Motion
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For other uses, see Motion (disambiguation).
Motion involves a change in position
In physics, motion is the phenomenon in which an object changes its position over time.
Motion is mathematically described in terms
of displacement, distance, velocity, acceleration, speed, and time. The motion of a body
is observed by attaching a frame of reference to an observer and measuring the change
in position of the body relative to that frame with change in time. The branch of physics
describing the motion of objects without reference to its cause is kinematics; the branch
studying forces and their effect on motion is dynamics.
If an object is not changing relatively to a given frame of reference, the object is said to
be at rest, motionless, immobile, stationary, or to have a constant or time-
invariant position with reference to its surroundings. As there is no absolute frame of
reference, absolute motion cannot be determined.[1] Thus, everything in the universe can
be considered to be in motion.[2]:20–21
Motion applies to various physical systems: to objects, bodies, matter particles, matter
fields, radiation, radiation fields, radiation particles, curvature and space-time. One can
also speak of motion of images, shapes and boundaries. So, the term motion, in
general, signifies a continuous change in the positions or configuration of a physical
system in space. For example, one can talk about motion of a wave or about motion of
a quantum particle, where the configuration consists of probabilities of occupying
specific positions.
The main quantity that measures the motion of a body is momentum. An object's
momentum increases with the object's mass and with its velocity. The total momentum
of all objects in an isolated system (one not affected by external forces) does not
change with time, as described by the law of conservation of momentum. An object's
motion, and thus its momentum, cannot change unless a force acts on the body.
Contents
1Laws of motion
o 1.1Classical mechanics
o 1.2Relativistic mechanics
o 1.3Quantum mechanics
2List of "imperceptible" human motions
o 2.1Universe
o 2.2Galaxy
o 2.3Sun and solar system
o 2.4Earth
o 2.5Continents
o 2.6Internal body
o 2.7Cells
o 2.8Particles
o 2.9Subatomic particles
3Light
4Types of motion
5Fundamental motions
6See also
7References
8External links
Laws of motion[edit]
Main article: Mechanics
In physics, motion of massive bodies is described through two related sets of laws of
mechanics. Motions of all large-scale and familiar objects in the universe (such
as cars, projectiles, planets, cells, and humans) are described by classical mechanics,
whereas the motion of very small atomic and sub-atomic objects is described
by quantum mechanics. Historically, Newton and Euler formulated three laws of
classical mechanics:
In an inertial reference frame, an object either remains at rest or continues to move at a
First law:
constant velocity, unless acted upon by a net force.
Second In an inertial reference frame, the vector sum of the forces F on an object is equal to
law: the mass m of that object multiplied by the acceleration a of the object: F = ma.
If the resultant force F acting on a body or an object is not equals to zero, the body will have an
acceleration a which is in the same direction as the resultant.
Third When one body exerts a force on a second body, the second body simultaneously exerts a force
law: equal in magnitude and opposite in direction on the first body.
Classical mechanics[edit]
Part of a series on
Classical mechanics
Second law of motion
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Classical mechanics is used for describing the motion of macroscopic objects,
from projectiles to parts of machinery, as well as astronomical objects, such
as spacecraft, planets, stars, and galaxies. It produces very accurate results within
these domains, and is one of the oldest and largest in science, engineering,
and technology.
Classical mechanics is fundamentally based on Newton's laws of motion. These laws
describe the relationship between the forces acting on a body and the motion of that
body. They were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis
Principia Mathematica, first published on July 5, 1687. Newton's three laws are:
1. A body either is at rest or moves with constant velocity, until and unless an outer force is
applied to it.
2. An object will travel in one direction only until an outer force changes its direction.
3. Whenever one body exerts a force F onto a second body, (in some cases, which is
standing still) the second body exerts the force −F on the first body. F and −F are equal
in magnitude and opposite in sense. So, the body which exerts F will go backwards.[3]
Newton's three laws of motion were the first to accurately provide a mathematical model
for understanding orbiting bodies in outer space. This explanation unified the motion of
celestial bodies and motion of objects on earth.
