Mechanics
Mechanics
Mechanics is a branch of physics that deals with the motion of objects and the forces that cause this motion. It is divided into two main subfields: classical mechanics and quantum mechanics. Classical mechanics, based on Newton’s laws of motion, describes the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects such as spacecraft, planets, stars, and galaxies. Quantum mechanics, on the other hand, deals with phenomena on a very small scale, such as atoms and particles. Overall, mechanics is fundamental to the study of physics as it underpins much of our understanding of the physical world.
Classical mechanics
Classical mechanics, also known as Newtonian mechanics, is a branch of physics that deals with the motion of macroscopic objects under the influence of forces. It is based on the laws of motion that were first formulated by Sir Isaac Newton in the 17th century. Classical mechanics is used to describe the motion of planets, cars, projectiles, and other objects that are much larger than atoms and molecules.
Classical mechanics can be divided into three main parts: kinematics, dynamics, and statics.

Kinematics: This is the study of motion without considering the forces that cause it. For example, if a car is moving at a constant speed of 60 mph on a straight road, kinematics can be used to calculate how far the car will travel in a given amount of time.

Dynamics: This is the study of motion and the forces that cause it. For example, if a car is accelerating from rest, dynamics can be used to calculate the force required to produce the acceleration, given the mass of the car.

Statics: This is the study of forces on bodies at rest. For example, if a book is resting on a table, statics can be used to calculate the force exerted by the book on the table (which is equal to the weight of the book), and the force exerted by the table on the book (which is equal and opposite to the weight of the book).
Classical mechanics is based on several fundamental principles, including:

Newton’s three laws of motion: These laws describe how forces affect the motion of objects. The first law (the law of inertia) states that an object at rest tends to stay at rest, and an object in motion tends to stay in motion, unless acted upon by a force. The second law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The third law states that for every action, there is an equal and opposite reaction.

The principle of conservation of energy: This principle states that the total energy of an isolated system remains constant if no external forces act on it. For example, if a pendulum is swinging back and forth, the total energy of the pendulum (the sum of its kinetic energy and potential energy) remains constant, assuming no energy is lost due to air resistance or friction.

The principle of conservation of momentum: This principle states that the total momentum of an isolated system remains constant if no external forces act on it. For example, if two ice skaters push off each other and glide away in opposite directions, the total momentum of the two skaters remains constant.
Classical mechanics is a deterministic theory, which means that the future state of a system can be predicted exactly if its current state is known. However, classical mechanics fails to accurately describe the behavior of very small particles (such as electrons) and very high speeds (close to the speed of light). These situations are better described by quantum mechanics and special relativity, respectively.
Quantum Mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.
The word “quantum” itself refers to the smallest possible discrete unit of any physical property, usually of energy or matter. Quantum mechanics is based on the principle that matter and light have characteristics of both particles and waves, a principle known as waveparticle duality.
One of the key principles of quantum mechanics is the superposition principle which states that a physical system—such as an electron in an atom—exists partly in all its particular, theoretically possible states simultaneously; but when measured or observed, it gives a result corresponding to only one of the possible configurations.
For example, a quantum particle like an electron in an atom doesn’t exist in one state or another, but in all of its possible states at once. It’s only when we try to measure the position of the electron that we force the electron to have a definite position.
Another fundamental principle in quantum mechanics is the uncertainty principle, formulated by Werner Heisenberg, which states that the position and the velocity of an object cannot both be measured exactly, at the same time. The more precisely one of these is known, the less precisely the other can be known. This is not a statement about the limitations of a researcher’s ability to measure particular quantities of a system, but rather about the nature of the system itself.
Quantum entanglement is another strange and fascinating phenomenon predicted by quantum mechanics. It refers to a situation where multiple particles are linked together in such a way that the state of one particle is immediately connected to the state of the other, no matter how far apart they are.
For example, if two entangled particles are created and one is sent to the other side of the universe, a change in the state of one particle will instantly affect the state of the other particle. This “spooky action at a distance” was a phrase used by Einstein to describe quantum entanglement, and it has been the subject of much debate and testing in the scientific community.
Quantum mechanics has been extremely successful in explaining the behavior of very small systems like atoms and subatomic particles. However, it also has many strange and counterintuitive predictions, and there are still many open questions about how it fits in with the rest of physics. Despite these questions, quantum mechanics has a wide range of applications including in lasers, semiconductors, magnetic resonance imaging, and designing drugs for medicine.
Statistical Mechanics
Statistical mechanics is a branch of theoretical physics that uses probability theory and statistics to solve physical problems that involve large populations of particles. It provides a framework for relating the microscopic properties of individual atoms and molecules to the macroscopic or bulk properties of materials that can be observed in everyday life, therefore explaining thermodynamics as a result of the classical and quantum mechanical behaviors of systems.
Statistical mechanics is based on the fundamental assumption, known as the ergodic hypothesis, that all accessible microstates are equally probable over a long period of time. A microstate is a specific microscopic configuration of a system that the system may occupy with a certain probability in the course of its thermal fluctuations.
For example, consider a box of gas particles. The macrostate of the system could be defined by the overall properties like pressure, temperature, and volume. However, the microstate would describe the position and velocity of each individual particle. Even though we can’t track each particle, we can calculate the probability of the system being in a certain microstate.
There are two main types of statistical mechanics: classical statistical mechanics, which does not take into account quantum mechanical effects, and quantum statistical mechanics, which does.
Classical statistical mechanics can be used to derive the ideal gas law. For instance, by considering a large number of gas particles, their average kinetic energy, and the volume they occupy, we can derive the relationship PV=nRT, where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature.
Quantum statistical mechanics, on the other hand, is necessary for dealing with systems at very low temperatures or very high densities, where quantum effects become significant. For example, it can explain phenomena such as superconductivity and superfluidity, where materials conduct electricity without resistance or flow without friction, respectively.
In conclusion, statistical mechanics is a powerful tool in physics that allows us to bridge the gap between the microscopic world of individual particles and the macroscopic world of bulk materials. It has wideranging applications, from explaining the behavior of gases to predicting the properties of solids and liquids.