Simulation of upthrust
What this simulation shows
A 3-D block is dropped into a tank of fluid. Students watch it either float at equilibrium or sink to the bottom, driven by real physics running every frame.
Controls (right panel)
- Object — six materials: Cork (200), Wood (600), Ice (917), Wax (960), Aluminium (2700), Steel (7800) kg/m³
- Fluid — four options: Oil (800), Water (1000), Seawater (1025), Mercury (13600) kg/m³
- Object size — Small / Medium / Large (side length 7, 13, 19 cm)
- Drag the block up and down with mouse or touch to explore partial submersion live
What’s animated
- The block drops and settles with damped spring physics (frame-rate independent)
- Animated horizontal sine waves scroll through the fluid to show it’s a liquid, not a solid
- Rising bubbles appear when dense objects sink
- A dashed waterline marks the surface on a floating block
- The submerged portion of the block is shaded darker
Force arrows (live)
- Amber arrow ↓ — Weight (W = ρ_obj × g × V), always points down
- Cyan arrow ↑ — Upthrust (U = ρ_fluid × g × V_sub), grows as more of the block enters the fluid
- Arrow lengths are proportional — when U = W the arrows match and the block floats
Data panel
- Live Weight (N), Upthrust (N), Net force with direction, and Float/Sink state
- Formula U = ρ_f g V with live value shown on canvas
Accessibility — High contrast, large text, dyslexia spacing, reduce motion (♿ button)
Suggested class activity — 25 minutes
“Can you predict before you press?”
The goal is to build the habit of using the formula first, then verifying — not clicking and guessing.
Starter (5 min)
Write on the board: Upthrust = ρ_fluid × g × V. Ask: “What two things would make upthrust bigger?” Take hands — establish density of fluid and volume submerged before touching the simulation.
Part 1 — Float or sink? (8 min)
Hand out or project this table. Students complete the Predict column using the density rule (ρ_obj vs ρ_fluid) before anyone touches the screen.
| Object | Fluid | Predict | Simulation | Correct? |
|---|---|---|---|---|
| Cork | Water | |||
| Wood | Oil | |||
| Ice | Water | |||
| Wax | Seawater | |||
| Aluminium | Mercury | |||
| Steel | Mercury |
Ice in water is the deliberate trap — 917 < 1000 so it floats, which surprises many students. Wood in oil (600 vs 800) also floats, challenging the misconception that “wood always floats because it’s light.”
Part 2 — Arrow comparison (7 min)
Students use the simulation to answer these, looking only at the arrows:
- Set Cork in Water. Which arrow is longer — Weight or Upthrust? What does that tell you?
- Set Steel in Water. Now which is longer? What happens to the net force?
- Set Wood in Water. Drag the block halfway into the fluid. What do you notice about the cyan arrow as you push it deeper?
- Keep Wood in Water but switch to Oil. Does it still float? Why has the cyan arrow changed even though the block is the same?
Part 3 — Calculation check (5 min)
Students calculate U for one specific case, then verify against the simulation readout.
Example: Ice, Medium size, in Seawater.
- V = 0.13³ = 0.002197 m³
- At equilibrium, fraction submerged = 917 ÷ 1025 = 0.895
- V_sub = 0.002197 × 0.895 = 0.001966 m³
- U = 1025 × 9.8 × 0.001966 = 19.7 N
Students check this matches the Upthrust card in the panel. If it does, the formula is working and they’ve verified it themselves.
Plenary question (5 min)
“A ship is made of steel — density 7800 kg/m³. Steel sinks. So why does a ship float?”
This leads naturally to the key insight: it’s not the density of the material that matters — it’s the average density of the whole object including the air inside. A hollow steel hull displaces far more water than a solid lump, so U can exceed W.
SEND adaptations: Use the ♿ menu for high contrast and larger text. The drag interaction suits kinaesthetic learners. The “Predict before you press” structure supports students who need processing time before responding to visual stimuli.