1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

On the nature of vacuum and questions thereof

  1. Jun 11, 2013 #1
    i have been pondering something. this is it: if a fellow had a tube with a plunger in it, like a syringe but without an opening for a needle or such. say the plunger is at the bottom of the tube. If you start to pull/raise it, it is my understanding that the force required to lift it would be equal to the weight of the column of air in the atmosphere described by the area of the plunger's cross section. Would this mean then that after that force had been met that then the energy needed would plateau? Say you go to pick up a 50 lb weight, you strain until it is lifted but then it does not get any harder. Is this the same with vacuum? Another thing: I you are in space, since here are no forces on either side of the previously mentioned tube-and-plunger, would suction exist?
  2. jcsd
  3. Jun 11, 2013 #2
    Once the force of the air pressure pushing down on the "plunger" is equal to the force you are lifting with, velocity will be constant as long as you can continue to apply that force.

    In space, there is no air pressure, so no suction would exist.
  4. Jun 11, 2013 #3


    User Avatar
    Science Advisor

    The force needed to pull a plunger with a vacuum does indeed plateau. Given a syringe this is fairly easy to do. You can pull a vacuum in a small syringe with little difficulty.

    That's not the same as saying that there is a plateau in energy. The farther you pull against a fixed force, the more work you perform and so the more energy you are putting into the system.
  5. Jun 12, 2013 #4
    hmm. I have tried this quickly and i was only able to pull the syringe a pretty short distance. (need to work out i guess!) but it made me wonder whether i was missing something. However, If the amount of force becomes a constant after the weight of atmosphere is equaled, this might make a good clock spring right?
  6. Jun 13, 2013 #5
    The force does not become a constant it varies with air pressure.
  7. Jun 14, 2013 #6
    ah rats, you're right.
  8. Jun 15, 2013 #7
    The atmosphere can still be used as spring because of it variability and a pretty good one, atmospheric clocks run mainly on the variation of the atmospheres pressure and dont need winding for years.http://en.wikipedia.org/wiki/Atmos_clock
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - nature vacuum questions Date
I Hamiltonian as total energy for natural systems? Sep 23, 2017
B What is meant by natural frequency? Sep 15, 2017
B Confusion about nature of collision Apr 30, 2016
Electromagnetic nature of light Aug 21, 2015
Nature of vacuum Oct 20, 2005