Dr. Jie Zou PHY 1151G Department of Physics 1

Chapter 15

Fluids

Dr. Jie Zou PHY 1151G Department of Physics 2

Outline Density Pressure

Atmospheric pressure Pressure and depth

Applications: barometer Water seeks its own level

Dr. Jie Zou PHY 1151G Department of Physics 3

Density Definition of density, : The density of a

material is defined as the mass per volume. = M/V. SI unit: kg/m3.

The denser a material, the more mass it has in any given volume. Examples of density of different materials: gold,

19,300 kg/m3; fresh water, 1000 kg/m3; helium, 0.179 kg/m3 (See Table 15-1)

Example: Find the mass of a gallon of water.(1 gallon = 3.79 L = 3.79 10-3 m3)

Dr. Jie Zou PHY 1151G Department of Physics 4

The same force applied to the small area of a needle tip causes a large enough pressure to rupture the balloon.

This bird exerts only a small pressure on the lily pad because its weight spreads out over a large area by its long toes.

Pressure Definition of pressure:

Pressure is a measure of the amount of force per area. P = F/A. SI unit: N/m2. Pressure is increased if the

force applied to a given area is increased, or if a given force is applied to a smaller area.

Dr. Jie Zou PHY 1151G Department of Physics 5

Example

Find the pressure exerted on the skin of a balloon if you press with a force of 2.1 N using (a) your finger or (b) a needle (Assume the

area of your fingertip is 1.0 10-4 m2, and the area of the needle tip is 2.5 10-7 m2.)

(c) Find the minimum force necessary to pop the balloon with the needle, given that the balloon pops with a pressure of 3.0 105 N/m2.

Dr. Jie Zou PHY 1151G Department of Physics 6

Atmospheric pressure Atmospheric pressure, Pat, is a direct result

of the weight of the air above us. Pat = 1.01 105 N/m2 (about 14.7 pounds per square inch). A Shorthand unit for N/m2 is the pascal (Pa): 1 Pa

= 1 N/m2. A common unit for atmospheric pressure in

weather forecasting is the bar, defined as follows: 1 bar = 105 Pa ≈ 1 Pat.

Gauge pressure: Pg = P - Pat

Dr. Jie Zou PHY 1151G Department of Physics 7

Pressure and depth in fluids At a depth h, the pressure P

is given by: P = Pat + gh. : density of the fluid

If the pressure at one point is P1, the pressure P2 at a depth h below that point is the following: P2 = P1 + gh.

Pat

P

Dr. Jie Zou PHY 1151G Department of Physics 8

Example

The titanic was found in 1985 lying on the bottom of the North Atlantic at a depth of 2.5 miles. What is the pressure at this depth? (The density of sea water is 1025 kg/m3 and 1 mile = 1609 m.)

Dr. Jie Zou PHY 1151G Department of Physics 9

Application: barometer A simple barometer:

A device used to measure atmospheric pressure Atmospheric pressure

Pat is related to the height of fluid in the tube by the relation: Pat = gh.

Dr. Jie Zou PHY 1151G Department of Physics 10

Water seeks its own level Water seeks its own level. Example 15-4: A U-shaped

tube is filled mostly with water, but a small amount of vegetable oil has been added to one side. water is 1.00 x 103 kg/m3, and oil is 9.20 x 102 kg/m3. If the depth of the oil is 5.00 cm, what is the difference in level h between the top of the oil on one side of the U and the top of the water on the other side?

Dr. Jie Zou PHY 1151G Department of Physics 11

Homework See online assignment on

www.masteringphysics.com