Working with infinitesimal quantities and the motivation behind it

Question

So in my freshman physics class, in classical mechanics the homework was (it's solved already, this isn't a homework thread) the following:

"A thin, spinning ring is placed on a table, that divides into 2 halves, with different coefficients of friction ($ \mu_2 > \mu_1 $). What is the initial acceleration of the ring?"

We solved it, but it required a process that i didn't have experience with, and that is working with infinitesimal quantities throughout the whole problem.
I also found some other resources where such a technique is used

Below is the solution to the spinning ring problem, but the rules we can and can't do, and the way we motivated the equations seem arbitrary. I'm looking for any resource that deals with carrying infinitesimals and getting a solution by integrating over them at the end.

Solution:
A picture that gives the idea . From this we conclude that the ring won't move in the Y direction, so we can focus only on the X. Here comes the infinitesimal part:
We take a small line-segment dl with mass dm and compute the friction force that it experiences:
$$ d F_{fric} = \mu \cdot dm\cdot g $$ but since due to a uniform mass distribution $$ dm = \dfrac{M}{2R \pi} \cdot dl $$ and $$ dl \approx d \varphi \cdot R$$ $ d F_{fric} $ then becomes $$d F_{fric} = \dfrac {\mu M g}{2\pi} d\varphi $$ As said above we are intrested in the X component of this, for which $ d {F_{fric}}_{X} = d F_{fric} \cdot sin(\varphi) $
To get the net force on a semi-circle we integrated this so: $$ \int_{0}^{\pi} {F_{fric}}_{X} d \varphi =\dfrac{\mu M g}{\pi}$$
and therefore the net acceleration looks like : $ a_{net} =(\mu_1 -\mu_2) \dfrac{g}{\pi} $
Here's a picture that captures this all
But when can/should we try to approach problems with such tools? Also, as said before, it really seems that some of these equations are motivated out of thin air.

Answer 1

Let's imagine that we want to calculate the area of a circle of radius $R$, and we don't know the general formula $S = \pi R^2$, but do know that the circumference of a circle is $C = 2\pi R$. Then we can say that we will divide the circle into many concentric circles with radii in the range $(0,R]$, and then the surface between pair of such circles is $2\pi r dr$ where $r$ is the radius of the inner circle, just because this length times 'height'. Now this is not true. The real surface between pair of such circles is $\Delta S = \pi (r+dr)^2-\pi r^2 = 2\pi r dr + \pi dr^2$. However, it is true to linear order in $dr$. So if we take infinitesimal $dr$, omitting the quadratic term is ok and then we can integrate $S = \int_0^{R} 2\pi r dr = \pi R^2$ and we got the correct result!

The idea is similar when dealing with problems in physics. If we restrict ourselves only to small changes, described by infinitesimal quantities, we tend to get linear equations, which nonetheless capture the entire physics (sometimes...). Then we can either integrate or turn this equations into a differential equation describing change over small ranges, and get the full nonlinear behavior from these solutions/integrations. The linear behavior is more easy and intuitive to derive, as it involves only small steps, just like the area was length times height when we took small $dr$.