Virtual laboratory/Physics/Hydrostatics/Buoyancy — what floats and what sinks?

Buoyancy — what floats and what sinks?

What we'll do: We'll drop a body into a fluid and check whether it floats or sinks — then we'll shape the same steel into a bowl and see what changes.

What you see on screen: A vessel with fluid and a submerged body, with arrows for weight (down) and buoyancy (up), toggles for shape (solid body / bowl) and fluid, plus a density slider. The density bar compares the density of the body with that of the fluid — that is what decides the outcome. Readouts: body and fluid density, state, percentage submerged.

Buoyancy — what floats and what sinks?
Physics · Hydrostatics · Archimedes’ principle (FIZ.MECH.PR.25)
Material density ρ7800 kg/m³≈ steel
We drop a solid body (steel) into the “water” fluid — what happens?
body ρ (avg)
7800
fluid ρ
1000
weight vs buoyancy
weight >
state
sinks
submersion
100%
🎯 Challenge: floats or sinks?
streak: 0

I’ll give you a body (density, shape) and a fluid — guess what happens. The body’s average density versus the fluid’s density decides.

What just happened?

The same piece of steel sank as a solid ball but floated as a bowl. The material did not change — the shape did, and with it the average density. Floating is decided not by weight but by density.

Two forces fight over the outcome:

Weight pulls down, the buoyant force pushes up.

Fweight=mgFbuoyancy=ρfluid g VsubF_{\text{weight}} = m g \qquad F_{\text{buoyancy}} = \rho_{\text{fluid}}\, g\, V_{\text{sub}}

Where buoyancy comes from — Archimedes’ principle:

The body pushes the fluid aside, and the fluid responds with an upward force equal to the weight of the displaced fluid. The more fluid you displace, the harder it pushes you up.

Who wins? Compare the densities:

ρbody<ρfluid  ⇒  floats\rho_{\text{body}} < \rho_{\text{fluid}} \;\Rightarrow\; \text{floats}
ρbody>ρfluid  ⇒  sinks\rho_{\text{body}} > \rho_{\text{fluid}} \;\Rightarrow\; \text{sinks}
ρbody=ρfluid  ⇒  stays suspended\rho_{\text{body}} = \rho_{\text{fluid}} \;\Rightarrow\; \text{stays suspended}

What matters is not how much the body weighs, but how much it weighs compared with the same volume of fluid.

Why doesn’t a steel ship sink?

Solid steel (~7800 kg/m³) sinks. But formed into a hull it traps air inside. What counts is the average density of the whole (steel + air) — and that drops below the density of water. That is why a multi-ton ship floats while a small screw sinks.

How much of the body is submerged?

VsubVtotal=ρbodyρfluid\dfrac{V_{\text{sub}}}{V_{\text{total}}} = \dfrac{\rho_{\text{body}}}{\rho_{\text{fluid}}}

Ice (~917 kg/m³) → 91% of an iceberg is underwater; only the tip is above the surface.

Why you need this for the exam:

The floating condition and Fbuoyancy=ρgVF_{\text{buoyancy}} = \rho g V are all you need for problems about boats, balloons (the same thing in air) and hydrometry. Trick: set weight equal to buoyancy — there is your equation.

Remember: floating is decided by density, not mass. Just trap air inside the shape and even the heaviest material stays on the water.

Slow motion · NexTutor Virtual laboratory

Enjoyed the experiment? Share it.

It's free — send it to friends from school.

Theory & formulas

Buoyant force

A body submerged in a fluid experiences a buoyant force equal to the weight of the displaced fluid: Fw=ρc g VzF_w = \rho_c\,g\,V_z It depends on the density of the fluid and the volume of the submerged part — not on the density of the body itself.

Floating condition

The fate of the body is decided by comparing densities:

  • ρbody<ρc\rho_{body} < \rho_c — the body floats,
  • ρbody>ρc\rho_{body} > \rho_c — the body sinks,
  • ρbody=ρc\rho_{body} = \rho_c — it stays suspended in the fluid.

Why a ship floats

A steel hull traps air inside, so the average density of the ship is lower than that of water. A solid screw has the density of steel — it sinks.

Symbols: FwF_w — buoyant force, ρc\rho_c — fluid density, ρbody\rho_{body} — average body density, g≈9.81 m/s2g \approx 9.81\ \mathrm{m/s^2}, VzV_z — submerged volume.

Frequently asked questions

›

When does a body float, and when does it sink?

It floats when its average density is less than the fluid's density; it sinks when greater; it stays suspended when equal.

›

What is the buoyant force?

F = ρ_fluid · g · V_submerged — it equals the weight of the displaced fluid.

›

Why does a steel ship float but a screw sink?

The hull traps air inside, so the ship's average density is lower than water; a solid screw has the density of steel.

Teaching? Embed this experiment on your site.

Free widget — paste it on your school site, blog or notes. It links to the full version.

Teaching a class or tutoring? Guide students in NexTutor →

Related experiments

Bernoulli's principle — pressure in a flow →Hydrostatic pressure — the formula p = ρgh and the hydrostatic paradox →Communicating vessels — liquid level →Pascal's law — the hydraulic press →