Physical Body - in Classical Physics, Mechanics, Quantum Physics, and Cosmology

In Classical Physics, Mechanics, Quantum Physics, and Cosmology

Further information: Physics, Classical mechanics, and String theory

A physical body is an object which can be described by the theories of classical mechanics, or quantum mechanics, and experimented upon with physical instruments. This includes the determination of trajectory of position through space, and in some cases of orientation in space, over a duration of time, as well as means to change these, by exerting forces.

In classical physics, a physical body is a body with mass, not only energy, is three dimensional (extended in 3-dimensions of space), has a trajectory of position and orientation in space, and is lasting for some duration of time. It is the subject of study in an experiment and is the object referred to in a law of physics, or physical theory. It can be considered as a whole, but may be composed of a collection of smaller physical bodies, e.g. a weight, ball, proton, or planet.

For instance, the force of gravity will accelerate a body if it is not supported, thus causing a change of its position (that is, it falls freely). However, it is not necessary for there to be forces present for an object position to change - only the rate of change of the object's position, that is, its velocity, will change under the influence of forces.

But in Quantum physics and Cosmology, there is a debate as to whether some elementary particles are not bodies, but are mere points without extension in physical space within space-time, or are always extended in at least one dimension of space as in String theory or M theory.