General concepts

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Experiment. Even very young children perform rudimentary experiments in order to learn about the world.


An experiment is an orderly procedure carried out with the goal of verifying, refuting, or establishing the validity of a hypothesis. Controlled experiments provide insight into cause-and-effect by demonstrating what outcome occurs when a particular factor is manipulated. Controlled experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results. There also exist natural experimental studies. Theoretical physics. Theoretical physics is a branch of physics which employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experimental tools to probe these phenomena.

The advancement of science depends in general on the interplay between experimental studies and theory. In some cases, theoretical physics adheres to standards of mathematical rigor while giving little weight to experiments and observations. [a] For example, while developing special relativity, Albert Einstein was concerned with the Lorentz transformation which left Maxwell's equations invariant, but was apparently uninterested in the Michelson–Morley experiment on Earth's drift through a luminiferous ether. [citation needed] On the other hand, Einstein was awarded the Nobel Prize for explaining the photoelectric effect, previously an experimental result lacking a theoretical formulation. Overview History Examples. Units of measurement. For example, length is a physical quantity.

Units of measurement

The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre". The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day. Different systems of units used to be very common. State of matter. Historically, the distinction is made based on qualitative differences in properties.

State of matter

Matter in the solid state maintains a fixed volume and shape, with component particles (atoms, molecules or ions) close together and fixed into place. Matter in the liquid state maintains a fixed volume, but has a variable shape that adapts to fit its container. Its particles are still close together but move freely. Matter in the gaseous state has both variable volume and shape, adapting both to fit its container.

Its particles are neither close together nor fixed in place. The four fundamental states Solid A crystalline solid: atomic resolution image of strontium titanate. In a solid the particles (ions, atoms or molecules) are closely packed together. Physical quantity. A physical quantity (or "physical magnitude") is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.[1] Extensive and intensive quantities[edit] An extensive quantity is equal to the sum of that quantity for all of its constituent subsystems; examples include volume, mass, and electric charge.

Physical quantity

For instance, if an object has mass m1 and another has mass m2 then a system simply comprising those two objects will have a mass of m1 + m2. An intensive quantity is independent of the extent of the system; quantities such as temperature, pressure, and density are examples. To illustrate, if two objects having a given temperature are combined, together they still have the same temperature (not twice the temperature). Observation. Observation is the active acquisition of information from a primary source.


In living beings, observation employs the senses. In science, observation can also involve the recording of data via the use of instruments. The term may also refer to any data collected during the scientific activity. Observation in science[edit] The scientific method requires observations of nature to formulate and test hypotheses.[1] It consists of these steps:[2][3] Physical system. Complexity in physical systems[edit]

Physical system

Light. The Sun is Earth's primary source of light. About 44% of the sun's electromagnetic radiation that reaches the ground is in the visible light range. Visible light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.[1] Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10−9 m, to 700 nanometres – between the infrared, with longer wavelengths and the ultraviolet, with shorter wavelengths.[2][3] These numbers do not represent the absolute limits of human vision, but the approximate range within which most people can see reasonably well under most circumstances.

Various sources define visible light as narrowly as 420 to 680[4][5] to as broadly as 380 to 800 nm.[6][7] Under ideal laboratory conditions, people can see infrared up to at least 1050 nm,[8] children and young adults ultraviolet down to about 310 to 313 nm.[9][10][11] Speed of light Optics where. Gravitation. Gravitation, or gravity, is a natural phenomenon by which all physical bodies attract each other.


It is most commonly recognized and experienced as the agent that gives weight to physical objects, and causes physical objects to fall toward the ground when dropped from a height. During the grand unification epoch, gravity separated from the electronuclear force.