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I Simple machine classification (can o worms?)

  1. May 21, 2016 #1
    Hello all. I have a question concerning the classification of simple machines, of which classically there are considered to be 6. I have joined the camp which claims there are only 2 true simple machine categories at the most fundamental level. The lever and the inclined plane.

    I will be happy to make the case in more detail as to why there are only these 2 if anyone is interested in more on that... (Some argue there are 4, but if you argue there are four I can't help but think you've made correlations in one place but decided to disregard similar correlations in other places)

    To the point now. Another form of mechanical advantage recently crossed into my awareness and had me questioning why it is not considered a distinct simple machine? The Hydraulic press. (pascals law).

    One answer I have found is that the 6 simple machines are 'classic' and that the press is just more modern. But in thinking more in terms of physics than history, would this not obviously be a simple machine? And if so, where would you place it (number 7 of 6 existing, number 3 of 2 existing, or within one of the existing 2/6?

    The similarity I see between wheel and axle, pulley, and lever is not apparent to me with the hydraulic principle. It seems distinct in large part due to its lack of fulcrum and rotational component. Similarly I can't draw any correlation between an inclined plane and the hydraulic piston.

    Lastly, I wonder if this can truly be discussed in terms of physics, or is it merely a question of engineering application, and where we draw the lines as to what is 'fundamental' is arbitrary?
  2. jcsd
  3. May 21, 2016 #2


    Staff: Mentor

    I cannot think of any way in which it matters if there are 2 or 3 or 6 or 7 "fundamental" simple machines. If you like 3 then say there are 3.
  4. May 21, 2016 #3
    Fairly recent discoveries and their applications I think would have added a number of fundamental machines to the classical ones.
    Transistors and other semiconductor devices?
  5. May 21, 2016 #4
    Well does it matter if we delineate any machines for that matter? Does it matter if we differentiate between an inclined plane and a lever. Why not say there is only one form of simple machine? If I understand your reply, you are implying that it is indeed all arbitrary, physically speaking?
    For me, it matters, at least a little bit, in part, because it helps to understand the fundamental way in which the 'machine' is working. Understanding how a pulley is actually acting as a class II lever (albeit transforming rotational force into linear motion) helps me to understand the mechanics of it.

    Are those mechanical machines? I've always assumed simple machines referenced mechanics.
  6. May 21, 2016 #5


    Staff: Mentor

    That is indeed what I am implying. If Bill and Bob both correctly use Newton's laws to analyze the same machine then they will always get the same answers for all measurable results. This even if they disagree about the classification of the machine.

    For me, the fundamental understanding is given by the laws.

    I do see pedagogical value in drawing analogies or connections between different machines. I do not see any pedagogical value in insisting on a rigid classification system.
  7. May 21, 2016 #6
    Fair enough. Perhaps my focus is misplaced. For what its worth, my interest is indeed on the pedagogy of mechanics.

    For my own educations sake, which are the laws that would hold merit, physically speaking, when describing mechanical systems? Or perhaps they are too numerous? Why did anyone decide it was a good idea to divide machines into 6 categories in the first place, or perhaps the question of more pertinence to my original question: for those that do feel its worth dividing machines into categories, is there a reason that hydraulic pistons are not included in the list?
  8. May 21, 2016 #7
    Source: http://mysite.du.edu/~jcalvert/tech/machines/machines.htm

    Well i just read this ^ which states that hydraulic systems are not 'machines' because they are not resistant bodies? I guess that would explain why they are not included in the list (however long a list you want to make). Can't say I know exactly what is meant by resistant bodies, but I assume that a fluid is not considered one.
  9. May 21, 2016 #8


    Staff: Mentor

    I would say Newton's laws, the law of gravitation, Hookes law, and equations for static and dynamic friction.
  10. May 22, 2016 #9


    User Avatar
    Science Advisor
    Gold Member


    You are confusing fundamental machine actions with actual mechanisms .

    Think about transfer functions .
  11. May 22, 2016 #10
    Care to explain more? Which of my statements / implied notions confuses these two? My notion that there are only 2 simple machines? Or my notions about hydraulics? Or others...?

    My guess is that you are saying there is a difference between the fundamentals of machines (of which we can draw similarities in a number of ways) and the actual application of machines? I.e. there are indeed 6 SM's because they are applied differently? If this is what you are saying I would have to argue along the lines of what Dale is saying and that the applications are so numerous, to say there are 6 would be as arbitrary as anything. My goal would be to get down to the fundamentals. But perhaps that is not at all what you are getting at?

    Thanks for all the replies. Hopefully I'll gain a better understanding of the core elements of machine's.
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