Rigid body

In physics, a rigid body (also known as a rigid object[2]) is a solid body in which deformation is zero or so small it can be neglected. The distance between any two given points on a rigid body remains constant in time regardless of external forces or moments exerted on it. A rigid body is usually considered as a continuous distribution of mass.

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Classical mechanics
${\displaystyle {\textbf {F}}={\frac {\mathrm {d} }{\mathrm {d} t}}(m{\textbf {v}})}$ Second law of motion
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In the study of special relativity, a perfectly rigid body does not exist; and objects can only be assumed to be rigid if they are not moving near the speed of light. In quantum mechanics, a rigid body is usually thought of as a collection of point masses. For instance, molecules (consisting of the point masses: electrons and nuclei) are often seen as rigid bodies (see classification of molecules as rigid rotors).