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Physics · 2. Forces and fluids

Buoyancy and density

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

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

Driving question

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

Curriculum coverage

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

Model scope and limitations

The model assumes a stationary, uniform-density fluid and constant g within a run.

Learning objectives

  • Identify the variables that affect upthrust.
  • Use Fᵦ = ρgV to predict and test results.
  • Distinguish mass, weight and apparent weight.

Prerequisite knowledge

  • Density ρ = m/V
  • Weight W = mg

Core knowledge

Magnitude of upthrust

Upthrust equals the weight of displaced fluid: Fᵦ = ρfluid·g·Vsubmerged.

Apparent weight

For an object at rest in a fluid: apparent weight = mg − Fᵦ.

Float or sink

Compare the object’s average density with the fluid; shape can change the displaced volume.

Worked example

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

Misconceptions to avoid

  • In a uniform fluid, upthrust on a fully submerged object does not increase merely because it is deeper.
  • A floating object settles when upthrust balances weight; upthrust does not remain greater than weight at equilibrium.

Virtual experiment procedure: Archimedes force

  1. Keep the fluid and g fixed; vary submerged volume in equal steps.
  2. Record upthrust and submerged volume for at least five trials.
  3. Change the fluid and repeat to compare the slope of the Fᵦ–V graph.

Safety and cautions

  • The simulation omits waves, surface tension and viscosity.
  • In a real lab, wipe spills and do not overload the force meter.

Evidence to collect

  • A unit-labelled Vsubmerged–Fᵦ table with at least five readings.
  • A graph and a statement relating its gradient to fluid density.
Open the virtual experiment

Diagnostic check

1. Upthrust on a submerged object directly depends on which quantity?

  1. Fluid density and submerged volume
  2. Only object mass
  3. Only depth
Show answer and explanation

Correct answer: Fluid density and submerged volume

In a uniform fluid, Fᵦ = ρgV and does not directly depend on depth.

2. What is the SI unit of density?

  1. kg/m³
  2. N/m³
  3. kg·m
Show answer and explanation

Correct answer: kg/m³

Density is mass per unit volume: kg/m³.

3. To test the effect of submerged volume, what should be held constant?

  1. Fluid type and g
  2. Submerged volume
  3. Upthrust
Show answer and explanation

Correct answer: Fluid type and g

A fair test changes only the independent variable, submerged volume.

Application practice

1. Water has ρ = 1000 kg/m³ and g = 9.8 m/s². An object displaces 0.002 m³. What is the upthrust?

  1. 19.6 N
  2. 4.9 N
  3. 1960 N
Show answer and explanation

Correct answer: 19.6 N

Fᵦ = 1000 × 9.8 × 0.002 = 19.6 N.

2. If submerged volume doubles in the same fluid, how does ideal upthrust change?

  1. It doubles
  2. It is unchanged
  3. It halves
Show answer and explanation

Correct answer: It doubles

Fᵦ is directly proportional to V when ρ and g are fixed.

3. An object weighs 30 N and experiences 12 N upthrust. What does the supporting force meter read?

  1. 18 N
  2. 42 N
  3. 2.5 N
Show answer and explanation

Correct answer: 18 N

Apparent weight = 30 − 12 = 18 N.

Academic references

  1. OpenStax Physics — peer-reviewed high-school physics
  2. 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