Motion
Motion is the continuous change of position in space.
In physics, motion refers to how an object or fluid shifts its location over time, measured against a chosen reference frame. Mathematically, it is described using vector quantities like displacement (which includes both distance and direction), velocity (speed with direction), and acceleration. The study of motion without considering its causes is called kinematics, while dynamics examines how forces influence motion. An object is considered at rest, motionless, or stationary relative to a given frame if its position does not change over time. Because modern physics recognizes no absolute frame of reference, Isaac Newton’s idea of absolute motion cannot be determined; everything in the universe can be seen as being in motion. Motion applies to physical systems such as objects, particles, fields, radiation, spacetime curvature, and even abstract concepts like images, shapes, and boundaries. It generally means a continuous change in position or configuration within space—for instance, the motion of a wave or a quantum particle, where configuration involves probabilities of occupying specific positions.
The motion of massive bodies is governed by two sets of laws: classical mechanics for objects larger than atoms (e.g., cars, planets, cells) and quantum mechanics for atomic and subatomic objects (e.g., protons, electrons). Historically, Isaac Newton and Leonhard Euler formulated three laws of classical mechanics, first compiled by Newton in his work *Philosophiæ Naturalis Principia Mathematica*, published on July 5, 1687. These laws are: a body at rest stays at rest, and a body in motion stays in motion, unless acted on by an external force (the law of inertia); force equals the change in momentum over time, and for constant mass, force equals mass times acceleration; and for every action, there is an equal and opposite reaction—meaning one body exerting a force on another experiences an equal force in the opposite direction. Newton’s laws were the first to mathematically model orbiting bodies in space, unifying the motion of celestial objects with that of objects on Earth.
Modern kinematics, developed alongside electromagnetism, relates all velocities to the speed of light. Velocity is reinterpreted as rapidity, a hyperbolic angle where the hyperbolic tangent of that angle equals the velocity divided by the speed of light.
- field
- Physics
- key_concepts
- Displacement, velocity, acceleration, kinematics, dynamics
- branches
- Classical mechanics, relativistic mechanics, quantum mechanics
- types
- Relative motion, absolute motion (undetermined), imperceptible motion
- applications
- Objects, bodies, particles, fields, radiation, space-time, waves, quantum particles
Lore & Background
Quantum mechanics describes motion at the atomic and subatomic level, encompassing the wave-like and particle-like behavior of both matter and radiation energy, a concept known as wave–particle duality. A defining characteristic of this realm is the Heisenberg uncertainty principle, which asserts that the complete state of a subatomic particle—for instance, its location and velocity—cannot be simultaneously determined. This contrasts with classical mechanics, where accurate measurements and predictions of an object’s state, such as its position and speed, are calculable. The motion of macroscopic objects, from projectiles to planets, is governed by classical mechanics, which is fundamentally based on Newton’s three laws of motion: the law of inertia, the relationship between force and change in momentum, and the principle of equal and opposite reaction. These laws, first compiled by Sir Isaac Newton, accurately model the motion of celestial bodies and objects on Earth. For objects moving at speeds significantly slower than light, classical mechanics produces very accurate results. At speeds approaching the speed of light, relativistic mechanics, developed through special and general relativity, describes motion using rapidity and Lorentz transformations, incorporating gravity through a curved universe. The concept of motion applies broadly to physical systems, including objects, particles, fields, radiation, and even space-time itself. Everything in the universe can be considered to be in motion, as there is no absolute frame of reference.
Reader's Guide
Motion is a fundamental concept in physics, underlying the description of everything from subatomic particles to galaxies. Its study is divided into kinematics (describing motion without causes) and dynamics (studying forces and their effects). Classical mechanics, based on Newton's laws, accurately describes macroscopic objects moving at non-relativistic speeds. Relativistic mechanics extends this to high speeds and gravity, while quantum mechanics handles atomic and subatomic phenomena. The concept of motion also applies to fields, radiation, and space-time itself. The expansion of the universe represents a form of motion that does not involve physical movement but a fundamental change in the nature of spacetime. Humans are in constant motion, though many motions are imperceptible without special tools.
Did You Know?
- Everything in the universe can be considered to be in motion, as there is no absolute frame of reference.
- In quantum mechanics, the Heisenberg uncertainty principle prevents the simultaneous determination of a subatomic particle's location and velocity.
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