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Study of mechanics

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The main principles of mechanics

Principle of energy conservation

For any physical object with mass, any change in its state results in the expenditure or dissipation of energy.

Principle of least action

The idea of ​​the principle of least action is that any change (in direction, speed, etc.) will be achieved by transferring as little energy as possible from one system to another, to achieve the change in question.

In other words, this change will be the one that will transfer the least energy among all the possible changes (principle of least action), and this change will not transfer more energy than what is strictly necessary (first principle of thermodynamics), if we neglect the losses due to friction, energy conversion, etc. (second principle of thermodynamics).


  • classical mechanics, also known as Newtonian which deals with the kinematic study (study of motion without being interested in its cause), static and dynamic of a system, whether it is a simple system (point mechanics, solid mechanics) or a complex system (general mechanics);
  • physical mechanics which studies the mechanical behaviors of undeformable solids or continuous media: mechanics of fluids or deformable solids, with:
    • static mechanics which is the branch of physics studying mechanical systems at rest in a Galilean frame of reference;
    • celestial mechanics which studies the movements of celestial bodies; we also find the term spatial mechanics to designate the study of space objects such as artificial satellites;
  • statistical mechanics which studies systems with a large number of components, for example gases, composed of billions of molecules;
  • relativistic mechanics which deals with systems moving at speeds close to that of light;
  • quantum mechanics which deals with the behavior of physical systems at the particle scale;
  • biomechanics which deals with the deformation of living bodies, in particular of the human body, for example in a situation of shocks (example: crash of a vehicle) or significant acceleration (example: air combat);
  • physical acoustics which is the branch of mechanics studying small vibrational movements in solids, liquids or gases;
  • mechatronics which deals with the engineering of active mechanical objects, including kinematics, deformations, dynamic behaviors are measured or even controlled by electronic (example: sensors, computers, actuators) and digital (example: software) devices.

Measurement units

The units of the International System used in mechanics are mainly:

  • The second(s): unit of time, (to measure durations)
  • The meter (m): unit of length (to measure positions in particular)
  • The meter per second (m/s also noted ms−1): unit of speed (to measure the increase in position per second)
  • The meter per square second (m/s2 also noted ms−2): unit of acceleration (to measure the increase in speed per second)
  • The kilogram (kg): unit of mass and quantity of matter (example: one mole of water with a mass of 18,0 g includes 6.02 × 1023 H2O molecules
  • The newton (N): unit of force (example: the force exerted by the Earth on a mass of 1 kg is worth 9,81 N)
    • When you press the push button of your pen = 1 N
    • When you push with your foot on the pedal of your bike = 100 N
    • What a rocket motor produces on the gases it ejects = 10.000.000 N
  • The joule (J): unit of work and energy (equal to 1 newton meter (N m) i.e. a force of 1 N working over 1 m)
  • The watt (W): unit of power (worth 1 J/s or a work of 1 J produced per second)

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