A force F on mass M1 causes the system to accelerate to the right with acceleration a. Show that a=M2*g/M3 by considering forces acting on M3 and M2. All surfaces are smooth. This is what one of the question from challenging Physics questions workbook. What I learn by solving this problem is how to push a system of 3 masses in this manner while keeping M3 'floating'. Not something that will win a Nobel prize or make a difference to life.

How about solving similar questions of boxes stacked in a moving lorry? That will make more sense in life.

Instead of random charges placed here and there? Why don't we look at electric field of ions in a solid arranged in different crystal structures?

It's tough coming out with original questions like these when majority of the references are the same old same old stuff... But it's worth doing it.