frictional force.

## Normal Contact Force

• Normal contact force, often referred to as the normal force, is the force exerted by a surface that supports the weight of an object resting on it. It acts perpendicular to the surface of contact.
• The term “normal” in this context refers to its mathematical meaning of “perpendicular”, not “usual” or “typical”.
• The normal force is exactly equal to the weight of an object when the object is at rest on a flat surface without any additional vertical forces.
• Normal force can change based on the angle of the surface (incline) or if there’s additional upward or downward forces.

## Relationship between Friction and Normal Force

• The maximum amount of static friction that can be exerted on an object, before it starts moving, is proportional to the normal contact force. This relationship is expressed mathematically as Fs_max = μsN, where Fs_max is the maximum static friction, μs is the coefficient of static friction, and N is the normal force.
• Similarly, the kinetic friction can be calculated using the relation Fk = μkN, where Fk is the kinetic friction, μk is the coefficient of kinetic friction, and N is the normal force.
• As the angle of incline increases, the normal force decreases whilst the component of weight acting parallel to the surface (and hence the friction) increases.

This underlines the importance of understanding both frictional force and normal contact force, as well as their relationship, when learning about mechanics. Always consider these forces when analysing real world problems involving motion.