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Motion (physics)

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

In physics, motion means a continuous change in the location of a body. Change in motion is the result of applied force. Motion is typically described in terms of velocity, acceleration, displacement, and time. An object's momentum is directly related to the object's mass and velocity, and is conserved within a system, as described by the law of conservation of momentum.

Everything in the universe is moving. As there is no absolute reference system, absolute motion cannot be determined and only motion relative to a point of reference can be determined; this is emphasised by the term relative motion.


Until the end of the 19th century, Isaac Newton's laws of motion, which he posited as axioms or postulates in his famous Principia were the basis of what has since become known as classical physics. Calculations of trajectories and forces of bodies in motion based on Newtonian or classical physics were very successful until physicists began to be able to measure and observe very fast physical phenomena.

At very high speeds, the equations of classical physics were not able to calculate accurate values. To address these problems, the ideas of Henri Poincaré and Albert Einstein concerning the fundamental phenomenon of motion were adopted in lieu of Newton's. Whereas Newton's laws of motion assumed absolute values of space and time in the equations of motion, the model of Einstein and Poincaré, now called the special theory of relativity, assumed values for these concepts with arbitrary zero points. Because (for example) the special relativity equations yielded accurate results at high speeds and Newton's did not, the special relativity model is now accepted as explaining bodies in motion (when we ignore gravity). However, as a practical matter, Newton's equations are much easier to work with than those of special relativity and therefore are more often used in applied physics and engineering.

In the Newtonian model, because motion is defined as the proportion of space to time, these concepts are prior to motion, just as the concept of motion itself is prior to force. In other words, the properties of space and time determine the nature of motion and the properties of motion, in turn, determine the nature of force.

In the special relativistic model, motion can be thought of as something like an angle between a space direction and the time direction.

In special relativity and Euclidean space, only relative motion can be measured, and absolute motion is meaningless.

Relative motion

Relative motion is a change in location relative to a reference point, as measured by a particular observer in a particular frame of reference. Essentially, an object is in relative motion when its distance from another object is changing. However, whether the object appears to be moving or not depends on the point of view. For example, a woman riding in a bus is not moving in relation to the seat she is sitting on, but she is moving in relation to the buildings the bus passes.

The place or object used for comparison to determine the change in position of an object is known as the reference point. Thus, if it is assumed that the reference point is stationary, an object can be said to be in motion if it changes position relative to a reference point. A classic misinterpretation of relative motion was the incorrect assumption that the Sun moved around the Earth rather than the other way around.

List of "imperceptible" human motions

Humans, like all things in the universe are in constant motion, 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. The smaller scales of these motions are too small for humans to sense.


  • 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.


  • The Milky Way Galaxy, is hurtling through space at an incredible speed. It is powered by the force left over from the Big Bang. 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.

Solar System

  • The Milky Way is rotating around its dense galactic centre, thus the solar system 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.


  • 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 (or 1040 mi/h).
  • 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 (or 67,000 mi/h).


  • 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. 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 (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 to range between 0.1 m/s and 0.45 m/s.
  • 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 or 0.036 m/s.
  • 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.


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

  • Cytoplasmic streaming is a way 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 cellulars substrate such as microtubules. Motor proteins are typically powered by the hydrolysis of 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.


  • 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 you up are vibrating, colliding, and moving. This motion can be detected as temperature; high temperatures (which represent greater kinetic energy in the particles) feel warmer to humans, whereas lower temperatures feel colder.

Subatomic particles

  • Within each atom the electrons are speeding around the nucleus so fast that they are not actually in one location, but rather smeared across a region of the electron cloud. Electrons have a high velocity, and the larger the nucleus they are orbiting the faster they move. In a hydrogen atom, electrons have been calculated to be orbiting at a speed of approximately 242,000 m/s
  • 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.


Light is both a photon and a wave, and moves at 186,000 miles per second (300,000 km per second). It is the fastest moving thing known to man, and, according to Einstein, a limit which nothing can travel faster than. Einstein also claimed that time would "slow down" for whatever was traveling near light speed; so, if a person was moving near light speed, they would age slower than someone who was not. Since light is what humans depend on to see the universe, there are tiny, imperceptible changes in what one observer is seeing compared to another. This is because, of course, that light still has to travel to get to an observer; so, if Observer #2 was twice as far from an object than Observer #1, Observer #2 would see it two times later than Observer #1. This can especially be seen when you look at stars many light-years away: you are actually seeing the past of that star, not what is happening at moment, since the light from that star must travel years and years to reach earth (depending on exactly how far away it was).


  • Simple harmonic motion - motion that is constantly being accelerated towards a midpoint (e.g. a swinging pendulum).
  • Linear motion - motion which follows a straight linear path, and whose displacement is exactly the same as its trajectory.
  • Reciprocation (i.e. vibration)
  • Brownian Motion (i.e. the random movement of particles)
  • Circular motion (e.g. the orbits of planets)
  • Rotary motion - a motion about a fixed point ex. the wheel of a bicycle
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