Many problems in mathematical physics, their solutions are connected with the application of cylindrical and spherical coordinates, solutions called Bessel functions resulted from a differential equation called the Bessel equation. The interesting application is related to a thin homogeneous circular membrane under small free oscillations, when the Bessel function is presented as a solution to describ the gravity by quantum field theory. This book contains simple problems and solutions as an attemp to help to approaching Bessel functions.