Physics · Grades 8–11
Displacement, density and buoyancy
Use displaced fluid weight and avoid shape-only explanations of floating.
Learn the idea
Buoyant force
The upward buoyant force equals the weight of displaced fluid in the static model.
Displaced volume
A fully submerged object's displaced volume equals its external submerged volume, not an arbitrary container reading.
Floating balance
An object floating at rest has buoyancy balancing its weight.
Density matters
Average density includes enclosed spaces. A hollow vessel can float even when its solid material is denser than water.
Worked examples
An object displaces 0.003 m³ of liquid of density 1000 kg/m³ with g = 10 N/kg. Find buoyant force.
1000 × 10 × 0.003 = 30 N.
30 N
An object floats motionless with weight 12 N. Its buoyant force is what?
Upward buoyancy balances downward weight in this model.
12 N
Check your understanding
An object has mass 0.6 kg and external volume 0.001 m³. Find average density.
Why can a hollow metal boat float when a compact piece of the same metal sinks?
Which density goes in ρgV for buoyant force?
- Only the object's solid-material density
- Fluid density
- Air temperature expressed as density
Answers and marking notes
Answers and marking notes
- 600 kg/m³
Density is 0.6/0.001 = 600 kg/m³.
- The boat can displace enough water before submerging; its overall average density includes enclosed space.
Credit displaced volume and total mass, not a claim that metal loses density.
- Fluid density
Buoyancy is the weight of displaced fluid.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.