Why does a boat float in water but a boat anchor sinks? What actually causes something to float?
The answer is the buoyant force. This is an upwards force that acts on any object submerged in a fluid. But there's nothing magical about it - the buoyant force is just the net force caused by the fluid pressure, and since pressure increases with depth the net force is upwards.
While taking a bath, an ancient Greek physicist named Archimedes discovered that the upward buoyant force on a submerged object is equal to the weight of the fluid that the object displaces (known as Archimedes' principle).
In this lesson we'll learn about the buoyant force, how it works and how to calculate it.
Answers
Free-Response Questions
Fluids (previously in AP Physics 2)
Note: These are questions from AP Physics 2 exams before 2025. For some questions only parts of the question are relevant to the Fluids unit in AP Physics 1 as noted below.
Buoyancy (PhET)
Jared Schmidt, Diana Lopez
Lab 1 - The Cartesian Diver
empty 2L plastic bottle with cap, drinking glass, water, plastic straw, scissors, paper clips, eyedropper (optional)
Rhett Allain
Lab 2 - Floating Eggs
2 glasses, 2 eggs, salt
Math and Science
Lab 1 - Fluid Dynamics
(Pages 123-124, Activity 3)
scale, object that sinks in water, object that floats in water, water and three other liquids with different densities, graduated cylinder, at least four objects with irregular shapes
Openstax - College Physics for AP Courses Lab Manual - Student Version
Lab 2 - Buoyant Force
force sensor with hook, support for force sensor, overflow can, brass cylinder, aluminum cylinder, scale, thread, beakers, glass stir rod, pen, dish soap, water, meterstick
Pasco
Why does a boat float in water but a boat anchor sinks? What actually causes something to float?
The answer is the buoyant force. This is an upwards force that acts on any object submerged in a fluid. But there's nothing magical about it - the buoyant force is just the net force caused by the fluid pressure, and since pressure increases with depth the net force is upwards.
While taking a bath, an ancient Greek physicist named Archimedes discovered that the upward buoyant force on a submerged object is equal to the weight of the fluid that the object displaces (known as Archimedes' principle).
In this lesson we'll learn about the buoyant force, how it works and how to calculate it.
