I am looking for references and free online books of solved problems in these topics. By applications in geometry I mean something like this
article about applications of integrals (something of that difficulty).
By applications in physics I mean simple classical mechanics (velocity, acceleration,...), combined with geometric representation.
Applications in percentage calculus should also include geometric representation.
Could someone suggest some good references and free online books with solved problems?
You might want to look into the Lagrangian, as that can spit out some nice differential equations. I'll let Wikipedia do the explaining: https://en.wikipedia.org/wiki/Lagrangian_mechanics.
If you like that, I suggest looking up Morin's Introduction to Classical Mechanics. It has some great stuff in there. I think the fourth chapter or so is on the lagrangian.
There are plenty more applications of differential equations, however. Whenever we want to model any process over time, we have to use a differential equation (I hope you see why a differential equation may be useful in that regard). ODE's are used across all the Sciences, and so they show up everywhere.