The pencil in these examples represents a rigid body, or link, which for purposes of
kinematic analysis we will assume to be incapable of deformation. This is merely a convenient
fiction to allow us to more easily define the gross motions of the body. We can
later superpose any deformations due to external or inertial loads onto our kinematic
motions to obtain a more complete and accurate picture of the body's behavior. But remember,
we are typically facing a blank sheet of paper at the beginning stage of the design
process. We cannot determine deformations of a body until we define its size, shape,
material properties, and loadings. Thus, at this stage we will assume, for purposes of
initial kinematic synthesis and analysis, that our kinematic bodies are rigid and
massless.