Buoyancy & floating, explained.
A fluid exerts an upward buoyant force equal to the weight of displaced fluid. A floating object settles where buoyancy equals its weight. Its submerged volume fraction then equals object density divided by fluid density, provided the object is less dense than the fluid.
The relationship to remember
A worked example
A 2 L object with density 500 kg/m³ weighs 9.81 N. In water of density 1,000 kg/m³, it floats with 1 L submerged: half its volume.
Inside the default experiment
- Volume = 2 L = 0.002 m³. Submerged volume = 0.001 m³.
- Buoyancy = ρfluid g Vsubmerged = 9.81 N. Weight = ρobject g V = 9.81 N.
- At equilibrium, submerged fraction = ρobject/ρfluid = 50%.
When this model applies
Uniform fluid and object. Submerged fraction is set by the control. Only gravity and buoyancy are shown; drag and any external force holding the object are excluded. g = 9.81 m/s².
Make a prediction. Then test it.
Submerge a wooden block completely. Which way does the net force point?
Try the interactive experiment ↗Frequently asked questions
Wondering about the why? Start here.
Why does a dense object sink?
If its density exceeds the fluid density, even full immersion cannot displace enough fluid for buoyancy to balance its weight. The net force is downward.
What happens if object and fluid densities are equal?
When fully submerged, weight and buoyancy balance. This is neutral buoyancy in the ideal uniform-fluid model.