What Decides Whether an Object Floats or Sinks in a Fluid?
Archimedes' Principle: Why Objects Float or Sink
Density decides, not weight. Both W = mg and FB = ρfluid·Vsub·g scale with g, so changing gravity (Moon → Jupiter) rescales every force but never flips float ↔ sink. For a floater, the submerged fraction equals the density ratio: Vsub/V = ρobject/ρfluid — independent of mass and of g.
Apparent weight is what a supporting scale reads: W − FB. A floating object reads zero; a sunk object resting on the bottom is held up by the floor with normal force N = W − FB. This is why objects feel lighter underwater — and why iron sinks in water (7870 > 1000 kg/m³) yet floats on mercury (7870 < 13600 kg/m³).
⚓Why Iron Sinks in Water but Floats on Mercury
According to Archimedes' principle, the buoyant force on an object equals the weight of the fluid it displaces: F_B = ρ_fluid · V_sub · g. An object floats when its density is less than the fluid's density and sinks when it is greater. A floating object settles where buoyant force equals weight, with its submerged fraction equal to ρ_object/ρ_fluid — independent of both mass and gravity, which is why the float-or-sink outcome is identical on the Moon, Earth, and Jupiter.
Use the sim to test it: drop wood into water, swap the fluid for mercury or honey, and change the mass to confirm the % submerged never budges. Sink an object and watch the normal force appear — apparent weight = W − F_B is exactly what a scale would read.
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