Classical mechanics

Besides this, many related specialties exist, dealing with gases, liquids, and solids, and so on. Classical mechanics is enhanced by special relativity for objects moving with high velocity, approaching the speed of light; general relativity is employed to handle gravitation at a deeper level; and quantum mechanics handles the wave-particle duality of atoms and molecules.

In physics, classical mechanics is one of the two major sub-fields of study in the science of mechanics, which is concerned with the set of physical laws governing and mathematically describing the motions of bodies and aggregates of bodies. The other sub-field is quantum mechanics.

The term classical mechanics was coined in the early 20th century to describe the system of mathematical physics begun by Isaac Newton and many contemporary 17th century workers, building upon the earlier astronomical theories of Johannes Kepler, which in turn were based on the precise observations of Tycho Brahe and the studies of terrestrial projectile motion of Galileo, but before the development of quantum physics and relativity. Therefore, some sources exclude so-called "relativistic physics" from that category. However, a number of modern sources do include Einstein's mechanics, which in their view represents classical mechanics in its most developed and most accurate form. The initial stage in the development of classical mechanics is often referred to as Newtonian mechanics, and is associated with the physical concepts employed by and the mathematical methods invented by Newton himself, in parallel with Leibniz, and others. This is further described in the following sections. More abstract and general methods include Lagrangian mechanics and Hamiltonian mechanics. Much of the content of classical mechanics was created in the 18th and 19th centuries and extends considerably beyond (particularly in its use of analytical mathematics) the work of Newton.

Description of the theory

The following introduces the basic concepts of classical mechanics. For simplicity, it often models real-world objects as point particles, objects with negligible size. The motion of a point particle is characterized by a small number of parameters: its position, mass, and the forces applied to it. Each of these parameters is discussed in turn.

In reality, the kind of objects which classical mechanics can describe always have a non-zero size. (The physics of very small particles, such as the electron, is more accurately described by quantum mechanics). Objects with non-zero size have more complicated behavior than hypothetical point particles, because of the additional degrees of freedom—for example, a baseball can spin while it is moving. However, the results for point particles can be used to study such objects by treating them as Composite objects, made up of a large number of interacting point particles. The center of mass of a composite object behaves like a point particle.

Displacement and its derivatives

Velocity and speed

$vec\{v\}\; =\; \{mathrm\{d\}vec\{r\}\; over\; mathrm\{d\}t\},!$.

In classical mechanics, velocities are directly additive and subtractive. For example, if one car traveling East at 60 km/h passes another car traveling East at 50 km/h, then from the perspective of the slower car, the faster car is traveling east at 60 − 50 = 10 km/h. Whereas, from the perspective of the faster car, the slower car is moving 10 km/h to the West. Velocities are directly additive as vector quantities; they must be dealt with using vector analysis.

Mathematically, if the velocity of the first object in the previous discussion is denoted by the vector $vec\{u\}\; =\; uvec\{d\}$ and the velocity of the second object by the vector $vec\{v\}\; =\; vvec\{e\}$ where $u$ is the speed of the first object, $v$ is the speed of the second object, and $vec\{d\}$ and $vec\{e\}$ are unit vectors in the directions of motion of each particle respectively, then the velocity of the first object as seen by the second object is:

$vec\{u\text{'}\}\; =\; vec\{u\}\; -\; vec\{v\},!$

Similarly:

$vec\{v\text{'}\}=\; vec\{v\}\; -\; vec\{u\},!$

When both objects are moving in the same direction, this equation can be simplified to:

$vec\{u\text{'}\}\; =\; (u\; -\; v\; )\; vec\{d\},!$

Or, by ignoring direction, the difference can be given in terms of speed only:

$u\text{'}\; =\; u\; -\; v\; ,!$

Acceleration

$vec\{a\}\; =\; \{mathrm\{d\}vec\{v\}\; over\; mathrm\{d\}t\}$.

Acceleration can arise from a change with time of the magnitude of the velocity or of the direction of the velocity or both. If only the magnitude, $v$, of the velocity decreases, this is sometimes referred to as deceleration, but generally any change in the velocity with time, including deceleration, is simply referred to as acceleration.

Frames of reference

The following consequences can be derived about the perspective of an event in two inertial reference frames, $S$ and $S\text{'}$, where $S\text{'}$ is traveling at a relative velocity of $scriptstyle\{vec\{u\}\}$ to $S$.

• $scriptstyle\{vec\{v\text{'}\}\; =\; vec\{v\}\; -\; vec\{u\}\}$ (the velocity $scriptstyle\{vec\{v\text{'}\}\}$ of a particle from the perspective of S' is slower by $scriptstyle\{vec\{u\}\}$ than its velocity $scriptstyle\{vec\{v\}\}$ from the perspective of S)
• $scriptstyle\{vec\{a\text{'}\}\; =\; vec\{a\}\}$ (the acceleration of a particle remains the same regardless of reference frame)
• $scriptstyle\{vec\{F\text{'}\}\; =\; vec\{F\}\}$ (the force on a particle remains the same regardless of reference frame)
• the speed of light is not a constant in classical mechanics, nor does the special position given to the speed of light in relativistic mechanics have a counterpart in classical mechanics.
• the form of Maxwell's equations is not preserved across such inertial reference frames. However, in Einstein's theory of special relativity, the assumed constancy (invariance) of the vacuum speed of light alters the relationships between inertial reference frames so as to render Maxwell's equations invariant.

Forces; Newton's Second Law

$vec\{F\}\; =\; \{mathrm\{d\}vec\{p\}\; over\; mathrm\{d\}t\}\; =\; \{mathrm\{d\}(m\; vec\{v\})\; over\; mathrm\{d\}t\}$.

The quantity $mvec\{v\}$ is called the (canonical) momentum. The net force on a particle is, thus, equal to rate change of momentum of the particle with time. Since the definition of acceleration is $vec\{a\}\; =\; frac\; \{mathrm\{d\}\; vec\{v\}\}\; \{mathrm\{d\}t\}$, when the mass of the object is fixed, for example, when the mass variation with velocity found in special relativity is negligible (an implicit approximation in Newtonian mechanics), Newton's law can be written in the simplified and more familiar form

$vec\{F\}\; =\; m\; vec\{a\}$.

So long as the force acting on a particle is known, Newton's second law is sufficient to describe the motion of a particle. Once independent relations for each force acting on a particle are available, they can be substituted into Newton's second law to obtain an ordinary differential equation, which is called the equation of motion.

As an example, assume that friction is the only force acting on the particle, and that it may be modeled as a function of the velocity of the particle, for example:

$vec\{F\}\_\{rm\; R\}\; =\; -\; lambda\; vec\{v\}$

with λ a positive constant.. Then the equation of motion is

$-\; lambda\; vec\{v\}\; =\; m\; vec\{a\}\; =\; m\; \{mathrm\{d\}vec\{v\}\; over\; mathrm\{d\}t\}$.

This can be integrated to obtain

$vec\{v\}\; =\; vec\{v\}\_0\; e^\{-\; lambda\; t\; /\; m\}$

where $vec\{v\}\_0$ is the initial velocity. This means that the velocity of this particle decays exponentially to zero as time progresses. In this case, an equivalent viewpoint is that the kinetic energy of the particle is absorbed by friction (which converts it to heat energy in accordance with the conservation of energy), slowing it down. This expression can be further integrated to obtain the position $vec\{r\}$ of the particle as a function of time.

Important forces include the gravitational force and the Lorentz force for electromagnetism. In addition, Newton's third law can sometimes be used to deduce the forces acting on a particle: if it is known that particle A exerts a force $vec\{F\}$ on another particle B, it follows that B must exert an equal and opposite reaction force, -$vec\{F\}$, on A. The strong form of Newton's third law requires that $vec\{F\}$ and -$vec\{F\}$ act along the line connecting A and B, while the weak form does not. Illustrations of the weak form of Newton's third law are often found for magnetic forces.

Energy

$W\; =\; vec\{F\}\; cdot\; Delta\; vec\{s\}$.

If the mass of the particle is constant, and W_total is the total work done on the particle, obtained by summing the work done by each applied force, from Newton's second law:

$W\_\{rm\; total\}\; =\; Delta\; E\_k\; ,!$,

where E_k is called the kinetic energy. For a point particle, it is mathematically defined as the amount of work done to accelerate the particle from zero velocity to the given velocity v:

$E\_k\; =\; begin\{matrix\}\; frac\{1\}\{2\}\; end\{matrix\}\; mv^2$.

For extended objects composed of many particles, the kinetic energy of the composite body is the sum of the kinetic energies of the particles.

A particular class of forces, known as conservative forces, can be expressed as the gradient of a scalar function, known as the potential energy and denoted E_p:

$vec\{F\}\; =\; -\; vec\{nabla\}\; E\_p$.

If all the forces acting on a particle are conservative, and E_p is the total potential energy (which is defined as a work of involved forces to rearrange mutual positions of bodies), obtained by summing the potential energies corresponding to each force

Rightarrow - Delta E_p = Delta E_k Rightarrow Delta (E_k + E_p) = 0 ,!.

This result is known as conservation of energy and states that the total energy,

$sum\; E\; =\; E\_k\; +\; E\_p\; ,!$

is constant in time. It is often useful, because many commonly encountered forces are conservative.

Beyond Newton's Laws

There are two important alternative formulations of classical mechanics: Lagrangian mechanics and Hamiltonian mechanics. These, and other modern formulations, usually bypass the concept of "force", instead referring to other physical quantities, such as energy, for describing mechanical systems.

Classical transformations

x' = x - ut

y' = y

z' = z

t' = t

This set of formulas defines a group transformation known as the Galilean transformation (informally, the Galilean transform). This group is a limiting case of the Poincaré group used in special relativity. The limiting case applies when the velocity u is very small compared to c, the speed of light.

For some problems, it is convenient to use rotating coordinates (reference frames). Thereby one can either keep a mapping to a convenient inertial frame, or introduce additionally a fictitious centrifugal force and Coriolis force.

History

Some Greek philosophers of antiquity, among them Aristotle, may have been the first to maintain the idea that "everything happens for a reason" and that theoretical principles can assist in the understanding of nature. While, to a modern reader, many of these preserved ideas come forth as eminently reasonable, there is a conspicuous lack of both mathematical theory and controlled experiment, as we know it. These both turned out to be decisive factors in forming modern science, and they started out with classical mechanics.

An early experimental scientific method was introduced into mechanics in the 11th century by al-Biruni, who along with al-Khazini in the 12th century, unified statics and dynamics into the science of mechanics, and combined the fields of hydrostatics with dynamics to create the field of hydrodynamics. Concepts related to Newton's laws of motion were also enunciated by several other Muslim physicists during the Middle Ages. Early versions of the law of inertia, known as Newton's first law of motion, and the concept relating to momentum, part of Newton's second law of motion, were described by Ibn al-Haytham (Alhacen) and Avicenna. The proportionality between force and acceleration, an important principle in classical mechanics, was first stated by Hibat Allah Abu'l-Barakat al-Baghdaadi, and theories on gravity were developed by Ja'far Muhammad ibn Mūsā ibn Shākir, Ibn al-Haytham, and al-Khazini. It is known that Galileo Galilei's mathematical treatment of acceleration and his concept of impetus grew out of earlier medieval analyses of motion, especially those of Avicenna, Ibn Bajjah, and Jean Buridan.

The first published causal explanation of the motions of planets was Johannes Kepler's Astronomia nova published in 1609. He concluded, based on Tycho Brahe's observations of the orbit of Mars, that the orbits were ellipses. This break with ancient thought was happening around the same time that Galilei was proposing abstract mathematical laws for the motion of objects. He may (or may not) have performed the famous experiment of dropping two cannon balls of different masses from the tower of Pisa, showing that they both hit the ground at the same time. The reality of this experiment is disputed, but, more importantly, he did carry out quantitative experiments by rolling balls on an inclined plane. His theory of accelerated motion derived from the results of such experiments, and forms a cornerstone of classical mechanics.

As foundation for his principles of natural philosophy, Newton proposed three laws of motion, the law of inertia, his second law of acceleration, mentioned above, and the law of action and reaction, and hence laying the foundations for classical mechanics. Both Newtons second and third laws were given proper scientific and mathematical treatment in Newton's Philosophiæ Naturalis Principia Mathematica, which distinguishes them from earlier attempts at explaining similar phenomena, which were either incomplete, incorrect, or given little accurate mathematical expression. Newton also enunciated the principles of conservation of momentum and angular momentum. In Mechanics, Newton was also the first to provide the first correct scientific and mathematical formulation of gravity in Newton's law of universal gravitation. The combination of Newton's laws of motion and gravitation provide the fullest and most accurate description of classical mechanics. He demonstrated that these laws apply to everyday objects as well as to celestial objects. In particular, he obtained a theoretical explanation of Kepler's laws of motion of the planets.

Newton previously invented the calculus, of mathematics, and used it to perform the mathematical calculations. For acceptability, his book, the Principia, was formulated entirely in terms of the long established geometric methods, which were soon to be eclipsed by his calculus. However it was Leibniz who developed the notation of the derivative and integral preferred today.

Newton, and most of his contemporaries, with the notable exception of Huygens, worked on the assumption that classical mechanics would be able to explain all phenomena, including light, in the form of geometric optics. Even when discovering the so-called Newton's rings (a wave interference phenomenon) his explanation remained with his own corpuscular theory of light.

After Newton, classical mechanics became a principal field of study in mathematics as well as physics.

Some difficulties were discovered in the late 19th century that could only be resolved by more modern physics. Some of these difficulties related to compatibility with electromagnetic theory, and the famous Michelson-Morley experiment. The resolution of these problems led to the special theory of relativity, often included in the term classical mechanics.

A second set of difficulties related to thermodynamics. When combined with thermodynamics, classical mechanics leads to the Gibbs paradox of classical statistical mechanics, in which entropy is not a well-defined quantity. Black-body radiation was not explained without the introduction of quanta. As experiments reached the atomic level, classical mechanics failed to explain, even approximately, such basic things as the energy levels and sizes of atoms and the photo-electric effect. The effort at resolving these problems led to the development of quantum mechanics.

Since the end of the 20th century, the place of classical mechanics in physics has been no longer that of an independent theory. Emphasis has shifted to understanding the fundamental forces of nature as in the Standard model and its more modern extensions into a unified theory of everything. Classical mechanics is a theory for the study of the motion of non-quantum mechanical, low-energy particles in weak gravitational fields.

Limits of validity

The Newtonian approximation to special relativity

$vec\{p\}\; =\; m\_0\; vec\{v\}$

$vec\{p\}\; =\; frac\{m\_0\; vec\{v\}\}\{\; sqrt\{1-v^2/c^2\}\}\; =\; m\_0\; vec\{v\}\; left(1+frac\{1\}\{2\}frac\{v^2\}\{c^2\}\; +\; ...\; right)$, where $v=|vec\{v\}|$

$frac\{v\}\{c\}=0$

For example, the relativistic cyclotron frequency of a cyclotron, gyrotron, or high voltage magnetron is given by $f=f\_cfrac\{m\_0\}\{m\_0+T/c^2\}$, where $f\_c$ is the classical frequency of an electron (or other charged particle) with kinetic energy $T$ and (rest) mass $m\_0$ circling in a magnetic field. The (rest) mass of an electron is 511 keV. So the frequency correction is 1% for a magnetic vacuum tube with a 5.11 kV. direct current accelerating voltage.

The classical approximation to quantum mechanics

$lambda=frac\{h\}\{p\}$

where h is Planck's constant and p is the momentum.

Again, this happens with electrons before it happens with heavier particles. For example, the electrons used by Clinton Davisson and Lester Germer in 1927, accelerated by 54 volts, had a wave length of 0.167 nm, which was long enough to exhibit a single diffraction side lobe when reflecting from the face of a nickel crystal with atomic spacing of 0.215 nm. With a larger vacuum chamber, it would seem relatively easy to increase the angular resolution from around a radian to a milliradian and see quantum diffraction from the periodic patterns of integrated circuit computer memory.

More practical examples of the failure of classical mechanics on an engineering scale are conduction by quantum tunneling in tunnel diodes and very narrow transistor gates in integrated circuits.

Classical mechanics is the same extreme high frequency approximation as geometric optics. It is more often accurate because it describes particles and bodies with rest mass. These have more momentum and therefore shorter De Broglie wavelengths than massless particles, such as light, with the same kinetic energies.

Notes

References

• Feynman, Richard (1996). Six Easy Pieces. Perseus Publishing. ISBN 0-201-40825-2.
• Feynman, Richard; Phillips, Richard (1998). Six Easy Pieces. Perseus Publishing. ISBN 0-201-32841-0.
• Feynman, Richard (1999). Lectures on Physics. Perseus Publishing. ISBN 0-7382-0092-1.
• Landau, L. D.; Lifshitz, E. M. (1972). Mechanics Course of Theoretical Physics , Vol. 1. Franklin Book Company, Inc.. ISBN 0-08-016739-X.
• Kleppner, D. and Kolenkow, R. J., An Introduction to Mechanics, McGraw-Hill (1973). ISBN 0-07-035048-5
• Gerald Jay Sussman and Jack Wisdom, Structure and Interpretation of Classical Mechanics, MIT Press (2001). ISBN 0-262-19455-4}
• Herbert Goldstein, Charles P. Poole, John L. Safko, Classical Mechanics (3rd Edition), Addison Wesley; ISBN 0-201-65702-3
• Robert Martin Eisberg, Fundamentals of Modern Physics, John Wiley and Sons, 1961
• M. Alonso, J. Finn, "Fundamental university physics", Addison-Wesley

See also

• History of classical mechanics
• Dynamical systems
• List of equations in classical mechanics
• List of publications in classical mechanics
• Molecular dynamics
• Newton's laws of motion
• Special theory of relativity

Branches

External links

• Binney, James. Classical Mechanics (Lagrangian and Hamiltonian formalisms)
• Crowell, Benjamin. Newtonian Physics (an introductory text, uses algebra with optional sections involving calculus)
• Fitzpatrick, Richard. Classical Mechanics (uses calculus)
• Hoiland, Paul (2004). Preferred Frames of Reference & Relativity
• Horbatsch, Marko, " Classical Mechanics Course Notes".
• Rosu, Haret C., " Classical Mechanics". Physics Education. 1999. [arxiv.org : physics/9909035]
• Schiller, Christoph. Motion Mountain (an introductory text, uses some calculus; see also Motion Mountain)
• Sussman, Gerald Jay & Wisdom, Jack & Mayer,Meinhard E. (2001). Structure and Interpretation of Classical Mechanics
• Tong, David. Classical Dynamics (Cambridge lecture notes on Lagrangian and Hamiltonian formalism)