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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.

## Laws of motion

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:

First law: In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force.
Second law: In an inertial reference frame, the 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: F = ma.

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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.

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:

 First law: In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force. Second law: In an inertial reference frame, the 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: 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 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 on the first body.