You cast some metal of density rm in a mold, but you are worried that there
might be cavities within the casting. You measure the weight of the casting
to be w, and the buoyant force when it is completely surrounded by water
to be B. (a) Show that is the total volume
of any enclosed cavities. (b) If your metal is copper, the casting’s weight
is 156 N, and the buoyant force is 20 N, what is the total volume of any
enclosed cavities in your casting? What fraction is this of the total volume
of the casting?