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In the discussion of many-body systems in the previous chapter no constraint was placed on the motion of the particles involved. A rigid body, on the other hand, is an idealization of a solid body of finite size in which the distance between any two given points of a rigid body remains constant, regardless of external forces exerted on it.

Such an object cannot exist physically for two reasons.

1. External forces will normally produce some deformation due to elasticity of the material from which the body is made. This effect, which will be discussed in Section 10.2, is usually small in the case of most solids.
2. As we will see in Chapter 9, objects cannot be assumed to be perfectly rigid if they are moving at speeds near the speed of light. Again this effect can be safely ignored at normal speeds.

Care needs to be taken not to confuse the notation for moment of a force (M) with the symbol for mass (M) used in this chapter. Such confusion should not arise, however, as the former is always a vector quantity and mass is a scalar.