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Question
Why is sliding friction less than static friction?

Answer
It has to be less than (or equal to) by definition.  If static friction is a force which prevents something from sliding and kinetic ("sliding") friction is a force resisting movement while an object is sliding along a surface, then static friction cannot be less than kinetic friction.  It would form a logical fallacy, since you could push with a force greater than static friction to get an object to start sliding, then the frictional force would jump up to the force of kinetic friction and the object would no longer slide.  It makes no logical sense at all.  Once you overcome the force keeping an object in place, you cannot replace that with a greater force or the object's acceleration would be against the direction in which it is pushed.  It would move the wrong way, which we know is physically impossible.  As to the reasons, there could be many factors (lubrication, relative average position of surface imperfections once sliding, vibration) which determine how much lower "sliding" friction will be...but it can never be greater than static friction.  

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Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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