In this chapter and in Chaps. 17 and 18, you will study the kinetics
of rigid bodies, that is, the relations existing between the forces acting
on a rigid body, the shape and mass of the body, and the motion
produced. In Chaps. 12 and 13, you studied similar relations, assuming
then that the body could be considered as a particle, that is, that
its mass could be concentrated in one point and that all forces acted
at that point. The shape of the body, as well as the exact location of
the points of application of the forces, will now be taken into account.
You will also be concerned not only with the motion of the body as a
whole but also with the motion of the body about its mass center.
Our approach will be to consider rigid bodies as made of large
numbers of particles and to use the results obtained in Chap. 14 for
the motion of systems of particles. Specifically, two equations from
Chap. 14 will be used:
(1.0K)
which relates the resultant of the external forces and the acceleration of the mass center
G of the system of particles, and (2.0K)
which relates the moment resultant of the external forces and the angular momentum
of the system of particles about G.
Except for Sec. 16.2, which applies to the most general case of
the motion of a rigid body, the results derived in this chapter will be
limited in two ways: (1) They will be restricted to the plane motion
of rigid bodies, that is, to a motion in which each particle of the body
remains at a constant distance from a fixed reference plane. (2) The
rigid bodies considered will consist only of plane slabs and of bodies
which are symmetrical with respect to the reference plane.† The study
of the plane motion of nonsymmetrical three-dimensional bodies and,
more generally, the motion of rigid bodies in three-dimensional space
will be postponed until Chap. 18.
In Sec. 16.3, we define the angular momentum of a rigid body in
plane motion and show that the rate of change of the angular momentum
(0.0K) about the mass center is equal to the product (0.0K) of the centroidal mass moment of inertia (0.0K) and the angular acceleration α of the body. D'Alembert's principle, introduced in Sec. 16.4, is used to prove that the external forces acting on a rigid body are equivalent to a vector (0.0K) attached at the mass center and a couple of moment (0.0K)
In Sec. 16.5, we derive the principle of transmissibility using only
the parallelogram law and Newton's laws of motion, allowing us to
remove this principle from the list of axioms (Sec. 1.2) required for
the study of the statics and dynamics of rigid bodies.
Free-body-diagram equations are introduced in Sec. 16.6 and will
be used in the solution of all problems involving the plane motion of
rigid bodies.
After considering the plane motion of connected rigid bodies in Sec.
16.7, you will be prepared to solve a variety of problems involving the
translation, centroidal rotation, and unconstrained motion of rigid bodies.
In Sec. 16.8 and in the remaining part of the chapter, the solution
of problems involving noncentroidal rotation, rolling motion, and other
partially constrained plane motions of rigid bodies will be considered.
† Or, more generally, bodies which have a principal centroidal axis of inertia perpendicular to the reference plane. |