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Physics is the natural science of matter, involving the study of matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. Physics is one of the most fundamental scientific disciplines, with its main goal being to understand how the universe behaves.
- Overview
- The scope of physics
- Mechanics
Physics is the branch of science that deals with the structure of matter and how the fundamental constituents of the universe interact. It studies objects ranging from the very small using quantum mechanics to the entire universe using general relativity.
Why does physics work in SI units?
Physicists and other scientists use the International System of Units (SI) in their work because they wish to use a system that is agreed upon by scientists worldwide. Since 2019 the SI units have been defined in terms of fundamental physical constants, which means that scientists anywhere using SI can agree upon the units they use to measure physical phenomena.
physics, science that deals with the structure of matter and the interactions between the fundamental constituents of the observable universe. In the broadest sense, physics (from the Greek physikos) is concerned with all aspects of nature on both the macroscopic and submicroscopic levels. Its scope of study encompasses not only the behaviour of objects under the action of given forces but also the nature and origin of gravitational, electromagnetic, and nuclear force fields. Its ultimate objective is the formulation of a few comprehensive principles that bring together and explain all such disparate phenomena.
(Read Einstein’s 1926 Britannica essay on space-time.)
Physics is the basic physical science. Until rather recent times physics and natural philosophy were used interchangeably for the science whose aim is the discovery and formulation of the fundamental laws of nature. As the modern sciences developed and became increasingly specialized, physics came to denote that part of physical science not included in astronomy, chemistry, geology, and engineering. Physics plays an important role in all the natural sciences, however, and all such fields have branches in which physical laws and measurements receive special emphasis, bearing such names as astrophysics, geophysics, biophysics, and even psychophysics. Physics can, at base, be defined as the science of matter, motion, and energy. Its laws are typically expressed with economy and precision in the language of mathematics.
The traditionally organized branches or fields of classical and modern physics are delineated below.
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Mechanics is generally taken to mean the study of the motion of objects (or their lack of motion) under the action of given forces. Classical mechanics is sometimes considered a branch of applied mathematics. It consists of kinematics, the description of motion, and dynamics, the study of the action of forces in producing either motion or static equilibrium (the latter constituting the science of statics). The 20th-century subjects of quantum mechanics, crucial to treating the structure of matter, subatomic particles, superfluidity, superconductivity, neutron stars, and other major phenomena, and relativistic mechanics, important when speeds approach that of light, are forms of mechanics that will be discussed later in this section.
In classical mechanics the laws are initially formulated for point particles in which the dimensions, shapes, and other intrinsic properties of bodies are ignored. Thus in the first approximation even objects as large as Earth and the Sun are treated as pointlike—e.g., in calculating planetary orbital motion. In rigid-body dynamics, the extension of bodies and their mass distributions are considered as well, but they are imagined to be incapable of deformation. The mechanics of deformable solids is elasticity; hydrostatics and hydrodynamics treat, respectively, fluids at rest and in motion.
The three laws of motion set forth by Isaac Newton form the foundation of classical mechanics, together with the recognition that forces are directed quantities (vectors) and combine accordingly. The first law, also called the law of inertia, states that, unless acted upon by an external force, an object at rest remains at rest, or if in motion, it continues to move in a straight line with constant speed. Uniform motion therefore does not require a cause. Accordingly, mechanics concentrates not on motion as such but on the change in the state of motion of an object that results from the net force acting upon it. Newton’s second law equates the net force on an object to the rate of change of its momentum, the latter being the product of the mass of a body and its velocity. Newton’s third law, that of action and reaction, states that when two particles interact, the forces each exerts on the other are equal in magnitude and opposite in direction. Taken together, these mechanical laws in principle permit the determination of the future motions of a set of particles, providing their state of motion is known at some instant, as well as the forces that act between them and upon them from the outside. From this deterministic character of the laws of classical mechanics, profound (and probably incorrect) philosophical conclusions have been drawn in the past and even applied to human history.
Lying at the most basic level of physics, the laws of mechanics are characterized by certain symmetry properties, as exemplified in the aforementioned symmetry between action and reaction forces. Other symmetries, such as the invariance (i.e., unchanging form) of the laws under reflections and rotations carried out in space, reversal of time, or transformation to a different part of space or to a different epoch of time, are present both in classical mechanics and in relativistic mechanics, and with certain restrictions, also in quantum mechanics. The symmetry properties of the theory can be shown to have as mathematical consequences basic principles known as conservation laws, which assert the constancy in time of the values of certain physical quantities under prescribed conditions. The conserved quantities are the most important ones in physics; included among them are mass and energy (in relativity theory, mass and energy are equivalent and are conserved together), momentum, angular momentum, and electric charge.
- One-dimensional motion. Introduction to physics: One-dimensional motion Displacement, velocity, and time: One-dimensional motion Acceleration: One-dimensional motion.
- Two-dimensional motion. Two-dimensional projectile motion: Two-dimensional motion Optimal angle for a projectile: Two-dimensional motion.
- Forces and Newton's laws of motion. Newton's laws of motion: Forces and Newton's laws of motion Normal force and contact force: Forces and Newton's laws of motion Balanced and unbalanced forces: Forces and Newton's laws of motion.
- Centripetal force and gravitation. Circular motion and centripetal acceleration: Centripetal force and gravitation Centripetal forces: Centripetal force and gravitation Newton's law of gravitation: Centripetal force and gravitation.
Learn the basics of physics, such as fundamental quantities, laws, and conservation principles. Explore topics like motion, forces, fluids, electricity, magnetism, optics, and relativity.
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- Does the study of physics alter what we discover in mathematics?For sure. Newton invented calculus to solve the specific problem of calculating planetary motion. Bessel functions were invented to solve the spher...
- This wasn't mentioned in the text, but what is dark matter? Why is there so much of it, but it is un...dark matter is matter that doesn't emit or absorb light. We can't directly measure it.
- What is electric force?An electric force would be a specific type of force (a push or pull, such as gravity or friction) that is caused by the electrical interaction of o...
- so if i finish a physics course on khan academy it wouldn't be enough to score 700 ON SAT physics? i...Just completing a course doesn't directly mean that you'll do good on a test. You still have to do applied practice for the test. The AP Physics 1...
- Can Newton’s 1st and 2nd law of motion both be active at the same time?Sure. Newton's first law states that an object at rest or moving with a constant velocity will stay at rest or stay moving with a constant velocity...
- why is there no gravity in space? and why do we have gravity?There actually is gravity in space: the whole reason the Earth orbits around the sun is because the sun is a 'heavy' object that makes an impressio...
Learn what physics is about, how it relates to math and other sciences, and how it helps us understand the world around us. Watch a video that covers the basics of physics, such as laws, models, and phenomena, and see examples and questions.
- 10 min
- How do you prove your theory in physics?"Proof" is a word used commonly but has no real meaning outside maths. Maths is the theory behind the idea in that maths is always right or wrong a...
- If the universe is infinite, expanding and accelerating: how is an explosion accelerating, where und...Good question; I like your way of thinking. I agree, it is strange that the universe should be expaning at an accelerting rate. If you want to know...
- what is dynamic system in physics?A system that is continuously changing .
- Why does a balloon filled with hydrogen rises while one filled with air sinks?The density of hydrogen is less than one-tenth that of air.
- Does a magnetic pendulum violate the second law of thermodynamics? Wouldn't a magnetic pendulum move...Air Resistance exists
- How to determine magnitude of displacemetmeasure the straight-line distance from start to finish
- the video talks about the law of gravitation and how its a force, but wasn't that proved wrong by ei...According to Newton Gravitation is a force. And according to Einstein's theory it is not a force it is geometry, but when we study higher level phy...
- How did Einstein show that light is fastest?Actually Einstein didn't show that light is the fastest. Because all measurements of the speed of light in a vacuum showed light going to same spee...
- What does Khan mean by "The whole world is just a mental model that we have"?We perceive the world as something that other beings do not. For example some beings can see ultraviolet light, which we cannot naturally. This mea...
Physics is a study of how the universe behaves. learning objectives. Apply physics to describe the function of daily life. Physics is a natural science that involves the study of matter and its motion through space and time, along with related concepts such as energy and force.
Learn about the realm, applications, and laws of physics, the science of describing the interactions of energy, matter, space, and time. Explore how physics is relevant to everyday situations, engineering, chemistry, biology, and more.
