PHYSICS MADE EASY - Friction- Solved problem - Block on Block

Hi reader, I created this Animation and Video of solved problem for Block-on-Block where the whole "system of two blocks" is sliding together on a horizontal ground plane -- in this numerical, there is ZERO friction between the Lower Block and ground plane. The problem here is asking us to find the right amount of Force to keep the Upper Block sticking to the Lower Block and not fall off. FREE BODY DIAGRAM is constructed, which tells you how forces works in this peculiar case. FREE BODY DIAGRAMS need a bit of patience to create, but once you get practice, you can solve almost every problem after lining up the Forces acting in various directions. The directions of the frictional forces have to be understood carefully. Imagining friction for the Upper Block is difficult and counter-intuitive, meaning for the Upper Block, Friction force direction is in the same direction as the direction of motion of the "SYSTEM of Blocks". However, if in case the Force acting on Lower Block exceeds a particular value, then the Upper Block will slide off and fall backwards in the middle of the motion, this happens when the Static Frictional Force between Upper Block and Lower Block is Not sufficient to hold the Upper Block sticking to the Lower Block. best regards, kaustubhan