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Good News -- Bad News -- Rolling Disk in a Rolling Ring



    Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 35
      Machinery Dynamics Research, 2017

Good News --- Bad News
Rolling Disk in a Rolling Ring



    Well, it looks like Mechanics Corner is back, at least in terms of an occasional post. It will probably be less frequent than previously, but there are just too many interesting things to talk about to remain entirely silent! The title for this post may leave you wondering what is the Good News, and what is the Bad News? Why is there both? Well, let me tell you about it ...




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I was reviewing some technical papers preparing for some rigging work that may come my way. An example far far simpler than the problem Dr D presents I decided to see if there was enough information provided to do it graphically. Well there was. I then returned to the two equations used to determine two different cable lengths. To my surprise the author had repeated the same equation for the two different lengths. I did not notice the equations were the same at first.

Any examples I find for a particular type of problem, I work out in detail to make sure I get the same answer and understand every step. I carry all units through from start to finish. If there is a step I do not understand or can't duplicate I look for other approaches. 

When I studied for my PE 34 years ago I used a review book that was the size of a large city telephone directory. I found many typos and errors. Often the answers were correct but the equations were missing a coefficient or variables had mixed up subscripts. If I blindly followed the method I would not arrive at the correct solution.

Just as in newspapers don't believe everything you read.

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Thanks for the comment, JAG. The importance of checking everything for yourself cannot be stressed too much; it is a critical engineering characteristic that all need to develop.


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This relates to the post above just how? I fail to see the connection.

That said, a kill switch is usually a device to shut off an engine. There may be different ways to do this, depending upon the engine type. For a spark ignited engine, it is often done by shorting the ignition, either at the magneto or coil, or sometimes directly at the spark plug. For a diesel engine, it must be done by shutting off the fuel supply.

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