# Motion (physics)

In physics, motion is a change in position of an object with respect to time. Motion is typically described in terms of displacement, distance, velocity, acceleration, time and speed.[1] 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.

If the position of a body is not changing with respect to a given frame of reference, the body is said to be at rest, motionless, immobile, stationary, or to have constant (time-invariant) position. An object's motion cannot change unless it is acted upon by a force, as described. Momentum is a quantity which is used for measuring motion of an object. An object's momentum is directly related to the object's mass and velocity, and 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.

As there is no absolute frame of reference, absolute motion cannot be determined.[2] Thus, everything in the universe can be considered to be moving.[3]:20–21

More generally, motion is a concept that applies to objects, bodies, and matter particles, to radiation, radiation fields and radiation particles, and to space, its curvature and space-time. One can also speak of motion of shapes and boundaries. So, the term motion in general signifies a continuous change in the configuration of a physical system. 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.

Motion involves a change in position, such as in this perspective of rapidly leaving Yongsan Station.

## Laws of motion

Main article: Mechanics

In physics, motion is described through two sets of apparently contradictory laws of mechanics. Motions of all large scale and familiar objects in the universe (such as 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.

### Classical mechanics

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 subjects in science, engineering, and technology. If a wheel is connected to a car and the car goes flying, then the wheel is in motion

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. His 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.[4]

Newton's three laws of motion, along with his Newton's law of motion, which 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.

Classical mechanics was later further enhanced by Albert Einstein's special relativity and general relativity. Motion of objects with a high velocity, approaching the speed of light; general relativity is employed to handle gravitational motion at a deeper level.

### Quantum mechanics

Main article: Quantum mechanics

Quantum mechanics is a set of principles describing physical reality at the atomic level of matter (molecules and atoms) and the subatomic particles (electrons, protons, and even smaller particles). These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation energy, this is described in the wave–particle duality.[citation needed]

In classical mechanics, accurate measurements and predictions of the state of objects can be calculated, such as location and velocity. In the 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.[citation needed]

In addition to describing the motion of atomic level phenomena, quantum mechanics is useful in understanding some large scale phenomenon such as superfluidity, superconductivity, and biological systems, including the function of smell receptors and the structures of proteins.[citation needed]

## List of "imperceptible" human motions

Humans, like all known things in the universe, are in constant motion,[3]:8–9 however, aside from obvious movements of the various external body parts and locomotion, humans are in motion in a variety of ways which 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: 1) Newton's laws of motion (particularly Inertia) which prevent humans from feeling motions of a mass to which they are connected, and 2) the lack of an obvious frame of reference which would allow individuals to easily see that they are moving.[5] The smaller scales of these motions are too small for humans to sense.

### Universe

• Spacetime (the fabric of the universe) is actually expanding. Essentially, everything in the universe is stretching like a rubber band. This motion is the most obscure as it is not physical motion as such, but rather a change in the very nature of the universe. 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 us ("Hubble's law") as predicted by a universal expansion.[6]

### Galaxy

• The Milky Way Galaxy, is moving through space. Many astronomers believe the Milky Way is moving at approximately 600 km/s 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 552 km/s.[7]

### Sun and solar system

• The Milky Way is rotating around its dense 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 210 and 240 km/s.[8] All planets and their moons move with the sun. Thus the solar system is moving.

### Earth

• The Earth is rotating or spinning around its axis, this is evidenced by day and night, at the equator the earth has an eastward velocity of 0.4651 km/s (1040 mi/h).[9]
• The Earth is orbiting 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 30 km/s (67,000 mi/h).[10]

### Continents

• The Theory of Plate tectonics tells us that the continents are drifting on convection currents within the mantle causing them to move across the surface of the planet at the slow speed of approximately 1 inch (2.54 cm) per year.[11][12] However, the velocities of plates range widely. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr[13] (3.0 in/yr) and the Pacific Plate moving 52–69 mm/yr (2.1–2.7 in/yr). At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr (0.8 in/yr).

### Internal body

• The human heart is constantly 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 0.1 m/s and 0.45 m/s.[14]
• The smooth muscles of hollow internal organs are moving. The most familiar would be peristalsis which is where digested food is forced throughout the digestive tract. Though different foods travel through the body at rates, an average speed through the human small intestine is 2.16 m/h (0.036 m/s).[15]
• Typically some sound is audible at any given moment, when the vibration of these sound waves reaches the ear drum it moves in response and allows the sense of hearing.
• The human lymphatic system is constantly moving 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 skin at approximately 0.0000097 m/s.[16]

### Cells

The cells of the human body have many structures which move throughout them.

### 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 molecules and atoms which 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 whom 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 feeling cold.[20]

### Subatomic particles

• Within each atom, electrons exist in an area around the nucleus. This area is called the electron cloud. According to Bohr's 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 '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 which far exceed the speed of light. However, there is no reason that one must confine one's self 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.[21]
• Inside the atomic nucleus the protons and neutrons are also probably moving around due the electrical repulsion of the protons and the presence of angular momentum of both particles.[22]

## Light

Main article: Speed of light

Light propagates at 299,792,458 m/s, often approximated as 300,000 kilometres per second or 186,000 miles per second. The speed of light (or c) 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, and information can travel. The speed of light is the limit speed for 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 the natural measurement unit for speed.

## References

1. ^ Nave, R. "Description of motion". Hyperphysics. Georgia State University. Retrieved 25 January 2014.
2. ^ Wahlin, Lars (1997). "9.1 Relative and absolute motion". The Deadbeat Universe (PDF). Boulder, CO: Coultron Research. pp. 121–129. ISBN 0-933407-03-3. Retrieved 25 January 2013.
3. ^ a b Tyson, Neil de Grasse; Charles Tsun-Chu Liu; Robert Irion (2000). The universe : at home in the cosmos. Washington, DC: National Academy Press. ISBN 0-309-06488-0.
4. ^ Newton's "Axioms or Laws of Motion" can be found in the "Principia" on page 19 of volume 1 of the 1729 translation.
5. ^ Safkan, Yasar. "Question: If the term 'absolute motion' has no meaning, then why do we say that the earth moves around the sun and not vice versa?". Ask the Experts. PhysLink.com. Retrieved 25 January 2014.
6. ^ Hubble, Edwin, "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae" (1929) Proceedings of the National Academy of Sciences of the United States of America, Volume 15, Issue 3, pp. 168–173 (Full article, PDF)
7. ^ Kogut, A.; Lineweaver, C.; Smoot, G. F.; Bennett, C. L.; Banday, A.; Boggess, N. W.; Cheng, E. S.; de Amici, G.; Fixsen, D. J.; Hinshaw, G.; Jackson, P. D.; Janssen, M.; Keegstra, P.; Loewenstein, K.; Lubin, P.; Mather, J. C.; Tenorio, L.; Weiss, R.; Wilkinson, D. T.; Wright, E. L. (1993). "Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps". Astrophysical Journal 419: 1. arXiv:astro-ph/9312056. Bibcode:1993ApJ...419....1K. doi:10.1086/173453.
8. ^ Imamura, Jim (August 10, 2006). "Mass of the Milky Way Galaxy". University of Oregon. Archived from the original on 2007-03-01. Retrieved 2007-05-10.
9. ^ Ask an Astrophysicist. NASA Goodard Space Flight Center.
10. ^ Williams, David R. (September 1, 2004). "Earth Fact Sheet". NASA. Retrieved 2007-03-17.
11. ^ Staff. "GPS Time Series". NASA JPL. Retrieved 2007-04-02.
12. ^ Huang, Zhen Shao. "Speed of the Continental Plates". The Physics Factbook. Retrieved 2007-11-09.
13. ^ Meschede, M.; Udo Barckhausen, U. (November 20, 2000). "Plate Tectonic Evolution of the Cocos-Nazca Spreading Center". Proceedings of the Ocean Drilling Program. Texas A&M University. Retrieved 2007-04-02.
14. ^ Wexler, L.; D H Bergel; I T Gabe; G S Makin; C J Mills (1 September 1968). "Velocity of Blood Flow in Normal Human Venae Cavae". Circulation Research 23 (3): 349–359. doi:10.1161/01.RES.23.3.349.
15. ^ Bowen, R (27 May 2006). "Gastrointestinal Transit: How Long Does It Take?". Pathophysiology of the digestive system. Colorado State University. Retrieved 25 January 2014.
16. ^ M. Fischer, U. K. Franzeck, I. Herrig, U. Costanzo, S. Wen, M. Schiesser, U. Hoffmann and A. Bollinger (1 January 1996). "Flow velocity of single lymphatic capillaries in human skin". Am J Physiol Heart Circ Physiol 270 (1): H358–H363. PMID 8769772. Retrieved 2007-11-14.
17. ^ "cytoplasmic streaming - biology". Encyclopedia Britannica.
18. ^ "Microtubule Motors". rpi.edu.
19. ^ Hill, David; Holzwarth, George; Bonin, Keith (2002). "Velocity and Drag Forces on motor-protein-driven Vesicles in Cells". American Physical Society, the 69th Annual Meeting of the Southeastern. abstract. #EA.002. Bibcode:2002APS..SES.EA002H.
20. ^ Temperature and BEC. Physics 2000: Colorado State University Physics Department
21. ^
22. ^ Chapter 2, Nuclear Science- A guide to the nuclear science wall chart. Berkley National Laboratory.