CM2110 Chapter 2



CM2110 Chapter 2

2.1 to 2.3) Units and Dimensions, Conversions of Units, Systems of Units

| |American Engineering System |System Internationale |CGS |

| |(English) |(SI) | |

|Length |ft |m |cm |

|Mass |lbm |kg |g |

|Time |s |s |s |

These are the base units.

Some conversion factors are on the front cover of the text and on p. 11 Table 2.3-1. Another reference is the back cover of Perry’s Chemical Engineers Handbook.

We’ll spend a lot of time in CM2110 converting between different units. Unit conversion is VERY IMPORTANT! You’ll find that in industry all three systems of units are used.

2.4) Force and Weight

a) Force

Newton’s 2nd law of motion

[pic]

m = mass

a = acceleration

gc= conversion factor

F= force

SI: Force= kg .m/s2 = Newton (or N): mass in kg and acceleration in m/s2

CGS: Force= g .cm/s2 = dyne : mass in g and acceleration in cm/s2

American Engr System: lbm. ft/s2

1 Newton = 1N = [pic]= 1kg .m/s2

1 dyne = [pic]= 1g .cm/s2

American Engr System:

Force= lbf = lbforce= defined as 1 unit mass (1 lbmass) times acceleration of gravity (ft/s2)

a = g = acceleration of gravity = 32.174 ft/s2 = 9.8066 m/s2= 980.66 cm/s2

1 lbf = 32.174 lbm .ft/s2

by definition :

SI CGS Amer. Engr.

gc = [pic]

b) Weight

Weight of an object is force exerted on that object by gravity.

[pic]

m = mass

a = acceleration( g = acceleration of gravity

gc= conversion factor

F= force

[pic]

2.5) Dimensional Homogeneity

Quantities can be added/subtracted only if their units are the same.

Example 1) V (m/s) = Vo (m/s) + g (m/s2) t (s)

Velocity

(m/s) = (m/s) + (m/s) Units are OK!

Example 2) V (m/s) = Vo (m/s) + g (m/s2) Units are NOT OK!

NOT a valid equation. Need all units to be the same!

2.6) Significant Figures

I’ll typically use 3 significant figures in CM2110.

Significant Figure Examples

2300. = 4 significant figures

0.03500 = 4 significant figures = 3.500 x 10-2

Chapter 2 Problems

Use the front cover of your text book to get conversion factors!

1. Verify that 1gal = 3.785 L = 0.003785 m3 = 231 in3

Add these to the front cover of your book.

1 gal [pic] = 3.785 L

1 gal [pic] = 0.003785 m3

1 gal [pic] = 231 in3

2. Verify that 1 g/cm3 = 62.4 lbm/ft3 = 8.34 lbm/gal = 1000 kg/ m3

Add these to the front cover of your book.

[pic][pic] = 62.4 lbm/ft3

[pic][pic] = 8.34 lbm/gal

[pic][pic] = 1,000. kg/m3

3. Convert 57.5 lbm/ft3 to kg/m3

[pic][pic] = 921 kg/m3

4. Re = Reynolds Number = dimensionless quantity used a lot by CM in fluid flow to determine if we have laminar or turbulent flow in a pipe.

Re< 2100 means laminar flow

Re> 2100 means turbulent flow

Re = [pic]

D = pipe diameter = 2.067 in

v = fluid velocity = 4.8 ft/s

[pic]= fluid density = 0.805 g/cm3

[pic]= fluid viscosity = 0.43 cP

1cP = 1 centipoise = 10-3 kg/(m.s)

What is Re? Is flow laminar or turbulent?

[pic][pic] =

143,700 >2100 so turbulent flow

5. A storage tank contains 10,000 gal of a liquid with a density of 1 g/cm3. How many m3 and ft3 and kg and lbm of liquid are in the tank? How much does the liquid weigh (lbf and N)?

10,000 gal [pic]= 37.9 m3

10,000 gal [pic]= 1,337 ft3

Density = Mass/Volume

Mass = Density . Volume

Mass:

37.9 m3 [pic]= 37,900 kg

1337 ft3 [pic]= 83,429 lbm

[pic]

Amer. Engr Units:

W= [pic]= 83,429 lbf

SI Units

1 Newton = 1N = [pic]

W = [pic]= 371,670 N

6. You can do additional Ch 2 problems on your own. Answers to selected problems are in the back of your book. In addition, there are more problems on the ‘Instructional Tutorials’ that came on the CD with your textbook.

7. Some selected answers to Ch 2 problems. You are to do these on your own. Do not turn them in. They are to help you study for tests and quizzes.

1. a) 1.1844 x 109 ms b) 26.0 mi/h c) 3.85 x 10 4 cm4/min. g

2. a) 340 m/s b) 57.5 lbm/ft3 c) 120 hp

8. a) 25.0 lbf b) 2.6 kg c) 9x 10 9 dynes

2.23 Re = 2x104

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