WebLab STEM

Physics · 2. Forces and fluids

Buoyancy and density

Investigate upthrust and the conditions for floating, sinking and neutral buoyancy.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

Why can a steel ship float while a solid steel sphere sinks?

Curriculum coverage

Vietnam 2018 and Cambridge: density, fluid pressure and upthrust.

Model scope and limitations

The model assumes static homogeneous fluid with constant g.

Learning objectives

  • Identify factors influencing upthrust.
  • Apply F = ρgV to predict and verify results.
  • Distinguish mass, weight and apparent weight.

Prerequisite knowledge

  • Density ρ = m/V.
  • Weight P = mg.

Core knowledge

Upthrust magnitude

Upthrust equals the weight of displaced fluid: F = ρ_fluid·g·V_submerged.

Apparent weight

When stationary in fluid: P_apparent = mg − F_upthrust.

Floating and sinking

Compare average object density with fluid; shape alters displaced fluid volume.

Worked example

In water, V = 1.50×10⁻³ m³ and g = 9.8 m/s² give F = 1000×9.8×1.50×10⁻³ = 14.7 N.

Misconceptions to avoid

  • In a homogeneous fluid, upthrust on a fully submerged object does not increase with depth.
  • A floating object has upthrust balancing its weight; upthrust is not greater at equilibrium.

Virtual experiment procedure: Archimedes upthrust and density

  1. Keep fluid and g constant; change submerged volume in equal steps.
  2. Record upthrust and submerged volume at least five times.
  3. Repeat with another fluid to compare F–V graph slopes.

Safety and cautions

  • Simulation neglects waves, surface tension and viscosity.

Evidence to collect

  • A table of V–F with units and at least five measurements.
  • A graph and comments on slope variation with fluid density.
Open the virtual experiment
📋 Embed this simulation in online lectures (For Teachers & LMS)

Copy the iframe code below to embed this interactive simulation into Canvas, Moodle, Google Sites or educational blogs:

<iframe src="https://visuallab.win/physics-lab/?experiment=g6-archimedes" width="100%" height="560" frameborder="0" allow="fullscreen" loading="lazy"></iframe>

Diagnostic check

1. What does Archimedes upthrust on a submerged object depend on?

  1. Fluid density and displaced volume
  2. Depth of object in fluid
  3. Density of the object
Show answer and explanation

Correct answer: Fluid density and displaced volume

F = ρ_fluid·g·V_submerged, independent of depth once fully submerged.

2. When an object floats in equilibrium, what does upthrust equal?

  1. Equal to the weight of the object
  2. Greater than the weight
  3. Zero
Show answer and explanation

Correct answer: Equal to the weight of the object

At floating equilibrium, F_upthrust = weight = mg.

Application practice

1. An object of volume 0.002 m³ is submerged in water (1000 kg/m³, g = 9.8 m/s²). Upthrust?

  1. 19.6 N
  2. 2 N
  3. 196 N
Show answer and explanation

Correct answer: 19.6 N

F = 1000 × 9.8 × 0.002 = 19.6 N.

2. A 4 kg wood block floats in equilibrium on water. Upthrust (g = 9.8)?

  1. 39.2 N
  2. 4 N
  3. 400 N
Show answer and explanation

Correct answer: 39.2 N

Floating in equilibrium means F = mg = 4 × 9.8 = 39.2 N.

3. Submerging the same object in oil (800 kg/m³) instead of water changes upthrust how?

  1. Decreases by 20%
  2. Increases by 20%
  3. Remains unchanged
Show answer and explanation

Correct answer: Decreases by 20%

Upthrust scales with fluid density: 800/1000 = 0.8 (20% reduction).

Academic references

  1. Cambridge IGCSE Physics 0625 — official syllabus and resources
  2. Cambridge AS & A Level Physics 9702 — official syllabus
  3. OpenStax Physics — peer-reviewed high-school physics
  4. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer