Fluids
Sink or float
Watch the dense ball sink — its weight exceeds the buoyant force.
Sink or float — interactive Fluids simulation. Watch the dense ball sink — its weight exceeds the buoyant force. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Negative buoyancy
A dense object sinks when its weight exceeds the maximum buoyant force the fluid can supply.
If the object's average density is greater than the fluid, the buoyant force (based on full displaced volume at immersion) never reaches the weight. The ball accelerates downward.
- F_b = ρ_fluid · V · g
- Sink: mg > F_b
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
A metal sinker is dropped into water and comes to rest on the bottom, fully submerged. Once it's fully submerged, does making it heavier change how submerged it is?
Predictions to weigh
- A heavier sinker submerges even more (over 100%).
- It's already fully submerged (100%), so more mass can't submerge it further.
- A heavier sinker submerges less because it drops faster.
Variable roles
What you set:
- Sinker mass m (kg)
What you measure:
- Weight mg (N)
- Submerged fraction
How the investigation runs
- Open the fluid-sink preset and press Reset. The sinker drops through the water and comes to rest on the tank floor.
- Enable the submerged-fraction probe on the sinker.
- Set the sinker's mass for each trial, let it settle, and record its submerged fraction.
Governing equation
Archimedes' Principle (at the maximum) — F_b = ρ·V·g
The largest buoyant force an object can ever receive is ρ·g·V_full — the weight of fluid it would displace if entirely submerged. Once an object's weight exceeds this cap, it sinks and stays fully submerged; adding more mass can't submerge it further because it already displaces its whole volume.
What the printable worksheet asks students to work out
- Compute the sinker's full volume from its radius (0.4 m) and the maximum possible buoyant force ρ·g·V_full using water density 1000 kg/m³. Compare it to each trial's weight.
- Explain why the submerged fraction stays capped at 1.0 (100%) once weight exceeds the maximum buoyant force, no matter how much heavier the sinker gets.
Where this shows up beyond the lab
- This sinker's maximum buoyant force is about 24.6 N — any mass under about 2.5 kg would float instead. Compare this to the fluid-float experiment: what determines whether an object floats or sinks?
- Seawater is denser than fresh water. Would a sinker that fully submerges in fresh water necessarily fully submerge in seawater? Explain using the buoyant-force formula.
- AP Physics 1 — Unit 8: Fluids
- General High School Physics — Fluids & pressure
- NGSS High School Physics — Forces and Newton's second law
- Welcome to Fluid Sink
- Select the sinker ball
- Press Play
- Sinking down
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.