### Physics - Mechanics

ANDREI SONIN

Department of Physics,

Moscow Polytechnic University

MECHANICS

(COURSE OF LECTURES)

Lecture 1

WHAT IS PHYSICS?

PLAN OF LECTURE:

I. Physics – Subjects and Methods.

II. How does Modern Physics Represent nature?

III. Physics and Other Sciences.

IV. How Physics is Studied at Technical Universities?

Hi! I am Professor Andrei Sonin from the Department of Physics, Moscow Polytechnic University.

I would like to present you my new course of lectures in physics.                                                                                                                                                                                  I have tried to make as easy understandable course as possible. It contains a minimum of mathematical formulas and a maximum of illustrations and animations. I hope that this course will be interesting to a large public, which would like to deepen its understanding of physics and expand its knowledge of this subject.

I will start with the first part of my course – Mechanics. Slide 1.

Today there will be my introductory lecture named “What is physics?”

Here is the brief plan of this lecture. Slide 2.

First, I will speak about subject and methods of physics.

So –

I. Physics – Subject and Methods.

Physics is the science, which studies nature. Slide 3. It formulates the most general laws of nature and investigates the world around us including ourselves.

Physics was born in ancient Greece. Indeed, the word “physics” means “nature” in Greek.                                                                                                                                          To obtain information about nature, physics explores its physical properties, – for example, sizes of objects, temperature, electric charge, magnetization, refractive indices, etc.

Physics is the experimental science. Usually, one makes an observation first and an experiment – second. Here is an example related to the great Italian physicist Galileo Galilei. Slide 4. According to some historical sources, he threw two balls of the same material but of different diameter and mass from the top of Pisa tower. He noticed that these balls fell to the ground in equal time. It was his observation. Then he had measured masses of balls and times of their descent, and he saw that his observation was correct. It was his experiment, which also proved the existence of gravitational acceleration.

To measure different physical properties (and related to them quantities) one uses two metric systems of units – the International system of units (its abbreviation is SI) and the Centimeter–gram–second system of units (its abbreviation is CGS). Slide. 5.

In physics, one utilizes seven base physical units having corresponding standards. Three of them are used in mechanics. These are length (l) with dimension “meter” in SI and – “centimeter” in CGS; mass (m) with dimension “kilogram” in SI and “gram” in SGS and time (t) with dimension “second” in both metric systems.                                                 It is easy to obtain the derived quantities from the base ones. For example, the derived quantity, force, can be written as F = ma according to Newton’s second law. Thus it follows for its unit – 1 N = 1 kg·m·s-2.                                                                                                                                                                                                                                    Physical laws express relations between physical properties written in the language of mathematics. There are universal and particular laws. Slide 6. Universal laws (for instance, Newton’s law of universal gravitation) are valid for all nature. Contrarily, particular laws work well only in some parts of nature. An example is the ideal gas law.       Here the torsion scales used by British physicist Henry Cavendish for checking the Newton law of universal gravitation is represented. Slide 7. The mutual attraction of small and big metal balls by gravity forces causes the rotation of a string on a small angle, which is detected by the motion of the light beam reflected from the miror.                                    Here the model of ideal gas is shown. Slide 8. Small red balls are the ideal gas molecules, while the bigger blue ball is some solid particle. The latter is pushed by actively moving ideal gas molecules and slowly moves. Such a motion is called the Brownian one, and it is the evidence of the molecular motion in a substance.                                              Modern physics it is not only an experiment science but also – a theoretical one. Slide 9.                                                                                                                                            Experimental physics deals with planning the future experiments, construction and use of the experimental setups, carrying out measurements, treatment and representation of experimental data. Theoretical physics deals with prediction of new physical phenomena and experiments, explanation of experimental data, making computer experiments and simulations. There is also a special branch of theoretical physics, which elaborates new mathematical methods for physics. It is called mathematical physics.

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II. How Does Modern Physics Represent nature?

Now, let us briefly overview the representation of nature by modern physics. We should note that physics is a material science and there is no place for God in it.                          We will start with the biggest scale objects and then will pass to the smaller ones.

Our Universe has appeared in the course of the Big Bang about 13.7 billion years ago. The Universe is expanding with acceleration, starting from its first moments. Only about 4 % of the Universe is observable, the resting 96 % are the so-called dark matter and energy. The Universe consist of galaxy filaments. Here they are looking as a white color network. Slide 10.

These filaments contain galaxies. Slide 11. Our galaxy called the “Milky Way”. It is a spiral galaxy of an average size.                                                                                              Our star is the Sun. Slide 12. This is a star of an average size and mass. It consists of plasma, and the temperature at its surface is around 6000 oC.                                            Our solar system includes eight planets. They revolve around the Sun in elliptical orbits. Some planets have gaseous composition, and others (like our Earth) consist of rocks. Slide 13

All listed objects – from the largest ones (the Universe, galaxies) to the smallest cosmic dust particles – are studied by astrophysics.

Our Earth has quite complex structure. Slide 14. It contains a magnetic core, mantle, soil (in 70 % covered by the ocean). All subjects related to Earth, are explored by geophysics.

The Earth is unique by the presence of life on its surface. The cell is the main building block for all living creatures. Slide 15. It has quite complex structure – internal nucleus, cell organs, membrane.

All living objects – the DNA and RNA molecules, viruses, bacteria, cells, plants, insects, animals and men are investigated by biophysics.

If we go to even smaller sizes, there we will find molecules (nonorganic and organic). Slide 16. Here some water molecules are represented. All molecules and their interaction are studied by molecular (or chemical) physics. This branch of physics also deals with the four states of matter: gas, liquid, solid and plasma. Slide 17. Here an ordered structure of a solid substance – sodium chloride (NaCl) crystal – is shown.

At even smaller scales, we will find atoms. Slide 18. This is the picture of the smallest hydrogen atom. It consists of a positively charged dense nucleus surrounded by a negatively charged electron cloud. Atoms and reactions between them are the topics of atomic physics.                                                                                                                       The atomic nucleus is built of protons (p) with positive electric charge and of electrically neutral neutrons (n). p and n are called the nucleons and they contain even smaller particles – quarks. Slide 19. Here is a schematic representation of proton, neutron and 3 different quarks inside each of them are given. Electrons, protons, neutrons and many other subatomic particles (about 400) are called the elementary particles. They are explored by nuclear physics (or physics of elementary particles).

Our picture of the world, which surrounds us, will be incomplete, if we do not consider the forces (or fundamental interactions) acting between all objects in nature. There exist four basic forces: gravitational, weak, electromagnetic and strong. Slide 20.  In this table, the energy of the weakest (gravitational) force is take for the unit. Then energies of another three forces will be much higher. Note that weak and strong forces manifest themselves only in subatomic world of elementary particles. So, weak interaction acts in atomic reactions, while strong interaction keeps together protons and neutrons in atomic nucleus. The range of both of these forces are very short. Contrarily, gravitational and electromagnetic interactions can be observed everywhere in macroworld. Their range is infinity. All four force fields should consist of elementary particles. The interaction carriers for strong, electromagnetic and weak forces are π-, ρ, ω-mesons and gluons, γ-quanta (photons) and W+-, W--, Z0-bosons, correspondingly. The gravitation carriers could be the hypothetical particles gravitons, but still there are no their experimental observations. The latter makes impossible for the moment to unify all four listed above force interactions.

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III. Physics and Other Sciences.

Physics interacts with many other sciences. Moreover, physics and all these sciences influence each other. Slide 21.                                                                                                First – what concerns physics and mathematics. You know already that mathematics is the language of physics. Moreover, the newly discovered physical phenomena can stimulate development of mathematics. Example – when Newton had developed the classical mechanics, he found that some novel mathematical methods were needed for description of mechanical motion. It were differential and integral calculus, which he had elaborated independently from Leibniz.                                                                                    Second – physics and technics. Of course, when an engineer creates some new technical machine, he must understand the physical principles of its functioning. And, vice versa, when some new machines appear it can stimulate the progress in physics. Example – creation of steam engines in the beginning of XIX century gave a strong impulse to development of thermodynamics and molecular kinetic theory.                                                                                                                                                                                            And, finally, physics can influence even some humanitarian sciences, – for example, economy. Indeed, statistical physics studies large ensembles of atoms and molecules, which are described by mathematical statistics. The same methods are applicable for analysis of some problems of economy, which depend on multiple factors. This is the so-called  physical economy. The multidisciplinary science synergetics, which explores the formation of ordered patterns from disordered ones in open, non-equilibrium systems, also uses some methods of statistical physics.

IV. How Physics is Studied at Technical Universities?

Normally, at technical universities, physics is studied from its easiest parts to more complicated ones – i.e. in chronological order. I will give you an approximate structure of the physics course covering three semesters. Slide 22.                                                                                                                                                                                                        In the first semester, one studies the most ancient part of physics – the classical mechanics (18th century). It is accompanied by short reviews of modern mechanics – Einstein’s STR and GTR (20th century). Further goes molecular-kinetic theory and thermodynamics (19th century).                                                                                                         The second semester is completely dedicated to electromagnetism (18–19th centuries).                                                                                                                                               And, finally, one studies wave (19th century) and quantum (20th century) optics together with atomic and nuclear physics (20th century) in the third semester.

Questions for Independent Work:

1. What are subject and methods of physics?                                                                                                                                                                                                                  2. How does modern physics represent nature?                                                                                                                                                                                                              3. What is relation of physics to other sciences?                                                                                                                                                                                                              4. How physics is studied at technical universities?

Slide 23.

That is all for today, and our first lecture is over. Thank you for your attention!