THE CENTRE OF GRAVITY IN TECHNICAL PRACTICE

The aim of this paper is to show the different methods of determination of a position of a centre of gravity in education: derivation of a formula for calculating the centre of gravity of a trapezoid and a derivation of a formula for calculating the volume of a truncated cylinder using gravity. The centre of gravity can be determined graphically, by calculation and experimentally. We use the calculation of a position of the centre of gravity by means of applying mathematics in engineering branches.


INTRODUCTION
The concept of "a centre of mass" in the form of the "a centre of gravity" was first introduced by the ancient Greek physicist, mathematician, and engineer Archimedes of Syracuse. He worked with simplified assumptions about gravity that amount to a uniform field, thus arriving at the mathematical properties of what we now call the centre of mass [1]. Archimedes showed that the torque exerted on a lever by weights resting at various points along the lever is the same as what it would be if all of the weights were moved to a single point their centre of mass. In work on floating bodies he demonstrated that the orientation of a floating object is the one that makes its centre of mass as low as possible. He developed mathematical techniques for finding the centres of mass of objects of uniform density of various well-defined shapes. Later mathematicians who developed the theory of the center of mass include Pappus of Alexandria, Guido Ubaldi, Francesco Maurolico, Federico Commandino, Simon Stevin, Luca Valerio, Jean-Charles de la Faille, Paul Guldin, John

MATERIAL AND METHODS
The experimental determination of the center of mass of a body uses gravity forces on the body and relies on the fact that in the parallel gravity field near the surface of the earth the center of mass is the same as the center of gravity. The center of mass of a body with an axis of symmetry and constant density must lie on this axis. Thus, the center of mass of a circular cylinder of constant density has its center of mass on the axis of the cylinder. In the same way, the center of mass of a spherically symmetric body of constant density is at the center of the sphere. In general, for any symmetry of a body, its center of mass will be a fixed point of that symmetry [2]. The term centre of gravity is introduced to pupils at elementary schools for the first time. Further information about the problem they get at secondary schools and universities. Table 1 shows different methods of teaching centre of gravity in connection with the particular type of schools.

RESULTS AND DISCUSSION
Calculate the coordinates of the centre of gravity of an isosceles trapezoid ABCD shown in the Fig. 1, . Obviously, the centre of gravity lies on the line segment MN , at a distance CG x from the side AB .
A coordinate T y can be found out easily, if we calculate the midpoint of AB or the midpoint of CD . Next, we concentrate on the calculation of the coordinates CG x .
In the calculation we use a double integral, therefore, we must determine the straight line of BC and AD.
We create them by applying the knowledge of an analytic geometry and subsequently we get . : , : The area S of a trapezoid can be calculated by a definite integral, so we get After adjusting we get From the Fig.1 we can see that  In terms of gravity and its applications, a task of calculating the volume of a truncated cylinder seems to be really interesting. Let`s suppose that the cylindrical body has a projection in the plane (x, y) and is bounded by a plane 0     d cz by ax from above. The cylindrical body is shown in the Fig. 3.  We adjust the given terms so that the formula calculating the centre of gravity of the shape acts in the last expression.
are the first moments of area with respect to the axis x (y) and P is a volume of an elementary area D, that is the volume of the base of the cylinder.
After some further adjustments we get Furthermore, from the definition of gravity and geometric shape, it is obvious that

CONCLUSIONS
Calculation of the solids centre of gravity plays a very important part in the educational process not only at elementary and secondary schools, but predominantly at technical universities. In this paper, we derive the formula for calculating the centre of gravity of a trapezoid, if sizes of its sides are given, next we derived the formula to calculate the volume of a truncated cylinder using gravity. It is important for teachers to have some kind of experience when explaining students a centre of gravity in different subjects (mathematics, physics, technical mathematics, statics, etc.)