1. Buoyancy
Properties of Matter

2. Buoyancy of Fluids Key Term: Key Concepts: Buoyancy
Archimedes’ Principle
Why do objects float?
How do you determine the pressure a gas exerts on its container?

3. Buoyancy of Fluids
Why do you feel lighter in the pool than you do on land?
The water in the pool exerts an upward force on you
Buoyancy is a measure of this upward force

4. Buoyancy of Fluids
A Greek in the 3rd century BC discovered that the buoyant force on an object is equal to the weight of the fluid displaced by the object.
This is called Archimedes' Principle.

5. Buoyancy of Fluids An example: A rock weighs 2.25N in air
It weighs 1.8N in water
The volume of the rock is 45mL
The buoyant force of the water on the rock is .45N (water’s density is 1.0 g/cm3)
In glycerin (density of 1.26 g/cm3), the rock weighs only 1.72N, because the buoyant force is larger.

6. Buoyancy of Fluids
The buoyant force explains why some objects float and others sink
In the previous example, in order to float the rock, the buoyant force would need to be greater than 2.25N
If the buoyant force is less than the weight, the object sinks
If the buoyant force is greater than the weight, the object floats
Neutral buoyancy occurs when the buoyant force exactly equals the weight of the object
Ex. SCUBA diving

7. Buoyancy of Fluids How can steel float?
If it were a steel block, it would only displace its volume in water and the buoyant force would be much less than the weight of the steel
When it is shaped into a boat, then it displaces much more water, making the buoyant force greater than the weight of the boat plus its cargo.

8. Buoyancy of Fluids Gas buoyancy
Floating hot air balloons displace a large volume of air.
The air weighs more than the entire balloon.
How does this work?
Hint: hot air – hot air has a larger volume than an equal mass of cool air, so it less dense.

9. Buoyancy of Fluids Helium Balloons Float because of buoyancy
Helium gas is less dense than air, so it takes up more space
The weight of the air displaced is greater than the total weight of the helium and the rubber balloon