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Hello sir

Sir in our friction chapter, it is given that friction is the force which opposes the relative motion of an object.And in next page it is given that Friction is caused by irregularities on the two surfaces in contact.

In first line it is stating that friction is a force whixh opposes the relative motion of an object and in secon line it is stating that friction is caused by irregularites. IAM NOT UNDERSTANDING THAT WHAT IS FRICTION AND HOW IT IS CAUSED BY IRREGULARITIES. sIR THIS IS NOT AN HOME WORK QUESTION AND P[LEASE DONT RESPOND THAT THIS IS OUT OF MY EXPERTISE.

Hello Aishwarya,

There is no contradiction in those 2 statements from the 2 pages.

I often take things to extremes, sometimes ridiculous extremes, to help me visualize a new concept. To help you visualize how irregularities cause friction: Imagine that you are pushing a box, which has an amazingly smooth bottom, on a floor, that is amazingly smooth - except that in one part of the room there is the head of a nail every 10 cm sticking up slightly. After pushing it some distance, you pick up the box to find that the nail heads have left slight scratches on the bottom of the box. You remember that it was harder to keep the box moving when pushing it over the part of the floor that had the nails heads sticking up. Those nail heads are examples of the irregularities that your book was referring to.

Think back to the part of the move of that box that was over smooth floor. If you decided to suddenly quit pushing, the box would coast for a bit but would come to a stop. That is because there was friction during that part too. (Explanation of why it came to a stop: Think of Newton's 1st Law. The box was in motion and when you quit pushing, it continued in motion. Newton said it would continue in motion forever - unless acted on by an outside force. Friction between the 2 smooth surfaces was the outside force that made the box come to a stop.) The friction was higher when going over the nails, but even when on the smooth floor, friction was more than zero.

Not all irregularities are as obvious as the nail heads. You know that the box and the floor is made up of atoms and perhaps of molecules. If you could look with a really great microscope, those atoms and or molecules would make the surface look irregular. So even as smooth as fine workmen can make them, those atoms and or molecules make the surfaces irregular.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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