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Friction, is a force acting opposite the force acting on a body.
Now , what does this necessary evil mean?
It means that it is a force which is necessary for us, but still an evil or bad.
It is a necessary as it helps us to walk, hold things and to maintain a grip between two objects and rubbing of hands produces heat making warm in winters..

It is evil or bad as it  causes wear and tear , wastes energy, produces heat which causes destroying of parts of a machine .
So, a necessary evil..
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Friction is the opposing force acting on a body which is in motion.Due to friction we can walk,hold the things,write etc.when the floor is very smooth its easy to fall down due to the lack of friction.but due to presence of friction there are some difficulties like the wear and tear of machine this case we use lubricants like oil to reduce friction