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Force in Newton's 3 laws

  1. Apr 18, 2015 #1
    Hi Mentor

    i am in little confusing about force, according newton third law force on action and reaction is equal but according hydraulic system are not equal.
    Thank you for guiding me to knowing the truth
  2. jcsd
  3. Apr 18, 2015 #2
    I think your are confusing pressure with force. A hydraulic system takes a relative small pressure and "magnifies" it but the force of the piston pushing is the same as the force on the cylinder.which is supporting the load..
  4. Apr 18, 2015 #3


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    That's not true.

    The pressure inside a hydraulic system is the same everywhere.

    What happens is that an input force applied to a small bore cylinder creates a given pressure in the system. When this pressure reaches a cylinder with a larger bore, a greater output force is created due to the same pressure being applied over the larger area of the piston.

    In any event, this forum is intended only for new members to introduce themselves.

    Technical questions should be posted in the appropriate forum.
  5. Apr 18, 2015 #4


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    Ironically it's you who confuses pressure with force.
  6. Apr 18, 2015 #5


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    Force by a piston on fluid is equal but opposite to force by fluid on that same piston. The same scheme, but eventually with a different force magnitude, applies for the other piston. That is Newton's third law.
    Last edited: Apr 18, 2015
  7. Apr 18, 2015 #6
    @SteamKing I didn't say or mean that the pressure in the cylinder is magnified. But the hydraulic cylinder does magnify a pressure at the the end of the piston and this magnification factor is the ratio of the area of the main cylinder to that of the piston.. And of course the force of the piston is equal and opposite to that of the cylinder..
  8. Apr 18, 2015 #7


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    It magnifies Force, not pressure. Force = Pressure times area
    Bigger area gives bigger force.
    But hydraulic systems and Newtons' Third law are not really related. There is no conflict as the OP seems to be suggesting.
  9. Apr 19, 2015 #8
    ph 3.jpg
    Thanks new friends.
    last night "struggling", finally comes with result, newton #3 law is work on solid material, because no mechanicl advantage there to be trade. The force is equal, like been used on punching machine. No matter the different size from input and output area, the force remain equal.
    Meanwhile the hydraulic system contain "trade". if the input area is smaller, the mechanical advantage force is > than 1. it's gain force by trading with distance. While if the input area is bigger, the mechanical advantage force is < then 1. it.s trade force with distance.
    there is no free lunch time :)
  10. Apr 19, 2015 #9


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    I don't think this has to do with solid and liquid, nor mechanical advantage.
    The hydraulic system shown is the equivalent of a mechanical lever with the left side having 1000lb force at 90cm from the fulcrum and the right side having 9000 lb force at 10 cm from the fulcrum
    . lever.gif
    IMO the correct application of Newton's 3rd law here is to say that, since there is 1000 lb pushing down on one side and 9000 lb pushing down on the other, then there must be an equal and opposite upwards force of 10,000 lb on the fulcrum (or on the mounting of the hydraulics in your eg.) In neither case are the 1000 lb and 9000 lb action and reaction to each other.
  11. Apr 19, 2015 #10
    See the force on the lever. on one side it force (action)100 N. And by the MA (9:1), the force in other side (reaction) is 900 N. ...and of course it didn't mean we put 1000 N to the level. because it sound false. this is not the Newton #3 law. this is Mechanical Advantage like on hydraulic system. we put 100 N force on input piston area (10 cm2) and gain 1000 N force (100 cm2), but we "lost/trade" distance. MA 100/10 = 10 ... 100 N x 10 = 1000 N

    In other side, if we put 1000 N (100 cm2) we only gain 100 N (10 cm2), but we also gain a distance as a trade. MA 10/100 = 0.1 ... 1000 N x 0.1 = 100 N

    But on the punching machine.. if we put 1000 N at the input area, and the output force are equal, no matter what size or design of the output are. because the distance while the punching machine input force move is equal with the punching plate to the material that punched. (it's a solid material as shown on my avatar)

    Meanwile if my avatar was a nozzle, the force on material (liquid or gas) betwen input and output will be diferent, depend on the size of the area

    If there's thing to be change on my mind is.. force for an solid and pressure for gas or liquid...
    Last edited: Apr 19, 2015
  12. Apr 19, 2015 #11
    Armand, not sure if you are familiar with "Work" (W), defined through W = F*s.
    A hydraulic system simply transfers work from one piston to the other, but it compacts the work over a much shorter distance at the receiving piston, thus increasing the force.
  13. Apr 19, 2015 #12
    Yes, the thing happend is MA (Mechanical Advantage) shorter distance as a trade of biggest force.
    but my "yesterday" problem is about Newton #3 Law, force on action and reaction is equal. and i found the clue is the law work on solid system
  14. Apr 19, 2015 #13
    yes, because the lever is a solid material, imagine the down side of the fulcrum is the mold, the earth surface is the punching area as the reaction, the lever is the force as the action.
    The lever representing the Mechanical Advantage of a simple machine
  15. Apr 19, 2015 #14
    Armand, while Mechanical Advantage is a neat number to specify a lever system, it after all really is just the fraction of input to output force.
    If you want to delve into why hydraulics works the way it does, you will need to embrace to more physical concepts such as Work, Power and such.
  16. Apr 19, 2015 #15
    Like Isaac Newton last words
    Sir Isaac Newton, British Scientist; died in 1727: 'I don't know what I may seem to the world. But as to myself I seem to have been only a boy playing on the seashore and diverting myself in now and then finding a smoother pebble or prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.'
    There always a new inventions by the application of the law of motion
  17. Apr 19, 2015 #16


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    There is a massive gap in those hydraulic diagrams. IF you want to include where and how Newton 3 applies, you need to have the force that is holding the hydraulics in place and the resulting extra upward force on it, when you press down on the pistons. Likewise for the diagram of the balanced moments in the other post. Without them, the systems will just accelerate downwards and the reaction force will just be Mass times the Acceleration.
  18. Apr 19, 2015 #17


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    No. The area does matter, because force = pressure * area. The pressure is the same, so the force is proportional to area.

    As already explained , the two different forces by the fluid on the pistons are not a 3rd law pair. Instead, each of them has a 3rd law partner exerted by the piston on the fluid, which are missing in those diagrams.
  19. Apr 20, 2015 #18
    What i found that Newton #3 law not precisely work on hydraulic system because it has a Mechanical Advantage where there was an exchange distance and force as result. but it work precisely on punching and molding machine, because it was a solid material. and yes the force per cm2 on the molding surface area are diferent depend the size of molding surface area but the summary of the total force on the surface molding area are equal with the force given to it.
  20. Apr 20, 2015 #19
    It works precisely in all situations. It is your understanding of it that needs a little more work. You are comparing forces that are not supposed to be pairs in teh sense of Newton's third law.
  21. Apr 20, 2015 #20


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    This is not correct. Read sophiecentaur's post again, and then carefully write down the forces (not pressures, forces) that are acting on the piston.
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