In physics, **classical mechanics** and quantum mechanics are the two major sub-fields of mechanics. Classical mechanics is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces. The study of the motion of bodies is an ancient one, making classical mechanics one of the oldest and largest subjects in science, engineering and technology.

Classical mechanics describes the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies. Besides this, many specializations within the subject deal with gases, liquids, solids, and other specific sub-topics. Classical mechanics provides extremely accurate results as long as the domain of study is restricted to large objects and the speeds involved do not approach the speed of light. When the objects being dealt with become sufficiently small, it becomes necessary to introduce the other major sub-field of mechanics, quantum mechanics, which reconciles the macroscopic laws of physics with the atomic nature of matter and handles the wave–particle duality of atoms and molecules. However, when both quantum mechanics and classical mechanics cannot apply, such as at the quantum level with many degrees of freedom, quantum field theory (QFT) becomes applicable. QFT deals with small distances and large speeds with many degrees of freedom as well as the possibility of any change in the number of particles throughout the interaction. To deal with large degrees of freedom at the macroscopic level, statistical mechanics becomes valid. Statistical mechanics explores the large number of particles and their interactions as a whole in everyday life. Statistical mechanics is mainly used in thermodynamics. In the case of high velocity objects approaching the speed of light, classical mechanics is enhanced by special relativity. General relativity unifies special relativity with Newton's law of universal gravitation, allowing physicists to handle gravitation at a deeper level.

A **physical quantity** (or "physical magnitude") is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.

The following outline is provided as an overview of and topical guide to physics:

**Physics** – natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.

In physics, **net force** is *the overall force* acting on an object. In order to calculate the net force, the body is isolated and interactions with the environment or constraints are introduced as forces and torques forming a free-body diagram.

The net force does not have the same effect on the movement of the object as the original system forces, unless the point of application of the net force and an associated torque are determined so that they form the resultant force and torque. It is always possible to determine the torque associated with a point of application of a net force so that it maintains the movement of the object under the original system of forces.

A **fictitious force**, also called a **phantom force**, **pseudo force**, **d'Alembert force** or **inertial force**, is an apparent force that acts on all masses whose motion is described using a non-inertial frame of reference, such as a rotating reference frame.

The force **F** does not arise from any physical interaction between two objects, but rather from the acceleration **a** of the non-inertial reference frame itself. As stated by Iro:

- Will an object moving at a constant velocity have no force based on force equals mass x acceleration?
- How will an object move if it is acted on by a nonzero net torque and a net force of zero?
- Is force required to keep an object moving at a constant velocity?
- Does a object moving with a constant velocity have a net force of zero?
- What is the net force on car moving in a straight line with a constant speed?
- If a nonzero net force is acting on an object, then the object is definitely, at rest, moving with a constant velocity, being accelerated or losing mass?
- If a nonzero net force is acting on an object, then the object is definitely at rest or being accelerated or losing mass or moving with a constant velocity?
- If an object is moving at constant velocity, is the net force 0 or not 0?

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