A solid cylindrical rod of mass 0.85kg and uniform cross-sectional area 4.0×10−4m2 is hung vertically from a light inextensible cord so that it is completely submerged and motionless in fresh water of density 1.0×103kgm−3. The rod does not touch the container walls or bottom.
Answer the following questions in sequence. Assume the density of air is negligible compared with that of water.
(a) Draw and label a free-body diagram for the rod while it is at rest in the water.
(b) Explain, without performing numerical calculations, whether the tension in the cord is greater than, less than, or equal to the weight of the rod.
(c) The cord is now slowly shortened until the top end of the rod just emerges into the air, leaving the remainder submerged and still at rest. Qualitatively describe how the tension in the cord changes during this process and justify your answer.