Lesson 15: Buoyant Force and Archimedes

Buoyant Force and Archimedes

"Buoyancy." The word that gets weirder the more you read it.

​Buoyant Force

​If you've ever floated on a raft or boat, you've experienced buoyant force. Gravitational force pulls down on the raft, but it doesn't sink. This is because the weight of the raft, person, and all the air above them are balanced out by the buoyant force which is an upward force applied by a fluid on an object in the fluid.

​Buoyant forces aren't just for water

​Balloons operate due to the buoyant force of air. A helium balloon floats because the gravitational force pulling down on it is less than the buoyant force pushing up on it. We also experience the buoyant force of air all the time, but the force isn't enough to overcome the gravitational force on us, so we don't float on air.

​Buoyant Force

​Buoyant force is the difference in upward and downward pressure on an object in a fluid. The upward pressure on an object is always greater than the downward pressure on the object, so the buoyant force is always upward. As the diver goes deeper, the pressure pushing down goes up, but so does the pressure pushing upward, so the buoyant force remains constant.

​Archimedes Principle

​The Archimedes Principle says that the weight of the fluid that an object displaces is equal to the buoyant force acting on the object. In other words, the more water an object displaces, the more the water was pushing back on the object as it submerged.

​Buoyancy and Weight

​Any object that sinks has a greater gravitational force acting on it than the buoyant force on it. Any object which floats on a fluid has a balanced buoyant force to the gravitational force acting on it.

​Buoyancy

​How do aluminum boats work? Aluminum is much denser than water, so why don't aluminum boats sink? Because the rest of the boat is filled with air which is less buoyant than water, and causes the boat to float.

​Buoyancy

So, if an object sinks, it is more dense than the fluid it is placed in. If it floats, it is less dense than the object it is placed in.​

Multiple Choice

_______________ says that the weight of a fluid that is displaced by an object is equal to the buoyant force acting on the object.

• Archimedes Principle

• Aristotle's Equation

• Aristophanes' Cloud

• Anaximander's Apeiron

Multiple Choice

I place an object made of an unknown substance into a bucket of water and it sinks. The object must be _________ dense than water.

• less

• more

Multiple Choice

_________________ Force is the difference between the downward force on an object in a fluid and the upward force acting on it.

• Buoyant

• Girlant

• Floatative

• Birant