1. Cutnell & Johnson “Physics” 6th Edition
11.1 Mass Density
Cutnell & Johnson
“Physics”
6th Edition

2. Fluids
Substances that can flow
Liquids
Gases

3. Mass Density Designated by the Greek letter rho
The mass density ρ is the mass m of a substance divided by its volume V
ρ = m/V
SI Units: kg/m3
Table 11.1 page 301

4. Mass Density Increasing mass density Gases → Liquids → Solids
Depends on how tightly packed the molecules of a substance are
Gases are very sensitive to changes in temperature and pressure

5. Example 1
The body of a man whose weight is about 690 N contains about 5.210-3 m3 (5.5 quarts) of blood.
Find the blood’s weight
Express it as a percentage of the body weight
m = rho * V = (1060 kg /m^3) (5.2 x 10^-3 m^3) = 5.5 kg
W = mg = (5.5 kg)(9.8 m/s^2) = 54 N
b) Percentage = 54 N / 690 N X 100 = 7.8%

6. Solution

7. Specific Gravity
A method of comparing mass densities of two substances
Specific = Density of substance Gravity Density of water at 4 C
= Density of substance  103 kg/m3

8. Cutnell & Johnson “Physics” 5th Edition John Wiley & Sons, Inc.
11.2 Pressure
Cutnell & Johnson “Physics”
5th Edition
John Wiley & Sons, Inc.

9. Air Pressure in a Tire
In colliding with the inner walls of a tire, the air molecules (blue dots) exert a force on every part of the wall surface.
If a small cube were inserted inside the tire, the cube would experience forces acting perpendicular to each of its 6 faces.

10. Pressure
The pressure P is the magnitude F of the force acting perpendicular to a surface divided by the area A over which the force acts.
P = F/A
SI Units: N/m2 = pascal (Pa)

11. Other Units 105 Pa = 1 bar lbs/in2 = psi 1 psi = 6.895  103 Pa
1 atm =  105 Pa = bar = psi = 760 torr
One atmosphere of pressure is a significant amount. If the air is pumped out of a can, the can will crumple.
Lynx paws are large so that they can walk across the snow. The cat’s weight is a constant. The pressure on the snow can be reduced by a larger cross-sectional area.

12. Direction of Force Due to Pressure
A static fluid cannot produce a force parallel to a surface
If it did, the surface would apply a reaction force to the fluid, consistent with Newton’s 3rd law
In response, the fluid would flow and then would not be static

13. Direction of Force Due to Pressure
The force generated by the pressure of a static fluid is always perpendicular to the surface that the fluid contacts.

14. Example 2: The Force on a Swimmer
P = 1.2105 Pa
Ahand = 8.4 10-3 m2
Determine the magnitude of the force that acts on the hand.
F = P * A = (1.2 x 10^5 N/m^2)(8.4 x 10^-3 m^2) = 1.0 x 10^3 N = 230 lb.

15. Solution