We have three cubes of identical volume but different density. We also have a container of fluid. The density of Cube A is less than the density of the fluid; the density of Cube B is exactly equal to the density of the fluid; and the density of Cube C is greater than the density of the fluid. When these objects are all completely submerged in the fluid, as shown, which cube displaces the largest volume of fluid?

1. Cube A
2. Cube B
3. Cube C
4. The cubes all displace equal volumes of fluid

Each cube displaces a volume of fluid equal to its own volume, and the cube volumes are equal so the volumes of fluid displaced are all equal.

Which object has the largest buoyant force acting on it?

1. Cube A
2. Cube B
3. Cube C
4. The cubes have equal buoyant forces

The buoyant force is proportional to the density of the fluid, and the volume of fluid displaced. These are equal for the cubes so they all experience equal buoyant forces.

Lesson 2: The buoyant force is proportional to the volume of fluid displaced.

Now we change the situation so the cubes are no longer at the same level in the fluid. Rank the cubes based on the buoyant forces they experience, from largest to smallest.

1. A = B = C
2. A > B > C
3. C > B > A
4. Some other order

As long as our fluid is incompressible, so the density of the fluid does not change as a function of depth, the buoyant forces do not change. All three cubes still feel equal buoyant forces because they all displace the same volume of fluid.