Most students at first predicted correctly that the reading on the scale would be the same for cases 1-4 (because all objects on the scale belong to the same system) and case 5 has the highest reading because of an external force acting on the scale. On the other hand. some students thought that cases 1-4 would give different readings (1 > 4 >3 > 2) because the tube is buoyant up by the water. However,when the students who had the correct answer were asked to consider the fact that water buoys up the tube, quite a few began to hesitate because they thought the weight of the less-dense tube(mg) is annihilated by the buoyancy force.