Question 2091 Deck Safety

The correct answer is C) The moment of inertia would be 1/4 its original value. The reason for this is that the moment of inertia of a free surface is proportional to the cube of the dimension perpendicular to the axis of rotation. By placing a bulkhead in the center of the tank, running fore-and-aft along the 36-foot axis, the effective width of the free surface is reduced from 36 feet to 18 feet. Since the moment of inertia is proportional to the cube of the width, reducing the width by a factor of 2 will result in the moment of inertia being reduced by a factor of 2^3 = 1/4. The other options are incorrect because A) the moment of inertia would change, and B) the reduction would be by a factor of 2, not 1/2.