Entries by wrobel

An Example of Servo-Constraints in Mechanics

Servo-constraint was invented by Henri Beghin in his PhD thesis in 1922. For details see the celebrated monograph in rational mechanics by Paul Appell. To understand what this is, we consider the following example. A trolley of mass ##M## can move freely along the horizontal ground in the standard gravity field. A pendulum is placed […]

Explaining the General Brachistochrone Problem

Consider a problem about the curve of fastest descent in the following generalized statement. Suppose that we have a Lagrangian system $$L(x,\dot x)=\frac{1}{2}g_{ij}(x)\dot x^i\dot x^j-V(x),\quad x=(x^1,\ldots, x^m)\in M.\qquad (*)$$ Here ##M## is a smooth configuration manifold, ##\mathrm{dim}\,M=m##. The functions ##g_{ij}## form a Riemann metric in ##M##. Assume also that our system is constrained with the […]

Basic Kinematics in Classical Mechanics

There is an interesting thing in teaching of Classical Mechanics. Several theorems which presented below form a core part of kinematics for all Russian textbooks in Classical Mechanics (excluding Classical Mechanics for physicists)  but European and USA textbooks contain this material just partly and from time to time. Perhaps I just do not know enough […]

Presenting a Rare Kinematic Formula

Here we present some useful kinematic fact which is uncommon for textbooks in mechanics. Consider a convex rigid body (RB) rolling without slipping on a fixed plane. (The plane can actually be replaced with a some other  fixed surface.) In the picture RB is a filled with dots oval .  At a given moment of time […]

Elementary Construction of the Angular Velocity

Physics books  seldom contain accurate definition of the angular velocity of a rigid body. I believe that the following construction is as simple as possible and also rigorous. Assume we have a rigid body which is moving in the space. Theorem. In each moment there exists a unique (pseudo)vector ##\boldsymbol\omega ## such that for any […]