1. Chapter 8 Objectives Define a fluid. Distinguish a gas from a liquid.
Section 1 Fluids and Buoyant Force
Chapter 8
Objectives
Define a fluid.
Distinguish a gas from a liquid.
Determine the magnitude of the buoyant force exerted on a floating object or a submerged object.
Explain why some objects float and some objects sink.

2. Chapter 8 Defining a Fluid
Section 1 Fluids and Buoyant Force
Chapter 8
Defining a Fluid
A fluid is a nonsolid state of matter in which the atoms or molecules are free to move past each other, as in a gas or a liquid.
Both liquids and gases are considered fluids because they can flow and change shape.
Liquids have a definite volume; gases do not.

3. Density and Buoyant Force
Section 1 Fluids and Buoyant Force
Chapter 8
Density and Buoyant Force
The concentration of matter of an object is called the mass density.
Mass density is measured as the mass per unit volume of a substance.

4. Section 1 Fluids and Buoyant Force
Chapter 8
Mass Density

5. Density and Buoyant Force, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Density and Buoyant Force, continued
The buoyant force is the upward force exerted by a liquid on an object immersed in or floating on the liquid.
Buoyant forces can keep objects afloat.

6. Buoyant Force and Archimedes’ Principle
Section 1 Fluids and Buoyant Force
Chapter 8
Buoyant Force and Archimedes’ Principle

7. Displaced Volume of a Fluid
Section 1 Fluids and Buoyant Force
Chapter 8
Displaced Volume of a Fluid

8. Density and Buoyant Force, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Density and Buoyant Force, continued
Archimedes’ principle describes the magnitude of a buoyant force.
Archimedes’ principle: Any object completely or partially submerged in a fluid experiences an upward buoyant force equal in magnitude to the weight of the fluid displaced by the object.
FB = Fg (displaced fluid) = mfg
magnitude of buoyant force = weight of fluid displaced

9. Buoyant Force on Floating Objects
Section 1 Fluids and Buoyant Force
Chapter 8
Buoyant Force on Floating Objects

10. Section 1 Fluids and Buoyant Force
Chapter 8
Buoyant Force

11. Density and Buoyant Force, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Density and Buoyant Force, continued
For a floating object, the buoyant force equals the object’s weight.
The apparent weight of a submerged object depends on the density of the object.
For an object with density rO submerged in a fluid of density rf, the buoyant force FB obeys the following ratio:

12. Chapter 8 Sample Problem Buoyant Force
Section 1 Fluids and Buoyant Force
Chapter 8
Sample Problem
Buoyant Force
A bargain hunter purchases a “gold” crown at a flea market. After she gets home, she hangs the crown from a scale and finds its weight to be 7.84 N. She then weighs the crown while it is immersed in water, and the scale reads 6.86 N. Is the crown made of pure gold? Explain.

13. Sample Problem, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Sample Problem, continued
Buoyant Force
1. Define
Given:
Fg = 7.84 N
apparent weight = 6.86 N
rf = pwater = 1.00  103 kg/m3
Unknown:
rO = ?

14. Sample Problem, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Sample Problem, continued
Buoyant Force
1. Define, continued
TIP: The use of a diagram can help clarify a problem and the variables involved. In this diagram, FT,1 equals the actual weight of the crown, and FT,2 is the apparent weight of the crown when immersed in water.
Diagram:

15. Sample Problem, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Sample Problem, continued
Buoyant Force
2. Plan
Choose an equation or situation: Because the object is completely submerged, consider the ratio of the weight to the buoyant force.

16. Sample Problem, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Sample Problem, continued
Buoyant Force
2. Plan, continued
Rearrange the equation to isolate the unknown:

17. Sample Problem, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Sample Problem, continued
Buoyant Force
3. Calculate
Substitute the values into the equation and solve:

18. Sample Problem, continued
Section 1 Fluids and Buoyant Force
Chapter 8
Sample Problem, continued
Buoyant Force
4. Evaluate
From the table, the density of gold is 19.3  103 kg/m3. Because 8.0  103 kg/m3 < 19.3  103 kg/m3, the crown cannot be pure gold.

19. Chapter 8 Objectives Calculate the pressure exerted by a fluid.
Section 2 Fluid Pressure
Chapter 8
Objectives
Calculate the pressure exerted by a fluid.
Calculate how pressure varies with depth in a fluid.

20. Section 2 Fluid Pressure
Chapter 8
Pressure
Pressure is the magnitude of the force on a surface per unit area.
Pascal’s principle states that pressure applied to a fluid in a closed container is transmitted equally to every point of the fluid and to the walls of the container.

21. Section 2 Fluid Pressure
Chapter 8
Pascal’s Principle

22. Chapter 8 Pressure, continued Pressure varies with depth in a fluid.
Section 2 Fluid Pressure
Chapter 8
Pressure, continued
Pressure varies with depth in a fluid.
The pressure in a fluid increases with depth.

23. Fluid Pressure as a Function of Depth
Section 2 Fluid Pressure
Chapter 8
Fluid Pressure as a Function of Depth