https://www.youtube.com/watch?v=0jWwl0bt6aU&list=PLUl4u3cNGP61qDex7XslwNJ-xxxEFzMNV&index=128
In this class Professor talks about work done by frictional force. He talks about motion of the object as well. I know frictional force is force opposes the motion. In the free body diagram, he doesn't specify any force which create this motion. I know friction never create any motion. Then what is the point talking about work done by frictional force?I am learning classical mechanics with the help of these lecture videos. I am confused with this notion. May I get help?
Actually friction does not always opposes motion, take for example your cup of coffee over a piece of paper. If you slightly pull your piece of paper so that the cup moves in the same direction of the paper, the friction between the cup and the paper is making the cup moves, and performs a given work over the coffee cup.
Now, you maybe be tempted to ask why if your are applying a force to the paper then it does not accelerates but moves at constant speed. Well remember that there is a force of friction between the paper and the table which opposes the motion.
Analyzing the role of friction in many problems of mechanics may get far from obvious, if you are interested in some cool applications take a look at this https://www.youtube.com/watch?v=jyQwgBAaBag.