Problem: Welcome to Mechanical PE exam prep, Extra Practice Module on Fluids on Dan Malay.
Given: 66.2 pounds mass per cubic foot; 144 inches squared per foot squared; 66.2 pound mass over cubic feet
Approach: Let's jump right into the first problem.
Key formula: formula is I want to see the units cancel
Calc: Blood has a dynamic viscosity of 3.3 CP and a density of 66.2 pounds mass per cubic foot.
Calc: And then the density of water is 62.4.
Result: And that is answer choice C.
Office Hours
5
Student questions asked in live office hours about this problem
OH 51: HVAC: Fluids-1
Q: Why does the solution use CP to Pascal seconds conversion instead of CP to pound-force-second per foot-squared, and why do I get different answers using both conversions?
A: Both conversions are valid—it's just different unit conversion paths. You likely made a small error converting hours to seconds (used 60 instead of 3,600); if you divide your answer by 60, you'll get the same result as the solution. Don't worry, this is a common mistake even Dan makes.
OH 68: HVAC: Fluids Module #1
Q: In a kinematic viscosity problem, why didn't the solution use G_c as a conversion factor, and how do we derive the conversion factor one pound force equals 32.2 pound mass·ft/s² from first principles?
A: In US units, Newton's second law F=ma naturally gives pound force when mass is in slugs and acceleration is in ft/s². Since one slug equals 32.2 lbm, we get pound force = 32.2 lbm·ft/s². This derived relationship is your conversion factor for eliminating mixed pound mass and pound force units—it comes directly from the definition of a slug, not from G_c.
OH 95: HVAC: Fluids Module #1
Q: I used a different conversion factor from the reference manual to convert CP to pound mass per hour foot, and while the units work out, my answer is wrong—is there something fundamentally incorrect with this approach?
A: No, your approach is fine, but you made a simple time conversion error—you forgot that there are 60 minutes in an hour, so divide your answer by 60 again to get the right result. This is a common mistake, so you're definitely not alone.
OH 103: HVAC: Fluids Module #1
Q: Why can't we use the CP equals 2.419 pound per hour foot conversion for this viscosity problem?
A: You actually can use it—that conversion factor is in the reference handbook. I'd encourage you to try it yourself; you should get the same answer. If you've already checked it and confirmed it works, you're good to go.
OH 106: HVAC: Fluids Module #1
Q: Why can't I use the conversion formula from the reference manual (CP = 2.49 lb/hr·ft) to directly convert 3.3 CP to kinematic viscosity, rather than converting through Pascal's? When I do the conversion I get 0.002 ft²/s, which doesn't match the expected answer.
A: You're using the right approach, but you made an error in the final step—you converted hours to minutes (dividing by 60) when you needed to convert to seconds (dividing by 3600). Use 3600 seconds per hour instead of 60, which will make your answer smaller by an order of magnitude and get you to the correct range of 10^-4 to 10^-5 ft²/s.