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what is friction?

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Friction is an opposing force that acts always in the opposite direction of the applied force.
For instance a box pushed from point A to point B experiences a resisting force acting in the direction of the applied force(the push). That resistance is friction.
Friction can also be defined as the force between two surfaces.
Newton's 1st Law tells us that a body in motion will continue to move in a straight line at a constant speed unless acted on by a force.  Yet when we roll or slide things along a surface they do slow down and stop, so some force must be acting to stop it.  The "force" that slows down and stops the movement is friction.  Friction is a force that resists motion along or through it (in the case of air or a fluid).
In the case of an object sliding along a surface, the friction is proportional to the "load" or weight of the object on the surface:  Ff = μFn  where Ff is the force of friction, Fn is the load or gravitational force (weight) of the object on the surface, and μ is a constant that's different for every type of surface (glass, metal, etc.).  Note that if the surface is slanted (inclined plane), Fn will be the force normal to the incline.
There are other types of friction (static, fluid, etc) but this is the basic idea.  Hope that helps.