Competency #4
Answer and Explanation
Given: Wood and Aluminum have same volume. Aluminum and Lead have same mass. Objects are in a beaker of water.
Which object experiences the greatest buoyant force?
The aluminum block experiences the greatest buoyant force.
Buoyant force is an upward force that a fluid exerts on an immersed object. The magnitude of the buoyant force depends on the volume of the object that is submerged.
Looking at the given information, wood and aluminum have the same volume; you might be tempted to say they have equal buoyant forces. But if you look at the picture, only a small fraction of the wood is under the surface of the water, while all of the aluminum is submerged. This means that the aluminum has a greater submerged volume, so the buoyant force must be greater for the aluminum than the wood.
The second piece of information tells you that aluminum and lead have the same mass. This does not tell you much, as the buoyant force is independent of mass. However, if you look at the picture, you see that the lead block is smaller, which means it has a smaller volume. Since it has a smaller volume that is submerged, the buoyant force must be smaller for the lead versus the aluminum.
Therefore, the aluminum block experiences the greatest buoyant force.