Buoyancy is the net upward force a fluid exerts on an object submerged in it, equal to the weight of the fluid the object displaces (F_b = ρ_fluid · V_displaced · g). It comes from the pressure difference between the bottom and top of the object, and it depends on the fluid's density, not the object's.
Buoyancy is the upward force a fluid (liquid or gas) exerts on any object placed in it. Physically, it exists because pressure in a fluid increases with depth, so the fluid pushes up on the bottom of an object harder than it pushes down on the top. Add up all those pressure forces and you get one net upward force, the buoyant force.
Archimedes' principle gives you the number: the buoyant force equals the weight of the fluid the object displaces, F_b = ρ_fluid · V_displaced · g. Read that formula carefully. The density in it is the fluid's density, and the volume is only the submerged volume. The object's own mass and density never appear. Whether something floats or sinks comes down to a density comparison. If the object is less dense than the fluid, buoyancy can match its weight and it floats partially submerged. If it's denser than the fluid, buoyancy can't fully cancel gravity and it sinks (though it still feels lighter underwater).
Buoyancy is the focus of Topic 1.5 in Unit 1 (Fluids) of AP Physics 2. It's where the fluids unit hands you a force you have to treat with Newton's second law, so it ties Unit 1 directly back to the mechanics skills from AP Physics 1. The exam loves this combo. You'll draw free-body diagrams with a buoyant force arrow, set up ΣF = ma for floating (a = 0) or accelerating submerged objects, and reason about what happens when the fluid's density or the submerged volume changes. Conceptually, buoyancy is also your first big payoff from understanding pressure-versus-depth, since the whole force is just pressure differences doing their thing.
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Archimedes' Principle (Unit 1)
This is the rule that quantifies buoyancy. Buoyant force equals the weight of displaced fluid. Buoyancy is the force; Archimedes' principle is how you calculate it.
Density (Unit 1)
Float-or-sink questions are really density-comparison questions. An object floats when its average density is less than the fluid's, and the fraction submerged equals ρ_object / ρ_fluid for a floating object.
Fluids and Pressure (Unit 1)
Buoyancy isn't a brand-new force of nature. It's the net result of fluid pressure increasing with depth, so the upward push on an object's bottom beats the downward push on its top.
Drag Force (Unit 1)
Both are forces a fluid exerts on an object, and AP problems often combine them. A ball released underwater accelerates upward from buoyancy, then drag grows with speed until the ball hits terminal velocity.
Expect buoyancy in multiple-choice stems that ask you to compare buoyant forces on objects of different sizes, densities, or depths, and in free-response parts that ask you to draw a free-body diagram and apply Newton's second law to a floating or submerged object. Classic moves you should be ready for: setting F_b = mg for a floating object to find the submerged fraction, computing apparent weight (true weight minus buoyant force) for a sunken object, and explaining in words why the buoyant force doesn't change as a fully submerged object goes deeper. Justify-your-answer prompts reward the pressure-difference explanation, not just the formula.
Both are fluid forces, but they have different causes and behaviors. Buoyancy comes from the static pressure difference across the object and acts upward whether the object moves or not. Drag exists only when the object moves relative to the fluid, opposes that motion, and grows with speed. On a sinking-object problem, buoyancy is constant while drag increases until the object reaches terminal velocity.
The buoyant force equals the weight of the displaced fluid: F_b = ρ_fluid · V_displaced · g, using the fluid's density and only the submerged volume.
Buoyancy exists because fluid pressure increases with depth, so the upward push on the bottom of an object is larger than the downward push on its top.
An object floats if its average density is less than the fluid's density, and for a floating object the submerged fraction equals ρ_object / ρ_fluid.
For a fully submerged object, going deeper does not increase the buoyant force, because the displaced volume stays the same.
On FRQs, treat the buoyant force like any other force: put it on a free-body diagram and apply Newton's second law (ΣF = 0 for floating, ΣF = ma for accelerating objects).
Apparent weight underwater equals true weight minus the buoyant force, which is why objects feel lighter in water.
Buoyancy is the upward force a fluid exerts on an object in it, equal to the weight of the fluid displaced (F_b = ρ_fluid · V_displaced · g). It's covered in Topic 1.5 of Unit 1 (Fluids).
No. The buoyant force depends only on the fluid's density and the volume of fluid displaced. A 1 kg block and a 10 kg block of the same volume, fully submerged in the same fluid, feel the exact same buoyant force. The object's density only matters for deciding whether that force is enough to make it float.
For a fully submerged object, essentially no. The displaced volume doesn't change with depth, so F_b = ρVg stays the same (assuming the fluid is incompressible). Pressure increases with depth, but it's the pressure difference across the object that creates buoyancy, and that difference stays constant.
Buoyancy is the upward force itself; Archimedes' principle is the statement that lets you calculate it. The principle says the buoyant force equals the weight of the displaced fluid, so the two terms are two sides of the same idea on the AP exam.
Because floating depends on average density, not material density. A ship's hull encloses a huge volume of air, so the ship's average density (steel plus air) is less than water's. It displaces enough water for the buoyant force to equal its weight.