Sinking Objects: Weight vs. Maximum Buoyant Force
1 · Predict
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?
- 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.
2 · Set Up
- 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.
3 · Collect Data
| Sinker mass m (kg) | Weight mg (N) | Submerged fraction |
|---|---|---|
| 3.00 | ||
| 4.00 | ||
| 5.00 |
Plot submerged fraction (y-axis) against weight (x-axis) for your three trials.
4 · Analyze
- This simulation models the sinker as a disk of radius 0.4 m extruded through the tank's fixed slice depth of 0.005 m — compute its full volume V_full = π·r²·(slice depth) 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.
5 · Extend
- 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.
The Physics Behind This Experiment
Archimedes' Principle (at the maximum)
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.