Standard physics glossary

waves · intermediate

Natural Frequency

The frequency at which a physical system tends to oscillate freely when disturbed and left alone, without any continuous external driving force, determined by the system's mass, stiffness, or other physical properties. When an external force is applied at or near a system's natural frequency, the resulting oscillations can grow dramatically in amplitude, a phenomenon known as resonance. Understanding natural frequency is essential in designing structures such as bridges and buildings to avoid dangerous resonant vibrations.

particle · intro

Neutrino

An extremely light, electrically neutral subatomic particle classified as a lepton, produced abundantly in nuclear reactions such as those in the Sun, nuclear reactors, and radioactive decay. Neutrinos interact only through the weak nuclear force and gravity, making them extraordinarily difficult to detect, since they can pass through entire planets with only a tiny chance of interacting. There are three known types, or flavors, of neutrino, and they have been shown to oscillate between these types as they travel.

nuclear · intro

Neutron

A subatomic particle with no net electric charge and a mass slightly greater than that of a proton, found together with protons in the nucleus of atoms. Neutrons are composed of three quarks bound together by the strong force and are classified as baryons. Free neutrons outside a nucleus are unstable and decay into a proton, an electron, and an antineutrino within an average lifetime of about 15 minutes.

astro · intro

Neutron Star

An extremely dense stellar remnant formed when a massive star's core collapses during a supernova explosion, composed almost entirely of tightly packed neutrons and supported against further collapse by neutron degeneracy pressure. A typical neutron star packs more mass than the Sun into a sphere only about 20 kilometers across, giving it an almost unimaginable density. Many neutron stars are observed as rapidly rotating pulsars, emitting regular beams of radiation as they spin.

mechanics · intro

Newton's First Law

Also called: law of inertia

One of Isaac Newton's three laws of motion, stating that an object at rest stays at rest, and an object in motion continues moving at constant velocity in a straight line, unless acted upon by a net external force. Also called the law of inertia, it establishes that force is required to change motion, not to sustain it, correcting the earlier Aristotelian view that a continuous force was needed to keep objects moving.

mechanics · intro

Newton's Law of Gravitation

A law formulated by Isaac Newton stating that every pair of objects with mass attracts each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This single law successfully explained planetary orbits, the tides, and falling objects on Earth under one unified framework, and it remained the best description of gravity until superseded in extreme cases by Einstein's general relativity.

mechanics · intro

Newton's Second Law

One of Isaac Newton's three laws of motion, stating that the net force acting on an object equals the product of its mass and its acceleration, commonly written as force equals mass times acceleration. This law quantitatively links force, mass, and motion, allowing the prediction of an object's future motion if the forces acting on it are known. It forms the mathematical foundation of classical mechanics.

mechanics · intro

Newton's Third Law

One of Isaac Newton's three laws of motion, stating that for every action force one object exerts on a second object, the second object exerts an equal and opposite force back on the first. This pairing of forces always acts on two different objects, never canceling each other out for a single object, and explains phenomena such as rocket propulsion, walking, and the recoil of a fired gun.

mechanics · intro

Normal Force

The contact force exerted by a surface on an object resting on or pressing against it, directed perpendicular to the surface, preventing the object from passing through it. On a flat, horizontal surface, the normal force typically balances the object's weight, but on an inclined or curved surface, its magnitude and direction adjust to match the geometry and any other forces involved. Normal force is a key component in calculating friction, since friction is often proportional to it.

astro · intro

Nova

A sudden, bright outburst of light from a white dwarf star in a binary system, caused by the rapid, explosive nuclear fusion of hydrogen gas that has accumulated on the white dwarf's surface after being pulled from a companion star. Unlike a supernova, a nova does not destroy the star, and the same white dwarf can undergo repeated nova outbursts over time as more material accumulates and ignites.

nuclear · intro

Nuclear Fission

Also called: fission

The splitting of a heavy atomic nucleus, such as uranium-235 or plutonium-239, into two or more smaller nuclei, usually triggered by the absorption of a neutron, and releasing a large amount of energy along with additional neutrons. These released neutrons can trigger further fission events, creating a chain reaction that powers nuclear reactors and, when uncontrolled, nuclear weapons. Fission releases energy because the resulting smaller nuclei are more tightly bound than the original heavy nucleus.

nuclear · intro

Nuclear Fusion

Also called: fusion

The combination of two light atomic nuclei into a single heavier nucleus, releasing a large amount of energy because the resulting nucleus is more tightly bound than the two original nuclei. Fusion powers the Sun and other stars, where immense pressure and temperature force hydrogen nuclei together to form helium. Achieving controlled, net-positive fusion on Earth as a practical energy source remains an active area of research.

nuclear · intro

Nuclear Reactor

A device designed to initiate and control a sustained nuclear fission chain reaction, releasing energy in a regulated manner, primarily used to generate electricity or to produce isotopes for research and medicine. Reactors use control rods to absorb excess neutrons and regulate the reaction rate, and a coolant to transfer the released heat to a turbine system. Reactor designs vary widely, using different fuels, moderators, and coolant systems.

nuclear · intro

Nucleon

A collective term for the particles found in an atomic nucleus, referring to both protons and neutrons, which are structurally similar particles held together by the strong nuclear force. The total number of nucleons in a nucleus is given by the mass number. Nucleons themselves are composite particles, each made of three quarks bound together by gluons.

astro · intermediate

Nucleosynthesis

The process by which new atomic nuclei are formed from pre-existing protons and neutrons, occurring in several distinct settings, including the early hot phase of cosmic expansion, which produced the lightest elements like hydrogen and helium, and the interiors of stars, which fuse progressively heavier elements up to iron. Elements heavier than iron are primarily produced in violent events such as supernovae and neutron star mergers, through rapid neutron capture processes.

nuclear · intro

Nucleus

The small, extremely dense central region of an atom, containing protons and neutrons bound tightly together by the strong nuclear force, and accounting for nearly all of an atom's mass despite occupying only a tiny fraction of its volume. The number of protons in the nucleus determines the chemical element, while the number of neutrons can vary, producing different isotopes. The nucleus was first identified experimentally by Ernest Rutherford in 1911.