Fluids mechanics is also gas mechanics?

  • #26
Femme_physics
Gold Member
2,548
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I'm trying to understand this lever principle of fluid based on this:

http://img26.imageshack.us/img26/2637/levery.jpg [Broken]


Does that mean that I can apply a really small force on the right side and lifts a huge heavy car with it?


If so, it's incredible, and reminds me of the pulley lever!
 
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  • #27
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9


That diagram and others like it are a tad misleading.

Your force formulae are correct but the diagram suggest the piston moves the same distance as the plate under the car.

Of course the volume change is the same on both sides so

ALdL = ARdR

where d is the distance moved

So if the area of the plate under the car is 10 times the area of the piston the piston moves 10 times as far!

go well
 
  • #28
I like Serena
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Cool huh! :cool:

Although if you put a small pressure on the right side, it will become even harder to lift the heavy car! :smile:
 
  • #29
Femme_physics
Gold Member
2,548
1


That diagram and others like it are a tad misleading.

Your force formulae are correct but the diagram suggest the piston moves the same distance as the plate under the car.

Of course the volume change is the same on both sides so

ALdL = ARdR

where d is the distance moved

So if the area of the plate under the car is 10 times the area of the piston the piston moves 10 times as far!

go well

I see. So not as useful as I thought, but still pretty useful! :smile:



Cool huh! :cool:

Although if you put a small pressure on the right side, it will become even harder to lift the heavy car! :smile:


Noted!
 
  • #30
2,027
85


I'm trying to understand this lever principle of fluid based on this:

Does that mean that I can apply a really small force on the right side and lifts a huge heavy car with it?

That's how hydraulic jacks work, so yes.
It's also how brakes work.
 
Last edited:
  • #31
5,441
9


It is not fashionable to teach basic mechanics quantities these days but here are some that are applicable to this hydraulic lift and other purely mechanical things like levers and pulleys.

[tex]{\rm{VelocityRatio = VR = }}\frac{{{\rm{distance}}\,{\rm{moved}}\,{\rm{byload}}}}{{{\rm{distance}}\,{\rm{moved}}\,{\rm{byeffort}}}}[/tex]

[tex]{\rm{MechanicalAdvantge = MA = }}\frac{{{\rm{load}}}}{{\,{\rm{effort}}}}[/tex]

[tex]{\rm{Efficiency = }}\frac{{{\rm{MA}}}}{{{\rm{VR}}}}[/tex]

and finally what is really the law of conservation of energy

[tex]{\rm{load*distance}}\,{\rm{moved}}\,{\rm{by load = effort*distance}}\,{\rm{moved}}\,{\rm{by}}\,{\rm{effort}}[/tex]

Which you can see equals MA * VR
 
  • #32
I like Serena
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Similarly, it can sometimes be assumed that the viscosity of the fluid is zero (the fluid is inviscid). Gases can often be assumed to be inviscid. If a fluid is viscous, and its flow contained in some way (e.g. in a pipe), then the flow at the boundary must have zero velocity

This is because the fluid sticks to the walls, right?

Perhaps a bit late, but I'd like to answer this question anyway.

I'll stick my neck out and say: yes, it is because the fluid sticks to the walls.
Or rather, the friction between the fluid and the wall makes it stand still where it makes contact with the wall (in modelling).
 

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