Friction is the "evil" of all motion. No matter which direction something moves in, friction pulls it the other way. Move something left, friction pulls right. Move something up, friction pulls down. It appears as if nature has given us friction to stop us from moving anything. Friction is actually a force that appears whenever two things rub against each other. Friction is a force which causes the motion between two surfaces to be reduced. [see below for a more general discussion/ mw] Friction happens because most surfaces are not perfectly smooth. Even a table top which may appear smooth has little bumps in it if you looked at it with a really good microscope. When two surfaces try to move past each other these little bumps collide and slow the motion of the surfaces down causing what we call friction. The rougher a surface is the more and bigger bumps it has and the more friction will affect it. Some examples of this are sliding a wood block down a ramp. If you slide a wooden block down a ramp it is slowed by friction. If you cover the block in sand paper (making it rougher) the block will slide slower because friction is slowing it down more. Friction also increases if you push the surfaces together more. So a full suitcase will have more friction opposing it's motion if you try to slide it across the floor than an empty one. Other examples of friction are very numerous because friction happens any time you move two surfaces that are touching. Try looking around your house and experimenting on your own with rough and smooth surfaces. The frictional force is also presumed to be proportional to the coefficient of friction. However, the amount of force required to move an object starting from rest is usually greater than the force required to keep it moving at constant velocity once it is started. Therefore two coefficients of friction are sometimes quoted for a given pair of surfaces - a coefficient of static friction and a coefficient of kinetic friction. The force expression above can be called the standard model of surface friction and is dependent upon several assumptions about friction. Kinetic Friction is when two surfaces are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds. It also is the force of friction that allows you to provide less force to an object to keep it from moving or to make it move. Coefficient of Friction that is friction is typically characterized by a coefficient of friction which is the ratio of the frictional resistance force to the normal force which presses the surfaces together. In this case the normal force is the weight of the block. Typically there is a significant difference between the coefficients of static friction and kinetic friction. Normal Friction is frictional resistance forces are typically proportional to the force which presses the surfaces together. This force which will affect frictional resistance is the component of applied force which acts perpendicular or "normal" to the surfaces which are in contact and is typically referred to as the normal force. In many common situations, the normal force is just the weight of the object which is sitting on some surface, but if an object is on an incline or has components of applied force perpendicular to the surface, then it is not equal to the weight. A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction. Forces can be measured using a device called force meter. The unit of force is called the newton. IForces resulting from physical interaction are called 'Contact Forces' and examples include Frictional, Tension, Air resistance and Spring force.A force resulting from non-physical interaction is