1. Class Notes: Buoyancy & Density
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

2. Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10
Buoyancy is the phenomenon caused by the upward force of fluid pressure.
Remember, pressure increases with depth.
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

3. Buoyant Force on a submerged object
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

4. Archimedes’ Principle
The buoyant force on an object is equal to the weight of the fluid displaced by the object.
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

5. Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10
If the displaced water = 10 kg (mass)
Weight = Mass x Acceleration due to gravity
= 10 kg x 9.8 m/s/s
= 98 N
98 N is the weight of the displaced water, and is equal to the buoyant force on the object.
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

6. Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10
Density and Floating
To find an object’s density, you need to know the object’s mass and volume.
Density is the ratio of the mass of a substance to the volume of the substance.
Mass per unit volume is Density.
Density = Mass ÷ Volume
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

7. Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10
Will it sink or float?
An object sinks because it is denser than the fluid it is in.
An object floats because it is less dense than the fluid it is in.
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

8. Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10
Determining Density
First, find an object’s mass (kg or g), using a balance scale.
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

9. Second, find the volume of the object.
For a Regular solid - length x width x height (cm3, m3)
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

10. Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10
For an Irregular solid - use a graduated cylinder to measure water displacement
(mL, L)
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10

11. Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10
Finally, take the Mass (in g or kg) and divide it by the Volume (in cm3 or L) to determine your Density (in g/cm3 or g/mL)
Unit 6 (Ch. 14) Density/Buoyancy Bach 03/01/10