# Aggravated with partial explanations in Lindeburg practice problems



## I M A PE (Oct 1, 2013)

Alright... why are these solutions written so in a way that give only partial explanations. I'd rather have 1/8th of the practice problems if I had complete solutions where the logic could be traced step by step. For instance, problem 14-2 of the 12th edition practice problems. Need to calculate kinematic viscosity. No problem. A quick look in MERM index gives formula for KV. Need absolute viscosity for air at a certain temperature and pressure. No problem, I'll look it up in MERM. What? It's not in there? Stumped. Look at solution. Solution says "For air at 14.7 psia and 80F, the absolute voscosity independent of pressure = 3.85E-7 lbf-sec/ft^2."

Am I just supposed to log that into my memory? What if they ask about a different fluid on the test? I can't find anything providing help in MERM on figuring or looking up absolute viscosity???


----------



## uzairsyedahmed (Oct 1, 2013)

I hear ya... MERM isnt the best in providing explanations. For 14-2, absolutely viscosity doesnt change much with pressure and temperature. The kinematic viscosity does. ( this is mentioned in chapter 14). so you can read absolute viscosity from App 14-D then use the formula in Chapter 14.


----------



## I M A PE (Oct 1, 2013)

Thank you, uzair... That's all I needed. I look up viscosity in the index and there is nothing to point me to App 14-D. Gracias.


----------



## FatherTime (Oct 2, 2013)

Okay, so let me repeat what I think I heard. You're working in Chap. 14, which has a section on (absolute/dynamic) viscosity (section 9), and on p. 14.7, it says, "...the absolute viscosity of gases and liquids is usually considered to be essentially independent of pressure." And, a section on kinematic viscosity (section 10), and on p. 14.8 it says, "Unlike absolute viscosity, kinematic viscosity is greatly dependent on both temperature and pressure". And, "...even if absolute viscosity is independent of temperature or pressure...." And, chapter 14 has numerous appendices (including ones for air) containing viscosity tables with columns labeled "absolute viscosity". And, the solution to the problem you're working gives the temperature and pressure and the explanation/reason for using the value listed. And, you're aggravated because Lindeburg didn't explain enough or you couldn't find where that value came from?

Lindeburg isn't perfect. His, idea of preparation is on the excessive side of adequate for some people. People who are at the top of their game may not need everything in MERM. But, people on their way up will probably need every bit of it, and there's little chance of helping the second group without making the first group unhappy. But, I think that's a bum rap, just like I think your aggravation is a bum rap.

By the way, I have both MERM12 and MERM13, and MERM12 isn't perfect. But, MERM13 addresses all of the complaints that I have with MERM12, and it might be your ticket, also. For example, even if you didn't know anything about absolute viscosity, I note that MERM13 has index entries for "Absolute viscosity" and "Viscosity, absolute", as well as "Air, properties" and "Property, air" that lead you to chapter 14 and to the appendices. Who knows what other things MERM13 has anticipated that you and I will need?


----------

