Sink or Float

Density and Buoyancy

Whether an object sinks or floats in a liquid depends mainly on density, which is the amount of mass packed into a given volume. If an object's average density is less than the liquid's density, it floats. If its average density is greater, it sinks. This idea helps explain everything from ice cubes in water to ships floating in the ocean.

Floating and sinking are also connected to buoyant force, which is the upward force a fluid exerts on an object. An object floats when the buoyant force balances its weight, and it sinks when its weight is greater than the buoyant force. Some objects can stay suspended in the middle of a liquid if their density is nearly equal to the liquid's density. Shape matters too, because it changes how much fluid is displaced and can lower an object's average density.

Key Facts

Density = mass/volume
If object density < liquid density, the object floats.
If object density > liquid density, the object sinks.
If object density = liquid density, the object can remain suspended.
Buoyant force equals the weight of displaced fluid.
Weight = mg

Vocabulary

Density: Density is the mass of a substance divided by its volume.
Buoyant force: Buoyant force is the upward push a fluid exerts on an object placed in it.
Displacement: Displacement is the amount of fluid pushed aside by an object.
Mass: Mass is the amount of matter in an object.
Volume: Volume is the amount of space an object occupies.

Common Mistakes to Avoid

Thinking heavier objects always sink, which is wrong because floating depends on density compared with the liquid, not just total mass. A heavy ship can float if its average density is low enough.
Comparing mass without comparing volume, which is wrong because density requires both quantities. A small metal cube and a large wood block cannot be judged by mass alone.
Assuming floating means no gravity acts, which is wrong because gravity still pulls downward. The object floats only when buoyant force balances its weight.
Forgetting that the liquid matters, which is wrong because the same object may float in one liquid and sink in another. Salt water, for example, is denser than fresh water.

Practice Questions

1 A block has a mass of 200 g and a volume of 250 cm^3. What is its density, and will it sink or float in water with density 1.0 g/cm^3?
2 A metal piece has density 7.8 g/cm^3 and volume 10 cm^3. What mass of water does it displace when fully submerged, and does it sink or float in water?
3 A sealed hollow plastic ball floats, but the same plastic shaped into a solid chunk sinks. Explain how shape and average density account for this difference.