1. Buoyancy

2. Buoyant Force pushes up on all matter when in a fluid.
Buoyant force is equal to the weight of the volume of the fluid displaced by the object
This is known as Archimedes’ principle
Volume of rock
= 1000 cm3
Volume of displaced water
Buoyant force on rock
= 9.8 N
Weight of displaced water

3. The object’s volume determines the buoyant force
The strength of the buoyant force on an object depends on the volume of the object that is underwater

4. So you could have a basketball full of air….
and a basketball of cement…
…and the buoyant force would be the same if they were both completely submerged!!!
100 N
100 N

5. If the ball weighs more than the buoyant force, it will sink
If the ball weighs more than the buoyant force, it will sink! If it weighs less, it will float!
Weight
Greater
weight =
sinking!
Buoyant
Force!
Water Tank

6. This works with all fluids
In air, hot air balloons create buoyant force that overcomes gravity’s pull downward.
This is why astronauts train in swimming pools – the buoyant force simulates low-gravity!

7. Water Displacement and boats
Boats float because they displace lots of water
Increasing the volume without increasing the mass decreases the density.
Hollow steel box
Steel cube
Volume = 10 cm3
Mass = 7.8 g
Density = .78 g/cm3
Volume = 1 cm3
Mass = 7.8 g
Density = 7.8 g/cm3

8. Sink or float? Water has a density of 1 gram/milliliter
If an object is denser than this, it will sink, less dense and it will float.

9. Calculating density Density is equal to mass divided by volume.
So a piece of wood with a mass of 200g and a volume of 400 ml has a density of 0.5 g/ml – And will float!
V = m D
D = m V
m = DV

10. It really comes down to density
Density is how much things are packed together.
A cube filled with styrofoam is lighter than a cube filled with lead. This is because lead has a higher density than styrofoam.
The unit for density is expressed using a mass unit divided by a volume unit, such as g/cm3, g/mL, kg/m3, kg/L (1 mL = 1 cm3)
styrofoam
lead

11. Density is a property of matter
The density of an object such as steel is the same throughout.
If you have a 1000 g block of steel with a density of 7.8 g/cm3 and cut it in half, each half will also have a density of 7.8 g/cm3.
Mass = 1000 g
Volume = 128 cm3
Density = 7.8 g/cm3
Mass = 500 g
Volume = 64 cm3
Density = 7.8 g/cm3

12. Try these….
1. What is the density of a toy sub with a mass of 55g and a volume of 5 ml?
2. What is the mass of a lump of metal with a volume of 27 ml and a density of 8 g/ml?
3. How much volume is occupied by a rubber snake with a density of 3.4 g/ml and a mass of 380g? Will it float or sink in your neighbor’s swimming pool?