rigid body
(noun)Definition of rigid body
An idealized solid whose size and shape are fixed and remain unaltered when forces are applied; used in Newtonian mechanics to model real objects.
Source: Wiktionary  CC BYSA 3.0
Examples of rigid body in the following topics:

Motion of the Center of Mass
 We can describe the translational motion of a rigid body as if it is a point particle with the total mass located at the center of mass (COM).
 We can describe the translational motion of a rigid body as if it is a point particle with the total mass located at the COMâ€”center of mass.

The Physical Pendulum
 In contrast, a physical pendulum (sometimes called a compound pendulum) may be suspended by a rod that is not massless or, more generally, may be an arbitrarilyshaped, rigid body swinging by a pivot (see ).
 Gravity acts through the center of mass of the rigid body.
 In case we know the moment of inertia of the rigid body, we can evaluate the above expression of the period for the physical pendulum.
 The important thing to note about this relation is that the period is still independent of the mass of the rigid body.
 However, it is not independent of the mass distribution of the rigid body.

Center of Mass and Translational Motion
 We have referred to particle, object and body in the same way.
 By doing this, we have essentially considered a rigid body as a point particle.
 Center of Mass (COM) An actual body, however, can move differently than this simplified paradigm.
 Different parts of a body have different motions.
 Describing Motion in a Rigid Body We can describe general motion of an object (with mass m) as follows: We describe the translational motion of a rigid body as if it is a point particle with mass m located at COM.

General ProblemSolving Tricks
 A force on a particle is a bound vector. rigid extended.
 A force on an extended rigid body is asliding vector. nonrigid extended.
 A force on a nonrigid body is a bound vector.
 So you will want to include the following things in the diagram: The body: This is usually sketched in a schematic way depending on the body  particle/extended, rigid/nonrigid  and on what questions are to be answered.
 Internal forces acting on various parts of the body by other parts of the body.
 Free body diagrams use geometry and vectors to visually represent the problem.

Locating the Center of Mass
 In the previous atom on "Center of Mass and Translational Motion," we learned why the concept of center of mass (COM) helps solving mechanics problems involving a rigid body.
 Locating the Center of Mass The experimental determination of the center of mass of a body uses gravity forces on the body and relies on the fact that in the parallel gravity field near the surface of Earth the center of mass is the same as the center of gravity.
 The center of mass of a body with an axis of symmetry and constant density must lie on this axis.
 In the same way, the center of mass of a spherically symmetric body of constant density is at the center of the sphere.
 In general, for any symmetry of a body, its center of mass will be a fixed point of that symmetry.

Stability, Balance, and Center of Mass
 To quantify equilibrium for a single object, there are two conditions: The net external force on the object is zero: $\sum_i \mathbf{F}_i = \mathbf{F}_{net} = 0$ The net external torque, regardless of choice of origin, is also zero : $\sum_i \mathbf{r}_i \times \mathbf{F}_i = \sum_i \mathbf{\tau}_i = \mathbf{\tau}_{net} = 0$ Those two conditions hold regardless of whether the object we are talking about is a single point particle, a rigid body, or a collection of discrete particles.