Ch 12: Fluid Mechanics

Question 1

What is (average) density?

Answer 1

density = mass/volume

Question 2

What is the SI unit of density?

Answer 2

kilogram per cubic meter

Question 3

What is the specific gravity of a material?

Answer 3

the ratio of its density to the density of water at 4.0°C, which is 1000 kg/m³

Question 4

When a fluid is at rest, it exerts a force _____ to any surface in contact with it.

Answer 4

perpendicular

Question 5

If we think of an imaginary surface within the fluid, the fluid on the two sides of the surface exerts _____.

Answer 5

equal and opposite forces on the surface

Question 6

What is pressure?

Answer 6

dF⊥/dA; if pressure is the same at all points, then p=F⊥/A

Question 7

What is the SI unit of pressure?

Answer 7

the pascal, which is equal to 1 N/m²

Question 8

What is atmospheric pressure?

Answer 8

the pressure of the earth’s atmosphere at sea level, which is 1 atm = 1.013E5 Pa

Question 9

When an element of a fluid is at rest, the downward forces (_____) are equal to the upward force (_____).

Answer 9

force due to pressure on top of surface and weight of the fluid element; force due to pressure on the bottom of the surface

Question 10

As you move upward in a liquid, pressure _____.

Answer 10

decreases

Question 11

What is the equation for pressure difference between 2 points in a static fluid of uniform density?

Answer 11

p_2 - p_1 = -ρg(y_2 - y_1)

Question 12

What is the equation for pressure at a given depth h if p_0 is at the surface of the liquid?

Answer 12

p = p_0 + ρgh

Question 13

What is Pascal’s Law (conceptually)?

Answer 13

Pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel

Question 14

What is Pascal’s Law (relating to hydraulic lift)?

Answer 14

If p is the same everywhere, then F1/A1 = F2/A2

Question 15

What is gauge pressure?

Answer 15

excess pressure above atmospheric pressure; p - p_atm

Question 16

What is absolute pressure?

Answer 16

total pressure (p); gauge pressure + atmospheric pressure

Question 17

What is Archimedes’s Principle?

Answer 17

When a body is completely or partially immersed in a fluid, the fluid exerts an upward force on the body equal to the weight of the fluid displaced by the body

Question 18

What is the buoyant force?

Answer 18

the upward force exerted by any fluid upon a body placed in it, equal to the weight of the displaced fluid

Question 19

The surface of a liquid behaves like _____.

Answer 19

a membrane under tension

Question 20

Why are raindrops spherical?

Answer 20

Surface tension; the molecules of water exert attractive forces on each other, and so surface molecules are drawn into the volume, minimizing surface area; Spheres have the smallest surface area to volume ratio, and so that is the resulting shape.

Question 21

What are two properties of an ideal fluid?

Answer 21

It is incompressible (its density cannot change) and has no internal friction (no viscosity)

Question 22

What is laminar flow?

Answer 22

Adjacent layers of fluid slide smoothly past each other and the flow is steady

Question 23

What is turbulent flow and when does it occur?

Answer 23

There is no steady-state pattern; this occurs at sufficiently high flow rates, or when boundary surfaces cause abrupt changes in velocity, causing the flow to become irregular and chaotic.

Question 24

What is the continuity equation?

Answer 24

A1v1=A2v2 for incompressible fluid; multiply by ρ for compressible fluid.

Question 25

What is volume flow rate?

Answer 25

dV/dt = Av, the rate at which volume crosses a section of the tub