The Force of Friction

This video from The Physics Classroom presents the concept of friction in a common sense manner. Learn the answers to these questions: What is friction? What causes friction? What variables affect friction? And how can the mathematics of friction be used to analyze physical situations?

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[basharatmalik]

By the way, I saw your video series on light, it is wonderful, especially the refraction (portion).

[basharatmalik]

Sir hope you are doing well.
I have 2 questions. Please do answer them.
1) if a quantity say x is directly proportional to y, does it imply that y is also proportional to x. I mean that is proportionality between two variables both ways correct.
2) Is friction due to intermolecular force of attraction or repulsion. Since normal force and friction are both components of contact forces and normal force is a kind of repulsive ( push), shouldn't the friction also be due to repulsive intermolecular forces.
I want to understand the origin of friction at microscopic level.
Thank you

[sinetan]

At 8:23, it says how much force to get it started moving so wouldn't that mean the static force should be greater than the multiplication of the mu (coefficient) and normal force which would mean the applied force would be greater than 379N? How are they equal when the object is supposed to move?

[simonwesley]

There is a typo in the slide for the static friction problem ~8 min 379 N is the static friction, but it is labeled as kinetic friction although the audio describes it correctly.

[Nobukhosi]

absolutely loved this so helpful. thank you