Why are the forces in Newton's 3 laws equal, but not in hydraulic systems?

[armand]

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.
https://www.facebook.com/photo.php?fbid=1749685448591067&set=pb.100006488851023.-2207520000.1429362639.&type=3&theater
Thank you for guiding me to knowing the truth

 

[gleem]

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..

 

[SteamKing]

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..

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.

 

[A.T.]

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..

Ironically it's you who confuses pressure with force.

 

[A.T.]

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.

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.

 

[gleem]

@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..

 

[sophiecentaur]

@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..

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.

 

[armand]

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.
https://www.facebook.com/photo.php?fbid=1749685448591067&set=pb.100006488851023.-2207520000.1429362639.&type=3&theater
Thank you for guiding me to knowing the truth

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 :)

 

[Merlin3189]

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
.

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.

 

[armand]

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...

 

[rumborak]

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.

 

[armand]

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.

Yes, the thing happened 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

 

[armand]

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
.View attachment 82234
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.

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

 

[rumborak]

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.

 

[armand]

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.

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

 

[sophiecentaur]

View attachment 82186

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 :)

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.

 

[A.T.]

No matter the different size from input and output area, the force remain equal.

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.

 

[armand]

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.

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.

 

[nasu]

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.

 

[Nugatory]

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.

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.

 

[armand]

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.

sample on lever system
https://www.physicsforums.com/attachments/lever-gif.82234/
the force we put in effort arm is 1000 lb, it lift 9000 lb. it doesn't me we spend 10.000 lb, but the fulcrum representing the reaction of 10.000 lb from the lever.

 

[nasu]

Yes, this an example of your confusion. It was already pointed out and explained in previous posts. How come you persist in your error?
Newton's 3rd law applies to the forces of a specific interaction between two systems (or objects).
Here you have interaction of the hand (or whatever applies the 1000 lb force) with lever, of lever with the load and of the lever with the fulcrum.
For each one of this interaction, you have a pair of equal and opposite forces. This is Newton's 3rd law. The force at the fulcrum and the force applied by the hand has nothing to do whatsoever with N's 3rd.

 

[A.T.]

the fulcrum representing the reaction

The force at the fulcrum is not the reaction to the applied forces in the sense of Newton's 3rd Law. The thing that pushes the lever with 1000lbs gets pushed back with 1000lbs. That is Newton's 3rd Law, as explained to you multiple times here.

 

[armand]

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.

Thanks Menor Nugatory, yes, that's my yesterday problem, i got little confused betwen force and pressure. where F=MA and Pressure P=F/A. because both are "action" that able to move an object

 

[A.T.]

because both are "action" that able to moving an object

Forget that action/reaction terminology in the context of Newtons 3rd Law. It is meaningless.

 

[armand]

The force at the fulcrum is not the reaction to the applied forces in the sense of Newton's 3rd Law. The thing that pushes the lever with 1000lbs gets pushed back with 1000lbs. That is Newton's 3rd Law, as explained to you multiple times here.

yes, for the 1000lbs i put on the effort-side, it pushed me back with 1000lbs. meanwhile on the other load-side, there was 9000lbs lifted up by the system. overall the Earth pushing back 10.000lbs as reaction for action given by the system on the lever

 

[armand]

Forget that action/reaction terminology in the context of Newtons 3rd Law. It is meaningless.

yes, because on the hydraulic system, it was the Newton #1 law that working on it.

 

[armand]

Forget that action/reaction terminology in the context of Newtons 3rd Law. It is meaningless.

for molding and dies with a diferent system, impact/punching by solid material or pressing by hydraulic system of course i need to understanding. the diferent calculation.
For the impact/punching machine, I need to know how much force needed for mold to punching the dies or on the hydraulic system, i need to know how high the pressure i need to pushing the mold to the dies.

 

[sophiecentaur]

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.

You have two options. Either you have found a truth that no one has ever spotted before or you have misunderstood the situation. Which is more likely?

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.

@armand: Read this post and take his advice.

 

[rumborak]

The problem here, as I perceive it, is that Armand is trying to reason with "intuitive" quantities such as Mechanical Advantage, "punching" etc.
Armand, it should be rather obvious at this point that these quantities are failing you, and if you really want to proceed in your understanding, you need to switch to the well-established tools such as summation of forces etc. As sophie alluded to, the basic assumption in physics should be: If you think you found a hole in a well-established theory, it's 99.999% likely you misunderstood it.

 

[Nugatory]

I'm closing this thread now, as we are running out of new ways to say the same thing.

As always, PM me if you would like to add something new.