Rigid body dynamics extends the ideas of particle mechanics to bodies of finite size. In this chapter, momentum and energy are computed by summing the contributions of the particles that comprise the body. We introduce the typical particle construction as the basic tool for obtaining equations of motion. This method of derivation of equations provides a vital connection between rigid body and particle mechanics. This and the next chapter focus on planar motion of rigid bodies because this class of problems affords the consideration of rotation without the machinery required to manage it in three dimensions.