Friction - Physics 101 / AP Physics 1 Review with Dianna Cowern

Nice video, as always. Just want to point out that at 10:20, the formula given is for maximum static friction, not static friction in general. Static friction can be as small as 0 when there is no tendency of relative motion between 2 contacting surfaces, and it can be as large as the product of static friction coefficient and the normal force when the relative motion between the surfaces is about to start. Imagine a book resting on a table. When there is no horizontal force on the book, the static friction is actually 0. When there is a horizontal force applied yet the book remains stationary on the table, then the static friction must be equal to the applied force but in opposite direction so the net force on the book is zero. But if the applied force keeps increasing, eventually the book slides on the table because the force exceeds the value of maximum static friction and the static friction on the book can’t increase beyond that.