An object's motion changes A. only if the object is at rest when a force acts upon it.
B. when there are no forces acting upon it.
C. only when a nonzero net force is applied to the object.
D. when the forces acting on the object remain balanced.

Consider an insulating crystal, made up of layers of atoms. what form would you expect the temperature dependence of the phonon heat capacity to approach at extremely low temperatures if the interlayer coupling is i)very strong (rigid coupling), and ii) very weak. explain.