Observations and results
Did more and more of the ball end up below the top of the water as the ball's diameter decreased? Was about half of the ball below the water when the ball had a diameter of about 2.5 cm, and did the entire ball sink when its diameter was about 1.6 cm or smaller?
If an object is floating in water, the amount of water that gets displaced weighs the same as the object. Consequently, while it was floating, the ball should have displaced the same amount of water as it decreased in diameter, and so the buoyant force should have remained the same. However, the density of the ball was changing—it increased as the ball's diameter decreased.
Density is the mass per unit volume—it describes how much "stuff" is packed into a volume of space. When the aluminum ball had a diameter of 6.0 cm, the ball should have floated well because it had a density lower than that of water due to the air inside of the ball, just like steel ships that can float because their density has been lowered by encasing air inside the hull. And as long as the ship displaces enough water to create a strong buoyant force, it can stay afloat—even if it is loaded with cargo. As the diameter decreased and density increased, the ball should have sank more and more. When its diameter was about 1.8 cm or 1.6 cm, you may have seen it become 90 percent (just barely) submerged. This is when the ball had a density approximately equal to that of water. With a diameter of about 1.6 cm or smaller, the ball should have completely sank, indicating that its density was greater than that of water, thereby overcoming the buoyant force.
Pour the water down a drain and recycle the aluminum foil.
More to explore
Archimedes' Principle from Hila Science Videos
How Stuff Works
Archimedes' Principle by ORACLE ThinkQuest: Education Foundation
This activity brought to you in partnership with Science Buddies