# General concepts

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. Theoretical physics. Units of measurement. For example, length is a physical quantity.

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. Now there is a global standard, the International System of Units (SI), the modern form of the metric system. In trade, weights and measures is often a subject of governmental regulation, to ensure fairness and transparency.

In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Science, medicine, and engineering often use larger and smaller units of measurement than those used in everyday life and indicate them more precisely. History Systems of units State of matter. Historically, the distinction is made based on qualitative differences in properties.

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. 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 An extensive quantity is equal to the sum of that quantity for all of its constituent subsystems; examples include volume, mass, and electric charge.

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). There are also physical quantities that can be classified as neither extensive nor intensive, for example an extensive quantity with a nonlinear operator applied, such as the square of volume.[2] Symbols, nomenclature Examples. 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 The scientific method requires observations of nature to formulate and test hypotheses.[1] It consists of these steps:[2][3] Asking a question about a natural phenomenonMaking observations of the phenomenonHypothesizing an explanation for the phenomenonPredicting a logical consequence of the hypothesisTesting the hypothesis by an experiment, an observational study, or a field studyCreating a conclusion with data gathered in the experiment, or forming a revised/new hypothesis and repeating the process Senses are limited, and are subject to errors in perception such as optical illusions.

Observational paradoxes Biases Confirmation bias Processing bias Physical system. Complexity in physical systems

Light. A triangular prism dispersing a beam of white light.

The longer wavelengths (red) and the shorter wavelengths (blue) get separated Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which 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] Often, infrared and ultraviolet are also called light. The main source of light on Earth is the Sun.

Sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. Electromagnetic spectrum and visible light The behaviour of EMR depends on its wavelength. Speed of light Main article: Speed of light Optics Refraction where Table 1. 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.