You're floating in a small boat on a perfectly still lake. In the boat is a heavy iron anchor. You pick up the anchor and throw it overboard — it sinks straight to the bottom of the lake.
What happens to the water level of the lake?
The water level drops! Here's the key: when the anchor is in the boat, the boat sinks deeper to support the anchor's weight — displacing a lot of water. When the anchor is on the lake bottom, it only displaces its own small volume of water. Since iron is much denser than water, it displaces MORE water as cargo than as a sunken object. Net result: the lake level goes down.
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Drop different objects into the water. Watch the buoyant force arrow (blue, pushing up) fight against gravity (red, pulling down). When buoyancy wins, objects float. Watch the waterline rise as objects displace water!
F_b = ρ_fluid · V_displaced · g
Float or sink? If your average density is less than the fluid's, you float. If more, you sink. Simple as that.
A ship's hull is hollow — its average density (steel + air) is less than water. Same steel as a solid ball that sinks!
The Dead Sea is so salty (1.24 g/cm³) that humans can't sink — you float effortlessly like a cork.
Helium is less dense than air, so the balloon experiences a buoyant force that lifts it up — it 'floats' in the atmosphere.
Fish adjust a gas-filled bladder to match their density to the surrounding water — achieving neutral buoyancy at any depth.
“Archimedes figured this out in a bathtub 2,200 years ago. Eureka!”
Submarine Commander
Take command of a submarine! Control your ballast tanks to dive and surface. Fill them with water to increase density and sink. Blow them with compressed air to decrease density and rise. Find neutral buoyancy to hover at any depth — just like a real submarine captain.
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Start at the surface and slowly fill the ballast tanks. Watch your submarine sink as its average density exceeds water. Now try to find the perfect ballast level for neutral buoyancy — hovering motionless at mid-depth. Then blow the tanks to surface!
A submarine is a density-changing machine. By adjusting how much water is inside its ballast tanks, it controls whether its average density is greater than, less than, or equal to the surrounding water. This is Archimedes' principle in action — the same physics that explains why you float.