1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Very basic phisics, hydraulics and weights, wrong diagram?

  1. Aug 15, 2013 #1
    Hej Guys,

    I am just wondering. See the picture.


    If I put 5kg on the right the water lever increases and the new water level on the left should be as much higher as 5kg volume of water. Its so obvious isn't it?

    So if it is obvious can you please tell me that why it is possible that in a exactly the same
    - http://hyperphysics.phy-astr.gsu.edu/hbase/pasc.html#hpress - diagram they hold a car with a fraction of the weight of a car on the left side?
    I cant understand it how it would be possible...

    Last edited: Aug 15, 2013
  2. jcsd
  3. Aug 15, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    It's a force multiplier.

    From the link:
    For example, if the lift cylinder were 25 cm in diameter and the small cylinder were 1.25 cm in diameter, then the ratio of the areas is 400, so the hydraulic press arrangement gives a multiplication of 400 times the force. To lift a 6000 newton car, you would have to exert only 6000 N/400 = 15 N on the fluid in the small cylinder to lift the car. However, to lift the car 10 cm, you would have to move the oil 400 x 10cm = 40 meters. This is practical by pumping oil into this small cylinder with a small compressor.

    It works kind of like a wrench. A short wrench requires much more force to turn something than a longer wrench does. However, the longer wrench has to move a much greater distance. So you're trading distance moved for an increased force multiplier.

    It's the same concept as me giving you a huge lever and you lifting up a tank with just your body weight.
  4. Aug 15, 2013 #3
    I understand what you write but then the 5kg/5kg example why true as well? can you help me understand that?
    You see I feel troubled because this two example is somewhat opposing?

    I have a feeling that we talk aside.. See this example. I if put 100 newton force the other side 100 newton force must come out. Not less force is enough top hold against 100 Newton force.


    or here is an other example. THere is the 100N push downwards that is distributes the force on the watered surface and then on the other side there will be less pressure? No it have to be 100 N + water weight.

    In other way the pressure distribution on the wall doesn't make the object lighter down on the other side... Just think about this, its not possible thet a 10kg object would be lighter to hold up, because of....

    Last edited: Aug 15, 2013
  5. Aug 15, 2013 #4


    User Avatar
    Science Advisor

    Consider that work is force * distance. If you push a large amount of fluid with a large piston into a small piston, the small piston will move much farther than large piston. The work is conserved, so pushing on a large piston must reduce the force that the small piston pushes with, since the small piston moves farther.
  6. Aug 15, 2013 #5
    yeah this is clear thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook