Buoyancy
Drop a wooden block in water and it floats. Drop a steel block in water and it sinks. Drop a steel block in mercury and it floats again. The deciding rule is not weight, it is density. The fluid pushes up on whatever sits in it with a force $F_b = \rho_\text{fluid} V_\text{disp} g$, and Newton's second law settles the rest. The trap: that formula uses the fluid's density, not the object's.
Three places students lose this. Heavier object, bigger buoyant force: the natural feeling that a heavier block must get more lift from the fluid. It does not. $F_b$ depends on the fluid's density and the displaced volume, not on the object's properties. Heavy things sink, light things float: a five-thousand-tonne ship floats and a one-gram pebble sinks. Density decides, not weight. The buoyant force always equals the weight: that equality is what floating equilibrium looks like, not a universal rule. A sinking block has $F_b < W$, and a buoy held under by a tether has $F_b > W$.
The work
3 ways in · any order
Lesson
Buoyancy
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A ten-scenario applet that walks you through the buoyant force, the density rule for float vs sink, and Newton's second law in a fluid. Each scenario rotates between focus-and-choice, symbolic, and numeric prompts; first-try accuracy and progress save to your browser.
Diagnostic
10-item topic check
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Ten multiple-choice items, each tied to a specific misconception. About fifteen minutes. Your score and the misconception traps that caught you both save as you go and feed the targeted practice card below.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you've missed and grind it on its own. The round is adaptive: two correct in a row clears the misconception and you move on.