Drum $A$ of mass $M$ and radius $R$ is suspended from a drum $B$ also of

mass $M$ and radius $R,$ which is free to rotate about its axis. The suspension

is in the form of a massless metal tape wound around the outside of each

drum, and free to unwind, as shown in figure. Gravity is directed

downward. Both drums are initially at rest. Find the initial acceleration of

drum $A,$ assuming that it moves straight down. $[5$M$]$