Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why is a vacuum sucked toward another object

  1. Apr 18, 2014 #1
    From the result of an experiment it seems to be that suction does indeed cause a force that causes a vacuum to move toward what is being sucked. Why this is we do not know. Maybe it is related to the Bernoulli Effect, that because we are restricting the entry area so that the air has to be sucked into the vacuum from a predominantly right angle and the KE of the air is creating a region of low pressure where it has to turn down to go into the vacuum hose. However we do not understand how that region can be lower pressure than the vacuum pressure in the hose. If anybody thinks they understand this suction of the vacuum source itself from a fluid dynamic standpoint or other standpoint they should offer us their theory.

    The experiment we ran can be viewed on YouTube at:
     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Apr 19, 2014 #2

    Drakkith

    User Avatar

    Staff: Mentor

    Let me ask you this. Why would the low pressure region be at a lower pressure than the hose? If anything I'd expect the opposite to be true.
     
  4. Apr 19, 2014 #3
  5. Apr 19, 2014 #4
    Kurros I read the wiki article about the Feynman sprinkler. Interesting but I do not understand the solution that they propose that the Sprinkler does not turn in reverse because of conservation of Momentum, although they say that if you could have a frictionless system the sprinkler will turn backwards. At any rate I do not know how that applies to my problem. I saw on YouTube another video about a Physicist Inventor that invented a suction system he says is a lot better than a Venturri tube. He also says that he has proved that Bernoulli was wrong. I am wrestling with the back pressure implications of his idea.
     
  6. Apr 19, 2014 #5
    Well I couldn't quite see what was going on in your video, but it looks like you just have a straight vacuum cleaner tube and are seeing if it can "pull" itself along? I would expect the conservation of momentum argument still applies, i.e. you are causing a net motion of fluid into the pipe, and therefore "something" should move in the opposite direction (assuming the air expelled from the vacuum cleaner points, say, upwards).
     
  7. Apr 19, 2014 #6
    No more than a partial vacuum being created.The hose could be substituted for a plunger in a syringe with your finger on the end of it.
    When pulled and a partial vacuum made, the plunger will shoot back in when released.
    A high pressure region moveing towards a low pressure one.The hose is trying to do that.
    The outside of the hose is at a higher pressure than the inside and the other regions towards the bottle.
    Why should it matter if the region inside the hose or outside was at a lower pressure than the other as long as the net constant pressure inside and towards the bottle is lower than the outside.
     
    Last edited: Apr 19, 2014
  8. Apr 19, 2014 #7
    The vacuum causes the pressure to fall below atmospheric pressure in the space between the end of the hose and the end of the bottle. The pressure will be higher and closer to atmospheric pressure at more distant places such as at the opposite ends of the hose and bottle. The resulting pressure difference on the two objects results in forces that tend to push them closer together. By holding the bottle in place it's the hose that tends to move.
     
    Last edited: Apr 19, 2014
  9. Apr 19, 2014 #8
    Kurros my inquiry is all about conservation of momentum really and goes along with my previous posts of why a hair dryer does not increase the momentum of its output by adding heat. Quoting from the Wikipedia article on sunction, "Dust being "sucked" into a vacuum cleaner is actually being pushed in by the higher pressure air on the outside of the cleaner. The higher pressure of the surrounding fluid can push matter into a vacuum but a vacuum cannot attract matter."

    So my question becomes what air is pushing the vacuum hose away from the vacuum source. I suppose it is the fast moving air coming up the end of the vacuum hose pipe and that air flow, that air flow and the vacuum pipe are only friction coupled, is that causing the pipe to move? It is not like the air can get behind the pipe and push it.

    Of the responses I have seen so far I think the video I found on Facebook "Bernoulli's Principal Disproved by Physicist Robert D. Hunt" has given me the most to think about. I am going to quote a little of his description here in case your interested.

    "After 275 years, the real cause of pressure is correctly understood thanks to these important finding by Physicist and Inventor Robert D. Hunt's research. With his greater understanding of fluid dynamics, Mr. Hunt invented a Hunt Vacuum Valve that is much more efficient than a Venturi Valve in forming suction; and, he has invented a method to increase the pressure of a fluid being capable of continuous flow of a fluid to a higher elevation"
     
  10. Apr 19, 2014 #9

    Drakkith

    User Avatar

    Staff: Mentor

    Yes, because there is no "sucking" force. Instead, there is a difference in pressure and the higher pressure side is what applies the force to cause the air molecules to move into the hose.

    Actually I believe that may just be the case. The air pressure is the same everywhere around the hose except at the end where it is at a lower pressure. So you have unequal forces which push the hose towards the area of low pressure.

    I highly HIGHLY advise you to not watch any video on youtube claiming to disprove any scientific law, principle, or theory. Actual scientists who know what they are talking about practically never post youtube videos to disprove something, they publish papers in peer-reviewed journals instead. Conversely, crackpots, cranks, and other people who simply don't know what they're talking about regularly post videos claiming to have disproved something about science, invented perpetual motion, or some other nonsense.

    I don't believe Robert D. Hunt actually understands Bernoulli's principle. A quick look at wikipedia's articles on Bernoulli's principle and the Venturi effect leads me to this conclusion. I think he's relying solely on the Venturi Meter he's using and not looking at the physics itself, which can lead to misunderstandings of what the meter is actually showing. For example, he manipulates the meter by rotating the nozzle to face the incoming air and acts like this disproves Bernoulli's principle when the meter changes. Of course it's going to change, he's forcing high velocity air into it by pointing the nozzle opening at the oncoming airflow. If anything, this may be measuring the dynamic pressure of the fluid, not the static pressure.
     
  11. Apr 19, 2014 #10

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Thread closed pending moderation.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook