In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an observer, measuring the change in position of the body relative to that frame with a change in time. The branch of physics describing the motion of objects without reference to their cause is called kinematics, while the branch studying and their effect on motion is called dynamics.
If an object is not in motion relative to a given frame of reference, it is said to be at rest, motionless, immobile, , or to have a constant or time-invariant position with reference to its surroundings. Modern physics holds that, as there is no absolute frame of reference, Isaac Newton's concept of absolute motion cannot be determined.
Motion applies to various physical systems: objects, bodies, matter Particle, matter fields, radiation, radiation fields, radiation particles, curvature, and Spacetime. One can also speak of the motion of images, shapes, and boundaries. In general, the term motion signifies a continuous change in the position or configuration of a physical system in space. For example, one can talk about the motion of a wave or the motion of a quantum particle, where the configuration consists of the probabilities of the wave or particle occupying specific positions.
Equations of motion
Can
Laws of motion
In physics, the motion of bodies is described through two related sets of
scientific law of mechanics. Classical mechanics for super atomic (larger than an atom) objects (such as
,
,
, cells, and
) and
for
and sub-atomic objects (such as
helium,
protons, and
electrons). Historically, Newton and Euler formulated three laws of classical mechanics:
| First law: | In an inertial reference frame, an object either remains at rest or continues to move in a straight line at a constant velocity, unless acted upon by a net force. |
| Second law: | In an inertial reference frame, the vector Vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: .
If the resultant force acting on a body or an object is not equal to zero, the body will have an acceleration that is in the same direction as the resultant force. |
| Third law: | When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction onto the first body. |
Classical mechanics
Classical mechanics is used for describing the motion of
macroscopic objects moving at speeds significantly slower than the speed of light, from
projectiles to parts of
machinery, as well as astronomical objects, such as
spacecraft,
planets,
, and
Galaxy. It produces very accurate results within these domains and is one of the oldest and largest scientific descriptions 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 Isaac Newton in his work Philosophiæ Naturalis Principia Mathematica, which was first published on July 5, 1687. Newton's three laws are:
-
A Physical body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force. (This is known as the law of inertia.)
-
Force () is equal to the change in momentum per change in time (). For a constant mass, force equals mass times acceleration ( ).
-
For every action, there is an equal and opposite reaction. (In other words, whenever one body exerts a force onto a second body, (in some cases, which is standing still) the second body exerts the force back onto the first body. and are equal in magnitude and opposite in direction. So, the body that exerts will be pushed backward.)
[Newton's "Axioms or Laws of Motion" can be found in the "Principia" on p. 19 of volume 1 of the 1729 translation .]
Newton's three laws of motion were the first to accurately provide a mathematical model for understanding bodies in outer space. This explanation unified the motion of celestial bodies and the motion of objects on Earth.
Relativistic mechanics
Modern kinematics developed with study of
electromagnetism and refers all velocities
to their ratio to speed of light
. Velocity is then interpreted as
rapidity, the
hyperbolic angle for which the hyperbolic tangent function
.
Acceleration, the change of velocity over time, then changes rapidity according to Lorentz transformations. This part of mechanics is special relativity. Efforts to incorporate
gravity into relativistic mechanics were made by W. K. Clifford and
Albert Einstein. The development used differential geometry to describe a curved universe with gravity; the study is called general relativity.
Quantum mechanics
Quantum mechanics is a set of principles describing
Physical systems at the atomic level of matter (
and
) and the subatomic particles (
,
,
, and even smaller elementary particles such as
). These descriptions include the simultaneous wave-like and particle-like behavior of both
matter and
radiation energy as described in the wave–particle duality.
In classical mechanics, accurate and of the state of objects can be calculated, such as location and velocity. In quantum mechanics, due to the Heisenberg uncertainty principle, the complete state of a subatomic particle, such as its location and velocity, cannot be simultaneously determined.
In addition to describing the motion of atomic level phenomena, quantum mechanics is useful in understanding some large-scale phenomena such as superfluidity, superconductivity, and biological systems, including the function of smell receptors and the structures of protein.
Orders of magnitude
Humans, like all known things in the universe, are in constant motion;
however, aside from obvious movements of the various external
anatomy parts and locomotion, humans are in motion in a variety of ways that are more difficult to perceive. Many of these "imperceptible motions" are only perceivable with the help of special tools and careful observation. The larger scales of imperceptible motions are difficult for humans to perceive for two reasons: Newton's laws of motion (particularly the third), which prevents the feeling of motion on a mass to which the observer is connected, and the lack of an obvious frame of reference that would allow individuals to easily see that they are moving.
The smaller scales of these motions are too small to be detected conventionally with human
.
Universe
Spacetime (the fabric of the universe) is expanding, meaning everything in the
universe is stretching, like a
rubber band. This motion is the most obscure, not involving physical movement but a fundamental change in the universe's nature. The primary source of verification of this expansion was provided by
Edwin Hubble who demonstrated that all galaxies and distant astronomical objects were moving away from Earth, known as Hubble's law, predicted by a universal expansion.
Galaxy
The Milky Way Galaxy is moving through
space and many astronomers believe the velocity of this motion to be approximately relative to the observed locations of other nearby galaxies. Another reference frame is provided by the Cosmic microwave background. This frame of reference indicates that the Milky Way is moving at around .
[
]
Sun and Solar System
The Milky Way is
rotation around its
density Galactic Center, thus the
Sun is moving in a circle within the
galaxy's
gravity. Away from the central bulge, or outer rim, the typical stellar
velocity is between .
[
] All planets and their moons move with the Sun. Thus, the
Solar System is in motion.
Earth
The Earth is
rotation or spinning around its axis. This is evidenced by
day and
night, at the equator the earth has an eastward velocity of .
[ Ask an Astrophysicist . NASA Goodard Space Flight Center.] The Earth is also
around the
Sun in an orbital revolution. A complete orbit around the Sun takes one
year, or about 365 days; it averages a speed of about .
Continents
The Theory of
tells us that the
are drifting on convection currents within the mantle, causing them to move across the surface of the
planet at the slow speed of approximately per year.
[
][
] However, the velocities of plates range widely. The fastest-moving plates are the oceanic plates, with the
Cocos Plate advancing at a rate of per year
[
] and the
Pacific Plate moving per year. At the other extreme, the slowest-moving plate is the
Eurasian Plate, progressing at a typical rate of about per year.
Internal body
The human
heart is regularly contracting to move
blood throughout the body. Through larger veins and arteries in the body, blood has been found to travel at approximately 0.33 m/s. Though considerable variation exists, and peak flows in the
venae cavae have been found between .
additionally, the
of hollow internal
viscera are moving. The most familiar would be the occurrence of
peristalsis, which is where digested
food is forced throughout the
digestive tract. Though different foods travel through the body at different rates, an average speed through the human
small intestine is .
The human
lymphatic system is also constantly causing movements of excess
fluids,
lipids, and immune system related products around the body. The lymph fluid has been found to move through a lymph capillary of the
human skin at approximately 0.0000097 m/s.
[
]
Cells
The cells of the
human body have many structures and organelles that move throughout them. Cytoplasmic streaming is a way in which cells move molecular substances throughout the
cytoplasm,
various
motor proteins work as
molecular motors within a cell and move along the surface of various cellular substrates such as
microtubules, and motor proteins are typically powered by the
hydrolysis of adenosine triphosphate (ATP), and convert chemical energy into mechanical work.
Vesicles propelled by motor proteins have been found to have a velocity of approximately 0.00000152 m/s.
Particles
According to the laws of thermodynamics, all particles of
matter are in constant random motion as long as the
temperature is above
absolute zero. Thus the
and
that make up the human body are vibrating, colliding, and moving. This motion can be detected as temperature; higher temperatures, which represent greater
kinetic energy in the particles, feel warm to humans who sense the thermal energy transferring from the object being touched to their nerves. Similarly, when lower temperature objects are touched, the senses perceive the transfer of heat away from the body as a feeling of cold.
[ Temperature and BEC. Physics 2000: Colorado State University Physics Department]
Subatomic particles
Within the standard atomic orbital model,
exist in a region around the nucleus of each atom. This region is called the
electron cloud. According to
Bohr model of the atom, electrons have a high velocity, and the larger the nucleus they are orbiting the faster they would need to move. If electrons were to move about the electron cloud in strict paths the same way planets orbit the Sun, then electrons would be required to do so at speeds that would far exceed the speed of light. However, there is no reason that one must confine oneself to this strict conceptualization (that electrons move in paths the same way macroscopic objects do), rather one can conceptualize electrons to be 'particles' that capriciously exist within the bounds of the electron cloud.
Inside the
atomic nucleus, the
and
are also probably moving around due to the electrical repulsion of the protons and the presence of
angular momentum of both particles.
Light
Light moves at a speed of 299,792,458 m/s, or , in a vacuum. The speed of light in vacuum (or
) is also the speed of all massless particles and associated fields in a vacuum, and it is the upper limit on the speed at which energy, matter, information or causation can travel. The speed of light in vacuum is thus the upper limit for speed for all physical systems.
In addition, the speed of light is an invariant quantity: it has the same value, irrespective of the position or speed of the observer. This property makes the speed of light c a natural measurement unit for speed and a fundamental constant of nature.
In 2019, the speed of light was redefined alongside all seven SI base units using what it calls "the explicit-constant formulation", where each "unit is defined indirectly by specifying explicitly an exact value for a well-recognized fundamental constant", as was done for the speed of light. A new, but completely equivalent, wording of the metre's definition was proposed: "The metre, symbol m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly when it is expressed in the SI unit ."[See, for example:
]
Superluminal motion
Some motion appears to an observer to exceed the speed of light. Bursts of energy moving out along the
emitted from these objects can have a
proper motion that appears greater than the speed of light. All of these sources are thought to contain a
black hole, responsible for the ejection of mass at high velocities.
can also produce apparent superluminal motion.
This occurs owing to how motion is often calculated at long distances; oftentimes calculations fail to account for the fact that the speed of light is finite. When measuring the movement of distant objects across the sky, there is a large time delay between what has been observed and what has occurred, due to the large distance the light from the distant object has to travel to reach us. The error in the above naive calculation comes from the fact that when an object has a component of velocity directed towards the Earth, as the object moves closer to the Earth that time delay becomes smaller. This means that the apparent speed as calculated above is
greater than the actual speed. Correspondingly, if the object is moving away from the Earth, the above calculation underestimates the actual speed.
Types of motion
-
Simple harmonic motion – motion in which the body oscillates in such a way that the restoring force acting on it is directly proportional to the body's displacement. Mathematically Force is directly proportional to the negative of displacement. Negative sign signifies the restoring nature of the force. (e.g., that of a pendulum).
-
Linear motion – motion that follows a straight linear path, and whose displacement is exactly the same as its trajectory. Also
-
Reciprocal motion
-
Brownian motion – the random movement of very small particles
-
Circular motion
-
Rotatory motion – a motion about a fixed point. (e.g. Ferris wheel).
-
Curvilinear motion – It is defined as the motion along a curved path that may be planar or in three dimensions.
-
Rolling motion – (as of the wheel of a bicycle)
-
Oscillatory – (swinging from side to side)
-
Vibration
-
Combination (or simultaneous) motions – Combination of two or more above listed motions
-
Projectile motion – uniform horizontal motion + vertical accelerated motion
Fundamental motions
See also
External links
