Forces classification in physics

All the known forces of interaction existing in nature can be reduced to a small number of main types. Belonging to the first type are the gravitational and electromagnetic forces belonging to the last type are the forces of interatomic and intermolecular interaction pertaining to which macroscopic manifestation are elasticity forces. (Outside the scope of this book are the short-range nuclear forces, bonded nucleons in nuclei, and weak interactions, revealed in the decay of elementary particles.) 

The forces of gravitation are weaker than all the others. At the same time their action is realized through a gravitational field onto great distances. The expression for gravitational interaction between two point masses M and m is defined by the law of Newtonian attraction 

A body on the Earth s surface (r = R, i.e., R is the distance between the center of the earth and the body) experiences the attraction F = mg, or in another form (according to eq. (1.3.19)), F = G(mM/R ) it then follows that mg = G mMIR ), or g = G(MIR ). (This relationship can be used in order to simplify the solution to some problems.) 

We meet here for the second time the notion of mass. In this respect the mass is called gravitational. Generally speaking, this coefficient can be different from that appearing in the second Newtonian law. However, practice shows that, fortunately, inertial mass is just the same as gravitational mass i.e., the mass is the objective characteristic of a body exhibiting equally both inertial and gravitational laws. 

Solution To determine scale readings means to find the weight of the body mg (a), i.e., the force with which the body acts on a spring. This force, under the third Newtonian law in the inertial system connected to the earth, is equal on the modulo and is opposite in direction to the force of elasticity (force of a support reaction) from which the spring cup of the scales operates on a body N, P being the scale reading, that is mg = -N or in scalar form P = N. 

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