Motion (physics)

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

In physics, motion means a constant 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 velocity cannot change unless it is acted upon by a force, as described by Newton's first law. An object's momentum is directly related to the object's mass and velocity, and the total momentum of all objects in a closed system (one not affected by external forces) does not change with time, as described by the law of conservation of momentum.

A body which does not move is said to be at rest, motionless, immobile, stationary, or to have constant (time-invariant) position.

Motion is always observed and measured relative to a frame of reference. As there is no absolute reference frame, absolute motion cannot be determined; this is emphasised by the term relative motion. A body which is motionless relative to a given reference frame, moves relative to infinitely many other frames. Thus, everything in the universe is moving.

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Theories

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 a function of displacement length, direction, and duration, 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.

Universe

Galaxy

Solar System

Earth

Continents

Internal body

Cells

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

Particles

Subatomic particles

Light

Main article: Light
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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, besides the tachyon, and, according to Einstein, a limit which nothing can travel faster than. Lorentz's Equations predicted 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).

Types