1. The Normal Force (N): This is the force exerted by a surface perpendicular to the object in contact with it. Think of it as the force pressing the object against the surface. The greater the normal force, the greater the friction force.

2. The Coefficient of Friction (μ): This is a dimensionless number that represents the "roughness" of the two surfaces in contact. It's a property of the materials involved, not the force applied. A higher coefficient means more friction.

The formula for the magnitude of friction force is:

Friction Force (F_f) = μ * N

Types of Friction:

* Static Friction (F_s): This is the friction force that prevents an object from moving when a force is applied to it. It's always equal and opposite to the applied force, up to a certain limit.

* Kinetic Friction (F_k): This is the friction force that opposes the motion of an object that's already moving. It's usually slightly less than static friction.

Key Points:

* The friction force always acts in the opposite direction to the motion or the tendency of motion.

* The coefficient of friction is generally less than 1, but can be greater in some cases.

* The normal force is usually equal to the object's weight, but this is not always the case (e.g., on an inclined plane).

* Friction is a complex phenomenon that can be affected by factors like surface roughness, temperature, and the presence of lubricants.