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Mechanical Engineering


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Everything posted by DrD

  1. DrD

    A Calculus Challenge

    Dear Wasulia, Congratulations, you got the correct answer, almost. Two comments come to mind: (1) You made an error in factoring out R^2 in the last line; (2) You should learn that, when doing calculus, angles are always expressed in radians, so your factors 360, 180, etc are really errors. Learn to think in radians; it will help you a lot. You took the long way around, but you almost got here. Good effort. ----------------------------------------- Dear Jarek, You are correct in saying x-bar is zero. I'm amazed that no one else has seen that yet! Without saying yes or no to your result for y-bar, I have two comments for you: (1) I urge you to provide a sketch showing how you formed the integrals you offer as a route to the answer. (2) You already have an expression for the area, so I would urge you to substitute it and seek to simplify the result. DrD
  2. DrD

    A Calculus Challenge

    Dear Amin, There are a few problems with your answer. 1) If you are going to cite Roark, you should surely say which edition. There are many, and they are not all the same. I have two of them myself, but quite likely not the one you use. 2) You don't give your final result for the area. You simply cite Roark, but there is nothing here that I can check. Roark has been known to be wrong. 3) You give an answer for something you call "static moment," but you do not locate the centroid. 4) You say, "The same for moment of inertia." The same what? You have given any answer before (unless you meant your area result), so this is no answer at all. 5) You seem to think actually answering the question is beneath you because you know how to look in a book. What do you do if the book is not at hand? 6) You said, "When I had to deal with these kind of problems, I used to go to handbooks." Do you think handbooks have all the answers? Do you no longer do engineering? Overall, I'd say this is a fairly poor effort by someone who really is not very interested. DrD
  3. DrD

    A Calculus Challenge

    Well, as of now (4 Jan 2019), over 1000 people have looked at this problem, one person has worked one part of the problem statement, and another says he know how to do this without integration (and thus far, without results). You folks are real balls of fire!! DrD
  4. DrD

    A Calculus Challenge

    Dear Jarek, Your figures help a lot to explain your logic. Thank you. I'd say you definitely have the answer for the first part. Now, can you extend this to obtain the other answers ? DrD
  5. DrD

    A Calculus Challenge

    Dear Jarek, Your final result for the enclosed area is correct (if we ignore some notational inconsistency, r in place of R). Congratulations!! That said, I cannot follow your development. Would you explain your reasoning with some words, perhaps? In particular, you show the sum of two integrals equal to a third integral. How is that equality established? I cannot follow it. Also, you write in the integrand in one place xtgalpha; what is this? Is "tg" the tangent function? If so, it would help if you would enclose the argument in parentheses. Now, to all others, I hope you see that the problem is not impossible. Why are you so unwilling to try it yourself? DrD
  6. DrD

    A Calculus Challenge

    My word! An answer ... I'm simply stunned! If you are sure it can be solved without integration, go ahead. This will serve as a check on the results obtained by integration. If you use formulas from a book, please be sure to give their derivation as well. DrD
  7. DrD

    A Calculus Challenge

    Today, 29 December, there have been over 300 people look at this problem, and yet not a single comment! What is it with all of you? What are you thinking? Is the problem unrealistic, so that it would not be useful to know how to do it? I can cite cases where this geometry really appears. Is the problem too hard? You are supposed to be engineers, able to confront new situations and find ways to deal with them. Why is this any different? Is the problem too easy, and therefore beneath your dignity? If so, please humor an old man, and indulge him by answering the questions. Whatever the situation, somebody please say something!! DrD
  8. Sorry, JAG. It does not happen on my Win7 machine. I'm just old and past my "use by" date! DrD
  9. Wow!! Great article!! Just what language was that in, anyway?
  10. DrD

    A Calculus Challenge

    As I check this on 26 December, there have been over 200 people look at this problem. Thus far, not a single one has ventured even the first bit of an attempt. What do you folks do when confronted by a problem that is not in the textbook? DrD
  11. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #45 (c) DrD, 2018 It has been quite a while since I last posted anything here, but an interesting problem has come to mind that I wanted to share with you. If you really know calculus, this should be straight forward; if you don't know calculus, don't even try! MEForumChallenge.pdf *********************************************************************************************************** THE CHALLENGE IS NOW ENDED. I WILL NOT RESPOND TO FURTHER ANSWERS. i EXPECT TO POST A SOLUTION AND A FEW COMMENTS IN THE NEXT FEW DAYS. ***********************************************************************************************************
  12. It is possible that I may be able to help you, but again, I may not. Please answer a few questions for me. 1) What college level math courses have you taken? 2) What college level mechanics courses have you taken (statics, dynamics, vibrations, theory of machines, etc)? 3) Where are you located? 4) Are you able to pay for tutoring? DrD
  13. A brake is a brake is a brake. Any brake has the potential to stop any rotating object. Whether it is a good idea or not is another matter, but certainly an automotive caliper brake has the potential to stop a winch drum. You really need to learn the difference between "brake" and "break." They are entirely different words, even though they sound alike in English.
  14. Wow! That was one boring video! I could not stay interested enough to watch it to the bitter end. The whole idea seems ludicrous. If you want to generate energy by human effort, why not apply it directly? Perhaps simply ride a bicycle, rather than try to generate electric power by pushing things. Talk about grasping at straws ...
  15. I rather doubt that you will ever get to the point where you fully understand the problem. Most of us find that there is always something more we can learn. At the same time, it seems foolhardy to commit to designing something about which you know so little. There is nothing about which I know everything, yet I would undertake to design some things but not others. I would design those things about which I already know enough to make a good start. I would avoid attempting to design something about which I now know nothing. Do you see the difference? DrD
  16. What a fabulous post! Not a complete sentence in it, just a string of terms describing destruction and destructive devices. This makes no sense at all, so it is posted in exactly the proper place. DrD
  17. What a great idea to commit yourself to a project you know nothing about! This is utter foolishness!! Engineers do not do such things. DrD
  18. What are Z-Kinematics U-Kinematics? I've never heard these terms before, so I'd be interested to know what you are talking about.
  19. DrD

    Two Short Math Problems

    I really don't think I can help you. I don't know much about robots. DrD
  20. Why in the world do you want to put a flywheel on a DC motor? The purposes of a flywheel are (1) to smooth out irregularities in the rotational motion of the shaft (torsional vibration), and (2) to store energy in order to continue operation though a power interruption. You did not mention either of these, so I ask again, "why do you want to put a flywheel on a DC motor?"
  21. "It gives balance to engine and load is uniformly distributed in crankshaft." This statement is totally incorrect. Torque is delivered to the crankshaft as a series of pulses, in no way uniform. In a multicylinder engine, torque will be positive at one location while it may be negative at another. What is a "four stage engine"? Do you mean four cylinder, or perhaps four stroke, or something else? All the pretty animations in the world are not worth much if the accompanying statements are nonsense. DrD
  22. The PDF file below will show you how to solve this problem. DrDPlanetaryProb.pdf
  23. As a starting point, you must know the MMOI for each individual component; without that, you cannot begin. Assuming that the individual component MMOI values are known, do a kinematic analysis of the planetary train to determine the velocity of the CM of each component. The use that information to formulate the kinetic energy of the entire system, all moving according to the kinematic relations. You will have to choose some rotation as a reference value, usually either the input or output rotation. The kinetic energy will have the form T = (1/2) theta-dot^2 (term1+term2+...+termlast) where theta is the reference rotation. The sum of terms indicated is the effective MMOI for the gear train. DrD
  24. Wow!! That was super exciting!! DrD
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