A body having kinetic energy K moving on a rough horizontal surface is stopped in a distance x. The force of friction exerted on the body is

Let f be the force of friction and m, the mass of the body.∴ Retardation a=f/mLet v be the initial velocity of the body, thenv2=2ax  or  12mv2=maxor K = fx   (∵ma=f)∴ f=K/x.